Introduction

Carduus nutans also known as Musk Thistle or Nodding thistle is a widespread invasive species found throughout most of southern Canada and almost the entire USA. It is a herbaceous biennial plant in the Carduoideae subfamily of the Asteraceae (Sunflower) family. While in cooler climates it typically takes 2 years to produce seeds before the plant dies in warmer climates it may grow and go to seed in a single season. It is frequently found on roadsides, in pastures, meadows, empty lots, disturbed land, and waste places throughout its range.

Description of Carduus nutans

Leaves & Stems

These biennial plants at maturity will reach 150 – 250 cm tall and typically have multi-branched stems. Occasionally single-stem specimens can be found under poorer conditions or in areas with lots of competition. The stems are covered with spines and a woolly covering of hair with occasional small leaves. Stems and leaves sprout from a stout taproot up to 40 cm long.

Leaves develop from a basal rosette and reach 40 cm long when mature. When young the leaves are somewhat lobed and slightly pubescent hairy and more or less upright but as they mature they become prostrate and more heavily pinnately lobed and have prominent sharp yellowish to whitish spines at the lobe margins. The surface is dark green, waxy, and pubescent (hairy) on top with woolly veins on the lower surface.

Flowers & Fruits

Flowers form in large globose heads made of hundreds of tiny but showy reddish-purple disk flowers. Rarely, white variants can be seen. Flower heads are 3 – 5(-7) cm in diameter and form at the end of the stems. Upright immature heads begin to droop as they mature at 90° to 120° from the stem. Heads are surrounded by woolly spiny tipped phyllaries that often are purplish. Outer phyllaries reflex at maturity while the rest of them are constricted in the middle with blades that are narrower than their bases.

Fruits are cypselae which are typical for the Asteraceae family. They consist of a small angled seed (frequently mistaken for an achene) 3 – 4.5 mm long that is gold, fawn, or brown with fine wrinkles and a white pappus of numerous hairs up to 2 cm long. These fruits are suitable for dispersal by wind due to the small light seeds and light pappus that catch wind currents. Each flower head produces hundreds up to 1200 cypselae.

Similar Species Frequently Confused With

There are many thistle species in North America that Carduus nutans may be confused with. Some are native and some are introduced. Following is a list of the most similar-looking species and how you can differentiate them.

In the Carduus genus we have:

• Carduus acanthoides the Broad-Winged Thistle found in the eastern USA and Canada, central USA, and south-western Canada. Native to Eurasia it has similar leaves and growth habits but the flower heads are smaller and not globose or nodding like Carduus nutans. Instead, its mature flowers are more or less erect, 1.3 – 2.5 cm across and its achenes have a pappus that is 1.1 – 1.3 cm long. Hybrids between the two species have been observed in some of the eastern US states which may make identification difficult. However, since no Carduus species are native to North America this should not be a problem for control purposes as long as it is identified to the genus level.

Most of the other less closely-related species that also look similar are in the Cirsium genus. They can be differentiated as follows:

• Cirsium vulgare the Bull Thistle is also Eurasian and occupies almost the exact same range as Musk Thistle but extends further into Mexico and in the US is found in Vermont and Maine as well. It is fairly easy to tell apart, however, by its much more bulbous-shaped receptacle and smaller phyllaries that have many more layers and are more appressed to the receptacle. The flower heads are less rounded and generally a bit smaller than Nodding Thistle. Leaves are more of a grey-green and are generally smaller to only 30 cm.
• Cirsium arvense the Creeping Thistle is native to Eurasia and is now found throughout the northern half of the USA and most of southern Canada and some in the northern Territories and eastern Alaska. It tends to grow in clonal colonies by sending up shoots that grow into plants on more slender stems to only 150 cm tall that are smooth and hairless and generally not spiny. The spiny leaves are smaller only 20 cm long. Flower heads are also much smaller only 2.2 cm wide which grow in clusters on the tops of stems instead of singly.
• Cirsium horridulum the Bristle Thistle is a native plant found in the eastern US from New England south to Florida and west to Texas and Oklahoma as well as parts of Mexico and eastern Central America. It grows to similar heights and has leaves of similar sizes but the often purple-tinged leaves have thicker spines that are more densely placed on the leaves and up the stem. Flower heads are also much spinier and disk flowers may be pink, reddish, white, or even yellow.
• Cirsium pumilum the Pasture Thistle is native to the north-central and northeastern USA as well as in Ontario, Canada. It only grows to 100 cm tall and its leaves are shorter to 30 cm long. It has a much longer narrowly egg-shaped receptacle with more rows of phyllaries that are not reflexed. Flowers may be pink, purple, or white and flower heads are less dense generally containing fewer than 100 disk flowers.
• Cirsium pumilum the Wavy Leaf Thistle is native to most of western North America from BC east to Manitoba, Canada, and south to Durango, Mexico. It grows to similar heights but is typically less branched, if at all. Its leaves are much more undulated, more shallowly lobed, and grey rather than dark green. The flower heads are of similar size but generally much lighter pink or even white and the receptacle is much more elongated with more appressed phyllaries.
• Cirsium ochrocentrum the Yellow Spine Thistle is native to the Central Plains and western deserts of the US and northern Mexico. It grows to only 100 cm tall and its deeply lobed and spiny leaves only grow to 25 cm long. Its phyllaries are appressed to the receptacle and they are tipped with yellow spines. Flower heads contain white, pink, or lavender disk flowers.
• Cirsium texanum the Texas Thistle is native to the south-central US and northern Mexico. It is much smaller growing to only 80 cm tall and it has light purple or light pink disk flowers. Its phyllaries are not reflexed and are green and white and tipped with spines. The leaves have fewer spines and are dark green above but woolly white below.
• Cirsium discolor the Field Thistle is native to central and eastern Canada and the US. It grows to 200 cm tall but can easily be distinguished by its always solitary stem and its leaves that are green above but white and woolly below.
• Cirsium edule the Edible Thistle is native to the Pacific Northwest from Alaska south to Oregon and east to Idaho. Along with its narrow range, it can easily be distinguished by its spiny reflexed arachnoid phyllaries (covered with a mass of downy white hair).
• Silybum marianum the Milk Thistle is native to Eurasia but can be found in North America mostly in the coastal states on both the Atlantic and Pacific side with fewer plants in the central regions though they are still present. The easiest way to tell this one apart is by its distinctly variegated leaves mottled in pale green and white. To learn more about this invasive species check out my blog at https://lyraenatureblog.com/blog/milk-thistle-silybum-marianum/

Native Distribution of Carduus nutans

Musk or Nodding Thistle was originally native to much of Europe and western Asia as well as northern and eastern Africa where it grew in fields, meadows, and disturbed habitats.

Habitat Types Where Carduus nutans is Found

Carduus nutans typically grow in meadows and grasslands and are often found in heavily grazed pastures. Stout taproots allow it to easily survive in pastures as it can be more resistant to drought than other pasture plants. It is also found in any disturbed open soil such as roadsides, cleared land, and waste sites. Areas subject to natural disturbances such as landslides and flooding are also suitable habitats.

Carduus nutans prefers open sun and will not grow in excessively wet or dry areas or shady sites such as under the forest canopy. It tolerates neutral or acidic soils and grows from sea level to 2500 m elevation.

Human Uses of Musk Thistle

The pith of the stem can be boiled and eaten like asparagus and is said to have a pleasant taste.

Medicinally the flowers are sometimes used to lower fevers and as a blood purifier. In India studies done with the linoleic acid in the seed’s oil have been shown to help prevent atherosclerosis.

Distribution of Carduus nutans in North America

The species was first accidentally brought to eastern North America in the mid-1800s likely in a ship’s ballast water. Since then it has become very widespread and is found in most US states and most of southern Canada.

In Canada, Carduus nutans have been recorded in most of the southern provinces including British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, and Nova Scotia. Its status in Newfoundland is uncertain and it has not been reported in Prince Edward Island or Labrador nor in any of the Arctic territories.

In the USA, Musk Thistle is found in most of the continental USA excluding only Vermont, Maine, and Florida. It is also not currently found in Alaska or Hawaii.

In Mexico, Carduus nutans so far have only been reported in Mexico City and Mexico State. Given its proximity to the northern border, and the as-of-yet under-reporting of invasive species in Mexico this information will change in the near future.

Musk Thistle has been introduced on every continent except for Antarctica. It has been declared a noxious weed in Canada, the USA, Australia, New Zealand, and South Africa.

How Carduus nutans Spreads

Propagation is strictly by seed. Seeds remain viable for only a couple of years on the soil surface. However, if seeds are buried 20 cm they can last up to 80 years in the soil seed bank.

It is currently primarily spread through long distances as a contaminant in uncertified seeds sold out of the country. Within the same country, long-distance dispersal also results from seeds carried as a contaminant in hay as well as on equipment and vehicles.

Short-distance dispersal occurs through seeds dispersed short distances by wind or transported on vehicles, equipment, etc.

Habitats at Risk of Invasion in North America

Carduus nutans prefer meadows, fields, pastures, disturbed ground, farmland, and any open land with good exposure to sunlight. Since it grows up to 2500 m elevation all open areas to this elevation are at risk. Mature forests, permanent wetlands, and deserts are not at risk as they will not grow in the shade or in permanently wet or excessively dry soil.

Impacts of Invasion

In pastures and farmlands, Carduus nutans can negatively impact production by suppressing the growth of more desirable plants through competition. Carduus nutans plants do well in pastures because of the deep thick taproots that allow them to outcompete grasses and other forage plants for water and nutrients. Furthermore, in dense mature stands, they can become a physical barrier to livestock due to their thick sharp spines.

In natural environments, Musk Thistle can outcompete native plants for resources thereby reducing biodiversity. They can also become a barrier to native animals that live there.

They can also be a barrier in recreational areas impeding hiking, swimming, and other recreational activities.

Potential Benefits of Invasion

Musk Thistle is of benefit to bees, hoverflies, and butterflies that visit the flowers. The plant is also a food source for numerous caterpillars in the Lepidoptera order.

Methods to Remove Carduus nutans

As always prevention is the preferred method of control. While Musk Thistle is never intentionally planted by people it can be unintentionally introduced in uncertified seeds. This is particularly common in seeds imported from other countries with less strict guidelines on seed purity and invasive species. Do not buy or plant poor-quality seeds.

An important source of infestation is in hay fields and pasture land. One method to help prevent their infestation is to plant more drought-tolerant species so that they are less likely to be outcompeted by Carduus nutans. Furthermore, avoiding over-grazing during periods of drought will help prevent them from successfully spreading.

Physical Control of Nodding Thistle

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming but it usually causes the least amount of environmental damage.

The best time to remove Carduus nutans is when the plants are young or during flowering but before they have gone to seed. If your plants have seeds it is best to manually cut the seed heads off into a garbage bag to prevent them from spreading. Once the seed heads have been cut the plants can be dealt with by other physical means.

Physical methods to remove Nodding Thistle in less dense stands involve digging the plants up. This can be done with a shovel or hoeing the ground to 10 cm below the soil surface to ensure that the tap root is sufficiently removed or damaged. Fortunately, the plant cannot survive once a sufficient amount of its taproot has been removed. This works for both young and mature plants.

Mowing can be used on larger populations of Carduus nutans just prior to going to seed. The mowing will need to be repeated each month during the growing season due to the variation of maturity in the plants. Young plants that have not started blooming will not be affected by mowing so the area will need to be mowed for at least 2 – 3 years.

Disposal of the Plants Once Removed

If you have removed the seeds or have removed plants that are not yet in seed they can be disposed of in a compost heap or left on the soil where removed. Carduus nutans can only reproduce by seed so the rest of the plant is not a disposal concern. However, if you have plants that have seeds on them they must either be burned or solarized. To solarize put the shrubs under a thick black tarp, or into thick black garbage bags and leave them in the full sun for 8 weeks at least to be sure that all seeds are no longer viable.

Chemical Control of Carduus nutans

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used species riches significantly increase.

If using chemical control it must be done when the plants are young, actively growing, and before the basal rosettes get too large. This makes adequate timing in spring imperative. Furthermore, repeated applications in New Zealand are starting to show resistance to herbicides.

Chemical control is not recommended.

Biological Control of Musk Thistle

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day there are still rats in Hawaii but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective. Following is a list of biological control methods that have been used in North America in an attempt to help control Carduus nutans.

• Rhinocyllus conicus is a weevil that has been introduced to fight Carduus and several other invasive thistle species including those in the Cirsium genus. The problem in North America is that we have several native Cirsium species so the use of this method is not recommended by some scientists. Furthermore, the success rate has had mixed results so the benefits may not outweigh the risk.
• Trichosirocalus mortadelo is perhaps a better choice as it is probably restricted to Carduus nutans and it has been used with some success in Canada, the USA, Australia, and New Zealand. There is still some confusion as there may have also been T. horridus introduced and it is uncertain as of yet which one is effective or more effective. More research is ongoing.
• Domestic goats are not selective in their choice of forage greens and will eat anything including Carduus nutans. Domestic goats can be used in pasture lands to help control the spread of Nodding Thistle. They can also be penned in smaller areas and used in conjunction with physical removal and ongoing monitoring.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form this involves physical removal methods, possibly biological control methods, replanting with drought-tolerant species, and ongoing monitoring. Integrated management is required because the area needs to be monitored for new or surviving plants otherwise all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting is Crucial

In all cases of large-scale physical removal, the site should be replanted immediately because the bare soil will allow the seed bank to germinate and reinvade the patch they were removed from. A replanting program should already be planned and ready to implement immediately upon the removal of the Carduus nutans.

Ongoing Monitoring is Essential

In all cases of invasive Musk Thistle removal, ongoing monitoring is absolutely essential. Since they only reproduce by seed an aggressive monitoring program is not necessary. Apart from the repeated monthly mowings suggested in the physical removal of large patches most monitoring programs can be simply done on a yearly basis. Each year check for any surviving or new individuals and remove them so that they are not allowed to set seed.

References and Resources

CABI on Carduus nutans https://www.cabi.org/isc/datasheet/11259

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Eflora Plants of North America http://www.efloras.org/browse.aspx?flora_id=1

iNaturalist Plant Search https://www.inaturalist.org/home

Plants For A Future on Carduus nutans https://pfaf.org/user/Plant.aspx?LatinName=Carduus+nutans

USDA Plants Database https://plants.sc.egov.usda.gov/home

Willis, Lyrae (2022).  Plant Families of North America. Not yet published.

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If you are able to donate so that I can continue this non-profit work of supplying people with scientific information on the plant families, native plants, and invasive species found throughout North America, please donate using the GoFundMe link below. Thank you!

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Introduction

Convolvulus arvensis, commonly known as Field Bindweed, is a well-known plant that has been recognized as being invasive in North America for more than 100 years already. This is because of its detrimental effects on agriculture, which led to its early recognition as an invasive weed. Once established, Field Bindweed is notoriously difficult to eradicate. Instead, most removal programs should focus on control rather than eradication. It is already a significant part of North American ecology, so we will never rid ourselves of this weed. However, new patches can be eradicated before they become too well-established. Any growth in natural areas, particularly nature reserves and parkland, should be eradicated where possible in order to preserve the native species also growing there. Agricultural lands, on the other hand, can focus on controlling this invasive pest to mitigate the damage they are causing.

Description of Convolvulus arvensis

Leaves & Stems

Convolvulus arvensis is a herbaceous perennial trailing or climbing vine. It grows from a very deep persistent root system that develops shoots from buds on the roots down to depths of 1 m. It has a main tap root that can grow from 0.5 – 3 m long. Other vertical roots from the main root may penetrate from 5 – 9 m deep, depending on the soil and site conditions. The roots also spread laterally in all directions from the main tap root allowing it to spread adventitiously throughout all of its length. Lateral roots are shallower and generally only penetrate down to 30 cm deep.

The stems are slender and grow from 20 cm to 2 m long and are either hairless or finely pubescent. Stems twine anticlockwise allowing them to climb over structures and other plants.

Leaves are alternate, have a petiole (leaf stalk), and vary significantly in shape and size. They may be lanceolate, ovate, or narrow-oblong and range in size from 1 – 10 cm long and 0.3 – 6 cm wide. The tips are acute, and the leaves themselves may be entire or hastate–sagittate (arrow-shaped) at the base and may be hairless or lightly pubescent.

Flowers & Fruits

Flowers of Convolvulus arvensis are axillary and may be solitary or found in cymes of 2-3 flowers on peduncles (flower stalks). They are subtended by bracteoles 2-4 mm long. The flowers have sepals that are free and obtuse in shape and 2.5 – 4.5 mm long. It has a funnel-shaped corolla with 5 radial pubescent bands. The corolla is entire and not divided into individual lobes, 1 – 2.5 cm in diameter and 1 – 2.5 cm long, and may be found in white or pink varieties. It has 5 stamens that are inserted in the corolla tube and a single style with 2 oblong stigmas.

Field Bindweed produces a capsular fruit that is globular or ovoid with a persistent style at the base. It breaks open irregularly and releases four seeds that are 3 – 5 mm in diameter, dark brown or black, with a granular testa (outer coating).

Toxicity

Convolvulus arvensis is considered mildly toxic. It contains tropane alkaloids that can have toxic effects on the autonomic nervous system and have been found to cause intestinal fibrosis in horses. The seeds are especially toxic compared to the rest of the plant.

Similar Species Frequently Confused With

Many, many plants are often confused with Convolvulus arvensis. By far, most of them belong to the Convolvulaceae family and have similar bell-shaped flowers. Occasionally people mistake the unrelated Oenothera speciosa or Evening Primrose for Field Bindweed with its pink flowers. However, this can easily be differentiated by the fact that its corolla actually has 5 overlapping petals and is not at all entire like Convolvulus arvensis. The other non-related plant that is sometimes confused with it is Fallopia convolvulus Black-bindweed or Wild Buckwheat of the Polygonaceae with similar leaves but reddish stems and small lobed flowers. The more closely related and similar-looking species can be differentiated as follows:

• Calystegia sepium (sometimes as Convolvulus sepium) Hedge Bindweed of the Convolvulaceae family, has a sub cosmopolitan distribution with several subspecies native throughout North America. It has pale matt green sagittate leaves that are arranged spirally rather than alternately on the stem. When in bud, the flowers are surrounded by green bracts that are tinged with crimson, and its flowers, when open, are much larger, from 3 – 7 cm in diameter. Its fruit is an almost spherical capsule.
• Calystegia subacaulis of the Convolvulaceae family is a narrow-range species endemic to the North and Central California Coast Ranges and the San Francisco Bay Area, where it is found in woodland and chaparral scrub habitat. In its range, it can be differentiated by its hairy stems and leaves and its short stems that grow no more than 20 cm long.
• Calystegia macrostegia Bellflower of the Convolvulaceae family also has a narrow range found along the Pacific coasts of southern California and western Mexico. Its leaves are large and triangular and are often more than 10 cm wide. It also produces longer stems to 9 m in length and larger flowers 2 – 6 cm in diameter.
• Calystegia spithamaea of the Convolvulaceae family is native to eastern North America, primarily the northeastern USA. Its stems are mostly erect rather than trailing or twining, its light green leaves are quite hairy, and its flowers are not entire and instead have 5 shallow lobes.
• Convolvulus equitans the Wounded Bellflower is native to the southern US and northern Mexico. It can be differentiated by its usually lobed rather than entire leaves and its corolla of similar size but with 5 distinct though shallow lobes, each with a very distinctly acute apex. The flower also has a distinct pink or purple throat giving it the common name of Wounded Bellflower.
• Calystegia silvatica Greater Bindweed is native to Europe but has been introduced to the eastern and western US and Canada. It can easily be differentiated by its larger leaves that are arrow-shaped and its very large flowers up to 9 cm in diameter that are always white and never pink.
• Calystegia purpurata Pacific False Bindweed is a narrow endemic found along the Pacific coast of California. It has lobed leaves that are triangular in shape and larger flowers up to 5 cm in diameter that vary in color from white to pink to purple or cream and often have purple stripes.
• Evolvulus sericeus Silver Dwarf Morning Glory of the Convolvulaceae family is native to the southern USA, Mexico, and parts of tropical Americas. It can easily be differentiated by its very small size, only about 30 cm tall, and the fact that it grows erect rather than a vine. It also has linear to narrowly lanceolate light green or gray-green leaves that are generally hairy.
• Calystegia soldanella the Dunebell is native to beaches of western North America, Europe, and East Asia. It is found growing only on beach sand dunes making it easy to differentiate by its location. Its leaves are also very thick, glossy, and fleshy, producing large singular pink flowers.
• Calystegia occidentalis Chaparral False Bindweed is native to California and Oregon, where it grows in foothills and montane habitats. It has small leaves up to 4 cm that are typically hairy, lobed, and arrow-shaped. It produces 1-4 flowers on a single stalk, and each flower is fairly large, from 2 – 5 cm in diameter, and is white, cream, or yellow in color but never pink.
• Bouchetia erecta of the closely related Solanaceae family is native to the gulf coast of the US. It is a much smaller plant, never more than 30 cm tall, and it has lanceolate leaves and lobed corollas containing bright yellow stamens.
• Ipomoea imperati  Beach Snowdrop of the Convolvulaceae family is native to the Americas but is restricted to beach sand dunes. Its white flowers are similar in shape but are usually larger and may be tinged with blue. Its leaves are highly variable in shape but are glossy and fleshy.
• Ipomoea lacunosa  Snowdrop of the Convolvulaceae family is native to eastern North America, mostly in the USA. It has a much smaller taproot, but the vines can grow to similar lengths, 2 m long. Its leaves are always large (8 – 9 c long) and are ovate when young but become cordate (heart-shaped) when mature, and they are always on long petioles (leaf stalks) at least 3 cm long. The similar-sized, usually white but occasionally pink flowers may or may not be shallowly lobed. It has distinctive pinkish or purple anthers on white filaments, and it produces large seed capsules that are spherical and hairy.

Native Distribution of Convolvulus arvensis

Field Bindweed is native throughout much of Europe and Asia in temperate, tropical, and Mediterranean regions.

Habitat Types Where Field Bindweed is Found

Convolvulus arvensis grows successfully in a wide range of temperate, tropical, and Mediterranean climates. It is very problematic for agriculture in the temperate zones between 60°N to 45°S latitude, though it also grows in tropical regions as well. It is a serious pest in wheat, barley, corn, legume, and sugar beet fields as well as vineyards and tree crops.

While it is known mostly as an agricultural weed, it also is frequently found in natural areas, particularly in riparian habitats and open communities such as meadows and grassland. They thrive in areas with cleared and disturbed ground, such as agricultural areas and fallow fields, as well as new construction, land clearing, roadsides, and waste areas.

Human Uses of Field Bindweed

Convolvulus arvensis has a number of medicinal properties. A decoction of the root may act as a diuretic and a laxative. Tea from the flowers and or the leaves has been used to treat fevers and wounds. Caution should be used, however, as taking too much or for too long could potentially cause blood in the urine and toxic effects on the autonomic nervous system. In small doses over short periods of time, it is considered safe.

The stems are sometimes used as twine for tying up plants and other things. While it works well as twine, its usefulness is short-lived, so it should not be used for long-term storage of things, for example. A green dye is sometimes made from the whole plant.

Distribution of Convolvulus arvensis in North America

The species was first brought to the US likely as a contaminant in seed though it was also intentionally planted in baskets. It was first reported in the wild in Virginia in 1739 and, by the 1800s, had already spread throughout the eastern seaboard. By 1838 it had made its way west to California.

In Canada, Convolvulus arvensis has been recorded throughout the southern provinces of British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, and Prince Edward Island. So far, it has not been reported in Newfoundland or the northern territories.

In the USA, Field Bindweed is even more widespread. It has been reported in every state in the continental USA as well as in Hawaii. It is absent only in Alaska.

In Mexico, Convolvulus arvensis so far has been reported in Baja California Norte, Baja California Sur, Sonora, Sinaloa, Chihuahua, Coahuila, Nuevo Leon, Durango, Tamaulipas, San Luis Potosi, Jalisco, Zacatecas, Aguascalientes, Guanajuato, Queretaro, Hidalgo, Michoacan, Mexico State, Mexico City, and Puebla. Given that the climate is suitable for it in many other states on the Caribbean side and in the south, it is likely that its range is still spreading.

Eurasian Bindweed has been introduced on every continent except Antarctica.

How Field Bindweed Spreads

Convolvulus arvensis was first introduced to the Americas by accidental introduction as a contaminant in other seeds. It may have also been intentionally planted in flower baskets. Since it spread as a contaminant early in the days of European settlement, it has already invaded much of the habitable zones in North America suitable to its growth.

Short-distance dispersal occurs through contamination of its seeds in other plant materials, where it grows as a weed in fields with agricultural crops. Short-distance dispersal of seeds can also occur via water, clothing, animals, vehicles, and machinery.

Birds can also disperse the seeds short and long distances because the seeds can remain viable in the digestive tract for up to 144 hours making migratory birds an important source of long-distance dispersal.

Dispersal also occurs with poorly disposed of yard waste debris filled with seeds or simply cut plants and roots that can regenerate via fragments. Solarization of removed plant matter is critical to prevent further spread this way.

Habitats at Risk of Invasion in North America

Most habitats in North America at risk of invasion have already been invaded. These include any disturbed areas such as agricultural fields, pastures, roadsides, and waste areas. Its range in North America will likely continue to spread in parts of Mexico where it has not yet reached its potential or has not yet been accurately reported as already being there. As climate change continues its warming trend in northern latitudes, it seems likely that it will continue to spread northwards in the southern provinces of Canada and may, in some locations, particularly near the coast, even reach the southern parts of the Northern Territories.

Impacts of Invasion

Field Bindweed threatens agricultural crops where it has been found to compete for water and nutrients, resulting in reductions in yields anywhere in the range of 0 – 100%. While only studied in crops, it is logical to assume a similar threat to native plant communities. In addition to competition for water and nutrients, it is an aggressive grower and climbs over other vegetation, physically smothering it and reducing biodiversity in that way as well.

It has been reported to be mildly toxic to some grazing animals, horses in particular. However, sheep, pigs, and goats seem more resistant to the toxic effects.

Potential Benefits of Invasion

It does not supply any significant nutritional food sources for native or domestic species and threatens species richness. There are no potential benefits of its invasion.

Methods to Remove Field Bindweed

As always, prevention of invasive species introductions is the preferred method. However, in the case of Convolvulus arvensis it has already been well established throughout North America, so control becomes the next best option. Control, however, is made particularly challenging due to both the longevity of the seeds in the soil seed bank (20 or more years) and the ability of the plant to regenerate via fragmentation of the stems or roots.

Fortunately, no one appears to be selling Convolvulus arvensis, so deliberate introductions of new plants are not likely to happen. Sadly this is not the case with many invasive species. Perhaps because Field Bindweed is such a well-known pest plant and has been known as such for well over 100 years now, people have actually stopped intentionally planting it.

Physical Control of Convolvulus arvensis

Once already established, physical control is always the most effective means. Physical control is labor-intensive and time-consuming, but it usually causes the least amount of environmental damage. Eradication of Field Bindweed once well established is difficult to impossible, but control and mitigation can be achieved.

Physical methods to remove Field Bindweed generally involve removing mature individuals before they go to seed to reduce the amount of seed in the soil. Since seeds are resilient enough to last more than 20 years in the soil, this makes control particularly challenging. Each year ongoing monitoring will be needed to remove any new seedlings as they generate from the soil seed bank.

The other added factors that make the control of this plant particularly challenging are its ability to resprout after being cut down, as well as its ability to generate new plants via fragmentation. When mature individuals are removed, try to pull out as much of their extensive root system as possible. This includes their very long and deep taproot as well as the lateral roots it produces that generally remain in the top 30 cm of the soil. Use a weed puller or a shovel to dig the taproot. For lateral roots, try to trace them from the main taproot along the soil. Heavy or compacted soils make this challenging, but the more root you remove, the better.

Since it can resprout from any remaining fragments, this absolutely must be followed with ongoing monitoring several times throughout the growing season. This can be done via the cutting of new sprouts or the cultivation method. To use the cutting method, simply cut any shoots that are regenerating, being sure to cut about 8 cm below the soil surface and repeat this for several years. Eventually, fewer and fewer sprouts will regenerate until the food reserves in the root system are completely depleted, and they can no longer regenerate. The more frequent the cutting, the less the root system has a chance to restore any energy stores, and the less treatment time is required. At a minimum, 2-3 years of extensive treatments will be required.

Frequent cultivation is a popular method to deal with resprouting mature plants and new seedlings. If the area is cultivated with a hoe or using agricultural equipment every 14 days throughout the growing season, the Field Bindweed can sometimes be controlled in as little as 2 years. It is critical to return to the patch and cultivate every 14 days, however. If left too long, the new green leaves will supply more energy to the extensive root system and prolong the necessary treatment time.

Solarization of large patches is a less labor-intensive method, and it will both deplete the root reserves and destroy any new seedlings as they try to emerge. Cover the area with a large black or other dark tarp and weigh the tarp down, leaving it there for 3 – 5 years. With many invasive plants, 2 years is sufficient, but due to the extensive root systems of Convolvulus arvensis, a minimum of 3 years is absolutely necessary. Prematurely removing the plastic mulch will no doubt result in re-infestation. The area should also still be monitored around the tarp edges throughout the growing season to look for lateral roots that try to sprout new plants outside of the tarped area.

Disposal of Convolvulus arvensis Once Removed

All plant material, whether it has seeds or not, must either be burned or solarized. Even without seeds, Convolvulus arvensis can generate new plants via root and stem fragmentation, so all plant material must be destroyed before disposal to prevent new infestations from carelessly discarded plant waste. Burning is a quick and effective method to dispose of the waste. However, it is not allowed in all areas and certainly not in all seasons. The other option is to solarize the material. To solarize, put the shrubs under a thick black tarp or into thick black garbage bags and leave them in the full sun for a good 8 weeks at least to be sure that all seeds and fragments are no longer viable. Some sources recommend shorter solarization periods, but this varies with latitude, aspect, cloud cover, etc, so leaving as long as possible ensures complete sterilization of the plant debris. Once properly solarized, the material can then be discarded at your local dump but do still inform them of what the material is so that they can properly dispose of it.

Chemical Control of Field Bindweed

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used, species riches significantly increases.

Furthermore, there are no chemical control methods that effectively target only Field Bindweed. Also, Field Bindweed has been shown to become herbicide-resistant in some cases after repeated application. Due to the physical nature of Field Bindweed, multiple applications are always needed.

Chemical control is not recommended.

Biological Control of Convolvulus arvensis

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day, there are still rats in Hawaii, but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach.

Since we have so many native Convolvulus species in North America, and some of them narrow endemic species such as those in California in particular, biological control is not an option. Multiple biological control agents have been tested for Convolvulus arvensis, but so far, all of them have also impacted our native species.

Grazing can help control Field Bindweed, but cattle will not eat it, and horses should not eat it. Sheep and goats will eat it but prefer other plants, including native species. Pigs, however, seem to love the entire plant and will eat both the above-ground plant matter and the roots if given the time. If you have an area that needs to be controlled, build a fence around it. Then allow the pigs to graze unchecked for 2 years, and they should destroy all of the Convolvulus arvensis growing there. Using goats or sheep in a similar manner can also be effective. However, since sheep and goats do not eat the roots, they would need to remain fenced in the patch for 3-4 years instead of 2.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest, most effective, and least impactful form, this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. Integrated management is required because the area needs to be monitored for returning sprouts or seedlings; otherwise, all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting With Native Species is Crucial

In all cases of large patch removal, the site will need to be replanted immediately because the bare soil will allow the seed bank of Field Bindweed and other invasives in the soil to germinate and reinvade the patch they were removed from. A replanting program should already be planned and ready to implement immediately upon successful removal of Convolvulus arvensis. If you are just removing an isolated individual replanting with native species is generally not necessary as the native species around it will simply fill in the spot left behind.

Ongoing Monitoring is Essential

In all cases of invasive Field Bindweed removal, ongoing monitoring is absolutely essential. Regular monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. In the first 3 years, monitoring should be done every 2 weeks to ensure that resprouting plants are destroyed to deplete the energy reserves of their extensive root systems. After the first 3 years, monitoring can be cut down to 2-3 times per growing season to remove any seedlings that germinate or any rootstock still attempting to recover.

Monitoring is required whether the area is replanted or not. Do not assume that once replanted, Convolvulus arvensis will not still try to grow. Field Bindweed is aggressive and prolific and will outcompete planted vegetation if yearly monitoring is not put in place to remove young native plants before they have a chance to become established. Once the area is well established with mature native plants, a simple once-a-year monitoring program is sufficient to root out any new individuals that may try to pop up. And you get to view the successful rewards of your hard work!

References and Resources

CABI on Convolvulus arvensis https://www.cabi.org/isc/datasheet/15101

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Fire Effects Information System on Field Bindweed https://www.fs.fed.us/database/feis/plants/vine/conarv/all.html

iNaturalist Plant Search https://www.inaturalist.org/home

USDA Plants Database https://plants.sc.egov.usda.gov/home

Willis, Lyrae (Unpublished).  Plant Families of North America.

Currently Seeking Funding To Continue This Non-Profit, Ad-Free Work

If you are able to donate so that I can continue this non-profit work of supplying people with scientific information on the plant families, native plants, and invasive species found throughout North America, please donate using the GoFundMe link below. Thank you!

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Introduction

Wisteria sinensis Chinese Wisteria and Wisteria floribunda Japanese Wisteria are both popular garden ornamentals that have become invasive throughout North America. In Canada, it only grows in the warmest regions near the coast and so far has not become invasive there. However, it is becoming very invasive throughout the eastern USA, where the climate is similar to its native range in China. There you can see it growing over the top of trees and old buildings and smothering native vegetation. Even in areas where it has not yet become invasive, it is well known to be aggressive and difficult to keep in check, requiring routine maintenance in your garden to keep it from growing over your other plants. There are so many lovely native vines in North America that are not aggressive and invasive, do yourself a favor and research what is native to your area and grow those instead. For instance, if you live in the eastern USA, why not grow American Wisteria or Trumpet Creeper that are native to this area?

Many of the populations found to be invasive in the eastern US have recently been discovered to be hybrids of Wisteria sinensis and Wisteria floribunda. While this article focuses on Chinese Wisteria Wisteria sinensis, keep in mind that many invasive plants are hybrids. Furthermore, both of these species appear very similar and, once hybridized, are difficult to distinguish. Finally, both are not native to North America, so for the purposes of control and eradication, identification to simply Asian Wisteria (as opposed to the less aggressive and native American Wisteria Wisteria frutescens) is sufficient in most cases.

Description of Wisteria sinensis

Leaves & Stems

Chinese Wisteria is a climbing, twining, or trailing perennial vine of the Fabaceae (Legume) family. While it also occasionally can be seen growing more as a shrub, it typically is a vine, and its stems can reach 20 m up into the canopy and reach 38 cm in diameter. They twine clockwise, and they occasionally branch alternately along the stem.

The leaves of Wisteria sinensis are compound and about 0.3 m in length, and these, too, alternate along the stem. It has from 7 to 13 leaflets that are themselves attached opposite along the stock of the compound leaf. Leaflets are oblong in shape, have wavy edges and long tapering tops, and are from 2-6 cm long. When young, the leaflets are covered in silky hairs, but they become mostly hairless as they mature.

Flowers & Fruits

The flowers or Wisteria sinensis are borne on showing dangling racemes 10-50 cm long and 7-10 cm wide. Its flowers have the typical pea family morphology, are very fragrant, and are usually vibrant blue to lavender or violet in color though occasional white varieties are seen. The flowers all mature and open around the same time, unlike Japanese Wisteria (see Similar Species below).

Fruits of Chinese Wisteria are typical legume-type pods that are velvety brown and 10-15 cm long. The pods are narrowed toward the base and have constrictions between the seeds in the pods. The pods each contain from 1 to 8 round but flattened brown seeds roughly 1.2 – 2.5 cm in diameter. 

Toxicity

Many sources consider the flowers, leaves, fruits, and seeds of the Chinese Wisteria to be poisonous. This is not uncommon in the Fabaceae family. Accidental or purposeful ingestion may cause symptoms such as nausea, vomiting, and diarrhea. 

Similar Species Frequently Confused With

Occasionally people mistake Chinese Wisteria for non-related species such as:

• Paulownia tomentosa with its purple flowers, but its flowers are large and tubular and not legume-like, and it has massive simple leaves rather than compound leaves.
• Dermatophyllum secundiflorum is native to the southern US and northern Mexico and has similar legume-like racemes of purple blossoms. But it can easily be differentiated by its evergreen tree habit and its 6-15 cm long compound leaves of oval leaflets 2-6 cm long with rounded or indented tips rather than tapering.
• Apios americana is a vine of the Fabaceae family, also native to the eastern US, with compound leaves with leaflets that have tapering tips. However, its flowers are not at all similar. Instead, they are red-brown to somewhat purple and occur in dense clusters rather than elongated racemes. This plant is also not poisonous and was an important food source for native peoples before the arrival of the Europeans.
• Syringa vulgaris Common Lilac is also occasionally mistaken for Chinese Wisteria, but these are usually shrubs and can easily be differentiated by the distinctive fragrance of its blossoms and its 4 petal flowers that look nothing like a legume flower.

There are two species in the same genus that are more often confused with Chinese Wisteria.

• Wisteria floribunda Japanese Wisteria looks very similar to Chinese Wisteria, and both are closely related. They are also both found throughout the eastern US though Chinese Wisteria does have a broader distribution and is also occasionally found in the West. Chinese and Japanese Wisteria have often been found hybridizing throughout this range at times, making the actual distribution of each problematic to determine. However, given that both are non-native, when found, identification as an Asian and not American Wisteria is sufficient for management purposes. The two Asian Wisterias can be differentiated by their leaves and their twining habit. Japanese Wisteria has 13 to 19 leaflets on its compound leaves, while Chinese wisteria has 7 to 13 leaflets. Japanese Wisteria twines counter-clockwise while Chinese Wisteria twines clockwise. Furthermore, the flowers of Japanese Wisteria bloom sequentially from the bottom up, while Chinese Wisteria blooms all at the same time.
• Wisteria frutescens American Wisteria is native to the eastern USA, sharing much of the same range as Chinese Wisteria. It can usually be easily differentiated by its much smaller racemes, only 5-15 cm in length, which is why it is less popular as a garden ornamental despite its native status and non-invasive habit. It also does not grow as tall, it has a shorter bloom time, its blossoms are not fragrant, and the seed pods are smooth instead of velvety when they mature.

Native Distribution of Wisteria sinensis

Wisteria sinensis is native to China in Guangxi, Guizhou, Hebei, Henan, Hubei, Shaanxi, and Yunnan provinces. It has been widely introduced around the world outside of its native range.

Habitat Types Where Chinese Wisteria is Found

Chinese Wisteria tends to escape cultivation vegetatively when left unchecked. From there, they spread into forest edges, disturbed areas, roadsides, ditches, and riparian habitats. They tend to become invasive in warm temperate climates but remain more controllable in cool temperate climates, where they will suffer dieback from harsh winter temperatures. They will not survive in the colder climates in most of Canada, for example.

Wisteria sinensis prefer deep rich soils but have been found growing in a range of sites. They will not tolerate extended periods of drought but can tolerate some seasonal flooding. While they grow best in full sun, they also tolerate full shade and are capable of spreading in these conditions.

Human Uses of Chinese Wisteria

Chinese Wisteria is widely used as a popular garden ornamental for its abundant fragrant blossoms. They are used on porches, gazebos, fences, walls, and in gardens and parks when also pruned as a shrub.

Since it is poisonous, it is not used as a food source or medicinally.

Distribution of Wisteria sinensis or their hybrids in North America

The species was first brought to the Americas in 1816 as a garden ornamental, where it quickly became popular.

In Canada, Wisteria sinensis has not been recorded outside of cultivation. It is cultivated in the southern coastal regions, however, and may one day escape cultivation there, particularly with climate change and the warming of southern Canada.

In the USA, Chinese Wisteria is found outside of cultivation in Texas, Missouri, Arkansas, Illinois, Kentucky, Tennessee, Mississippi, Alabama, Georgia, Florida, North & South Carolina, Virginia, West Virginia, Delaware, Washington DC, Michigan, Pennsylvania, New Jersey, New York, Massachusetts, Connecticut, Vermont, and Hawaii. It has also been reported in the Pacific coastal states, but it is not known if it has truly escaped cultivation there yet or not.

In Mexico, Wisteria sinensis so far has only been reported in Baja California Norte.

Chinese Knotweed is cultivated in temperate climates on every continent except Antarctica and has been reported as naturalized in New Zealand, Argentina, Chile, Brazil, South Africa, and many European countries.

How Chinese & Japanese Wisteria Spreads

It is primarily spread over long-distance by deliberate human introductions as garden ornamentals that then escape from cultivation when left unchecked in a suitable climate.

Short-distance dispersal occurs primarily from unchecked garden specimens that escape cultivation through vegetative spread. Rooting occurs at nodes in the vines and easily produces new plants this way. Another source of short-distance dispersal is carelessly discarded yard waste dumped in waste sites or forest edges that regenerate and produce new populations. Seeds are produced when the conditions are right and short-distance dispersal can occur this way as well. Since nothing eats the seeds, this is limited to locations right next to the parent plant.

Habitats at Risk of Invasion in North America

All warm temperate areas with adequate rainfall in North America will be at risk of invasion. Cool temperate areas are also at risk but less so as the vine is less aggressive in those regions. It invades forests and riparian areas, in particular throughout these regions but is also a threat to forest edges and shrub meadows, providing there is adequate moisture. Given that these similar conditions exist on the west coast, but Wisteria sinensis so far has not been reported as invasive there, this is the region most at current risk of new invasion in North America. People in those areas should not plant Chinese Wisteria in their gardens and should control or remove any existing plants there before they become a problem.

Desert areas (except valley bottoms with permanent rivers) and montane regions about 1000 m in elevation are not at risk of invasion.

Impacts of Invasion

Wisteria sinensis invades forest edges and penetrates the forest reducing all the biodiversity within it. Chinese Wisteria climbs by twining around trees and killing them by girdling or simply smothering them so they are unable to photosynthesize. They are currently being managed in many state and national parks in the eastern USA because of their detrimental effects on native tree populations and biodiversity. Chinese Wisteria also smothers the native understory vegetation by growing over it and reducing the availability of light. In areas where it gains a foothold in the ecosystem, biodiversity is always significantly reduced.

Potential Benefits of Invasion

Other than the fact that bees still visit their flowers, the rest of the plant is poisonous and provides no wildlife value.

Methods to Remove Chinese Wisteria

As always, prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in most local garden stores. Do not buy or transport any Chinese Wisteria. Do not plant it in your yard. If it is already growing there, and especially if you live on the west coast, where it has not yet gained a foothold but certainly will if left unchecked, then please destroy it and replace it with a native vine.

If you see them being sold online or in your local garden stores, please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

Physical Control of Chinese and Japanese Wisteria

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming, but it usually causes the least amount of environmental damage.

Physical methods to remove populations of Chinese Wisteria generally involve the physical pulling of all above-ground vines from the soil and the trees. Then this is followed by repeated cutting of the vine down to its rootstock. Since they can resprout many times from their rootstock, this should be repeated every 2-3 weeks from spring until early fall. This will exhaust the rootstock, and it will no longer be able to resprout. It will, of course, need to be monitored the following spring for any surviving rootstocks.

Juvenile plants or isolated individuals can be controlled by digging out the entire rootstock, all roots, and runners. This can be done with a weeding tool on juvenile plants or a pulaski (ax with an adze in one tool that is used for both chopping and digging) on more mature isolated plants.

Since seeds are only responsible for a small portion of the spread of Chinese Wisteria, the plant can be removed at any time. However, it is always best to remove any invasive plant early in the spring or when in flower. If cut while in flower, as long as the flowers are not already turning into fruit, they will not go to seed after being cut.

Disposal of the Shrubs Once Removed

Since the vines can resprout from any nodes, they should either be burned or solarized. Burning is fast and effective. However, it is not allowed in all areas and certainly not in all seasons. To solarize, put the vines under a thick black tarp or into thick black garbage bags and leave them in the full sun for a good 8 weeks to be sure that all seeds are no longer viable. Some sources recommend shorter solarization periods, but in my experience, differences in exposure, latitude, cloud cover, etc, can all lead to differential success. Leave it to solarize as long as possible to ensure they are no longer viable, and then they can be disposed of accordingly.

Chemical Control of Chinese Wisteria

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used, species riches significantly increases.

Furthermore, there are no chemical control methods that effectively target only Chinese Wisteria. Chinese Wisteria is known to resist herbicide treatments, so multiple applications are always needed. While this may eventually kill the Wisteria, the chemical side effects often pave the way for other herbicide-resistant invaders to come in.

Chemical control is not recommended.

Biological Control of Chinese Wisteria

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study.

Currently, there are no known biological control methods for Chinese Wisteria. No information could be found on even the use of goats which will eat just about anything. Since all parts of the plants are considered poisonous, it is not recommended. Physical control is the preferred method.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form, this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. Integrated management is required because the area needs to be monitored for returning sprouts or seedlings. Otherwise, all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting With Native Species is Crucial

In all cases of large patch removal, the site will need to be replanted immediately because the bare soil will allow the seed bank in the soil to germinate and reinvade the patch they were removed from. A replanting program should already be planned and ready to implement immediately upon the removal of the Chinese Wisteria. The area will need to be monitored for any surviving fragments that resprout.

In the case of isolated individuals replating is probably not necessary. Simply remove the individual(s) and monitor over the following 2-5 years to ensure that no rootstock is remaining and resprouting.

Ongoing Monitoring is Essential

In all cases of invasive Chinese Wisteria removal, ongoing monitoring is absolutely essential. In the first year, monitoring should be done every 2-3 weeks to remove the aggressive regrowth from the rootstock. Then in the following 2-5 years, the area should be monitored at least every spring, summer, and early fall to ensure that any surviving individuals are removed so that the population is not able to recover. This is required whether the area is replanted or not. Monitoring will prevent the re-establishment of invasive species and prevent all your hard work in removal from being wasted. And the added bonus is that you get to watch the ecosystem recover.

References and Resources

CABI on Wisteria sinensis https://www.cabi.org/isc/datasheet/56852

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Fire Effects Information System on Chinese Wisteria https://www.fs.fed.us/database/feis/plants/vine/wisspp/all.html

iNaturalist Plant Search https://www.inaturalist.org/home

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia on Wisteria frutescens https://en.wikipedia.org/wiki/Wisteria_frutescens

Willis, Lyrae (Unpublished).  Plant Families of North America.

Currently Seeking Funding To Continue This Non-Profit, Ad-Free Work

If you are able to donate so that I can continue this non-profit work of supplying people with scientific information on the plant families, native plants, and invasive species found throughout North America, please donate using the GoFundMe link below. Thank you!

---

Introduction

Common Periwinkle Vinca minor and Bigleaf Periwinkle Vinca major are very popular garden ornamentals widely sold in nurseries and online stores throughout North America. They are part of the Apocynaceae family in the Gentianales order of dicots. They produce attractive foliage, lovely blue, lavender, or sometimes pink or white flowers, and they require little maintenance. Of course, other than trying to control them to keep them from spreading into your garden beds, lawns, forest, or other areas, they were not intended to grow. In some areas, these plants are not very invasive, but in other areas, they are quite invasive.

Since they spread only vegetatively, the primary source of invasion is through deliberate human introductions as a garden ornamental that they plant and forget about and allow them to escape. We have so many lovely native groundcovers, just do some research into native groundcovers in your region and plant those instead. Native species require no maintenance, they tend not to be invasive, and they offer added wildlife and biodiversity values that invasive species lack altogether.

Description of Vinca major & Vinca minor

Leaves & Stems of Vinca major & Vinca minor

Both species of Periwinkles are scrambling vines from stolons up to 1 m long from fibrous roots 3 – 8 cm long. Their vertical stems grow up to 30 cm tall and are semi-succulent but become semi-woody at the base (caudex).

The leaves of Vinca major Bigleaf Periwinkle are opposite, semi-evergreen, heart-shaped (cordate) to triangular in shape, and are covered with a waxy coating (cuticle) and tiny hairs. They are 4 – 8 cm long and 2 – 5 cm wide. Sometimes, they have a finely hairy (ciliate) margin; otherwise, they are entire.

The leaves of Vinca minor Common Periwinkle differ from Vinca major in that they are evergreen, glabrous (hairless), more leathery, narrowly elliptic in shape, and are only 2 – 4.5 cm long by 1 – 2.5 cm wide with an entire margin that is not ciliate.

Flowers of Vinca major & Vinca minor

The flowers of both Periwinkle species are violet to blue and each has 5 petals radiating in pinwheel-like angles from the floral tube (they are partially connate).

The edges of the petals are typically slightly fringed and occasionally the petals are even white or pink.

Bigleaf Periwinkle Vinca major has somewhat larger flowers than the Common Periwinkle, but otherwise, they appear very similar.

Fruits of Vinca major & Vinca minor

The fruits of Periwinkles are pairs of slender cylindrical follicles to 5 cm long in Vinca major or 2.5 cm long in Vinca minor.

When the follicles dry, they split open to release 3-5 naked seeds that have no tufts (coma), unlike many seeds released from follicles.

Toxicity of Vinca major & Vinca minor

All parts of the plants are considered poisonous due to the presence of toxic alkaloids, which can attach themselves to the microtubules of the cells and impair their ability to divide, causing cell death.

Periwinkles have been known to poison humans, pets, and livestock. Most wild animals seem to know better than to ingest it; even if it is the only forage material around, most still will not eat it.

Similar Species Vinca major & Vinca minor are Frequently Confused With

There are only two species of Vinca that have been introduced to North America so far. However, there are a few other genera native and introduced that can occasionally be confused for Vinca. They can be differentiated as follows:

• Catharanthus roseus – Madagascar Periwinkle appears superficially similar, but it is an upright shrub that does not have a scrambling habit. Also, its leaves are oval to oblong and 2.5-9 cm long, and they are hairless and arranged in opposite pairs. The flowers are larger, up to 5 cm in diameter, with a very long floral tube. The petals are not at right angles to the tube but may still appear somewhat pinwheel-like in shape. The corolla is also typically various shades of pink or white rather than lavender or blue. It is endemic to Africa but has been widely introduced throughout the southern half of the USA and throughout Mexico.
• Phlox divaricata – native to the eastern USA and Canada, this plant has strikingly similar flowers in the same color and sometimes with a somewhat pinwheel shape to the petals. However, it is not a vine but rather an upright herb 25-50 cm tall. The leaves are opposite but are lanceolate and lack a leaf stalk (petiole).
• Euonymus fortunei – the Fortune Spindle is an evergreen vine occasionally mistaken for Periwinkles. They are native to Asia but are widespread in the USA and Mexico. However, the individual vines grow much longer, to 20 m, and their ovate evergreen leaves are much larger than the Common Periwinkle and are not cordate or triangular like Bigleaf Periwinkle. When in flower, they are hard to mistake for Periwinkle because their flowers are much smaller and are white or greenish-white.
• Hedera helix – Eurasian Ivy is occasionally mistaken for Vinca major. However, its vines grow much, much longer than Vinca major, and its leaves are alternate rather than opposite, and they are typically larger, though occasionally they can have similar shapes. Their flowers are also much different, being numerous tiny greenish-white flowers in an umbel.

Native Distribution of Vinca major & Vinca minor

Vinca major is native to the Mediterranean region of Europe and the Middle East from Spain east to Turkey plus northern Africa.

Vinca minor is native to central and southern Europe from Portugal, France, Holland, and the Baltics, plus eastern Caucasus and Turkey.

Habitat Types Where Periwinkles Are Found

In its native environment, Periwinkles are usually associated with moist, fertile, or moderate soils that are moderately acidic to moderately alkaline.

In North America, it is found in those soil types but has also been found in moderately well-drained soil types, poor soils, and even acidic clays. They grow from sea level up to 2300 m in elevation.

Periwinkles are both found in riparian forests, open forests, grasslands, scrub, and roadsides.

Vinca minor is often found in the understory of successional forests, including mature forest types, while Vinca major is more often found in partially shaded open forests and riparian forests.

Both species can grow in full shade, part shade, and even full sun, provided the soil is moist. In arid climates, both Periwinkles are more restricted to riparian forest understory.

Human Uses of Periwinkles

Periwinkle has been long been used as a ground cover plant as a garden ornamental where it forms dense mats. It has also been used for short hedges and for filling around the bases of trees. Though it rarely remains contained to its original purpose.

The dried leaves, aerial parts, and sometimes the whole dried plant are used to extract medicines that are used to treat a number of different cancers as well as to improve blood circulation, brain function, and cardiovascular disorders.

Vinpocetine is a synthetic nootropic (brain-enhancing) drug derived from vincamine, an alkaloid responsible for much of the medicinal activity of the genus. However, unsupervised medicinal use is not recommended due to the toxicity of the plant, which can make you very ill, cause miscarriages, and possibly even death if consumed in sufficient quantities.

Distribution of Vinca major, Vinca minor in North America

Periwinkles were first brought to North America in the 1700s. It was documented in a review of flora as early as the late 1700s, showing that it was already well established in the eastern USA by then.

In Canada, Vinca major has been recorded in British Columbia. Vinca minor has been recorded in British Columbia, Ontario, Quebec, New Brunswick, Nova Scotia, and possibly Newfoundland (excluding Labrador).

In the USA, Vinca major is found in Washington, Oregon, California, Idaho, Utah, Arizona, New Mexico, Texas, Louisiana, Arkansas, Mississippi, Alabama, Georgia, South Carolina, North Carolina, Virginia, Maryland, Illinois, Kentucky, Tennessee, Ohio, Pennsylvania, New York, and Massachusetts.

In the USA, Vina minor is found throughout the entire eastern USA and most of the southern USA. The only states it has not been reported in yet in the continental USA are Idaho, California, Nevada, Wyoming, Colorado, New Mexico, North & South Dakota, and Oklahoma.

In Mexico, Vinca major so far has been reported in Baja California Norte, Sonora, Sinaloa, Durango, Nayarit, Jalisco, Colima, Michoacan, Oaxaca, Chihuahua, Nuevo Leon, Tamaulipas, San Luis Potosi, Guanajuato, Mexico State, Mexico City, Puebla, Morelos, Veracruz, and Chiapas.

In Mexico, Vinca minor has not yet been reported, but it is located on the northern border of the US in Tucson, Arizona, and San Diego, California, so it will likely be recorded there soon.

Periwinkle, particularly Vinca major, has been introduced on every continent except Antarctica.

How Periwinkles Spread

It is primarily spread through long-distance by deliberate human introductions as a garden ornamental. Despite its invasive status, this is still the primary cause of long-distance spread.

Short-distance dispersal is primarily through vegetative means. Both species spread readily through their stolons which can quickly grow 25 cm or more before producing another rooted node yielding another clone.

Short-distance dispersal also routinely occurs by fragments regenerating out of dumped yard waste. In its native range, ants sometimes spread the seeds, but in North America spreading by seeds appears to be negligible to nonexistent.

Habitats at Risk of Invasion in North America

Riparian forests and canyon bottomlands are most at risk throughout all of North America due to their preference for moist soil types.

States and provinces located next to the coasts are also at high risk due to the presence of moist soils in those areas. In those regions, all land types are at risk, closed forests, open forests, roadsides, waste areas, grasslands, shrublands, and riparian areas as well.

The habitats not at risk of invasion are open deserts and high mountainous areas above 2300 m elevation. Fortunately, due to its inability to spread by seed, this restricts its spread to vegetative spread as long as people stop deliberately planting it in their gardens.

Impacts of Invasion

Both Vinca major and Vinca minor can grow in poor soils and full shade, and both are also allelopathic, giving them a significant competitive advantage.

Allelopathic plants inhibit the germination and growth of plants of other species growing in their vicinity due to how they alter the soil conditions. As a result, both species tend to form dense mats that smother native understory species as well as prevent the germination of new trees and shrubs.

Reductions in species richness, however, varied from location to location. This has resulted in Periwinkle being viewed as a ‘limited invasive’ species that is ‘stable’. This may partly be due to its spread being limited to vegetative growth.

However, I have seen firsthand its invasive nature in coastal British Columbia, Canada, as well as Virginia and Georgia in the eastern US, where it does indeed create monocultures of dense mats that exclude all other vegetation.

In riparian corridors, its ability to reduce native vegetation reduces the available forage for both wild and domestic grazers. This is particularly problematic where it occurs in riparian habitats found in otherwise arid landscapes.

In California, Vinca major is an important year-round host to the bacteria causing Pierce’s disease, which is a serious threat to the vineyards located there.

Potential Benefits of Invasion

Vinca minor and Vinca major provide no wildlife values in North America, and due to their toxicity and tendency to form monocultures, there are no potential benefits of their presence here.

Methods to Remove Vinca major & Vinca minor

As always, prevention is the preferred method of control.

It, like most invasive species, is still widely sold online and in most local garden stores. Do not buy or transport any Periwinkle, and do not plant it in your yard.

If you see them being sold online or in your local garden stores, please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

Do some research into native ground covers for your area. You will be pleasantly surprised by the number of low to no-maintenance, non-invasive native species that will grow in your area.

Physical Control of Vinca major & Vinca minor

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming, but it usually causes the least amount of environmental damage.

Fortunately, Periwinkle seed production and spread are not an issue, so they can be physically removed at any time of year. Removal is easiest, however, when the soil is moist, as the roots cling less tightly, allowing the plants to be pulled more readily. This means that spring and fall are often the best times, but it depends on your location.

Physical methods to remove Periwinkle generally involve raking up the stolons and then pulling the plants out using hand tools.

When dealing with large infestations, rake it first as this pulls the stolons up, then the mowing macerates the vines. Given their ability to regenerate by fragmentation, this requires repeated treatments throughout the entire growing season and is only recommended for large infestations where hand removal is not feasible.

For smaller infestations raking followed by pulling plants is often sufficient, though, like all control methods, this too will have to be repeated.

Try to pull as many of the roots as possible, but you will not get them all. The starchy roots of the Periwinkles allow them to regenerate following any method of control. As a result, ongoing monitoring is essential.

Solarizing Small Patches

Sometimes a small patch could be solarized by covering it with heavy black tarps and leaving it there in the sun for 4-6 months. This is the least labor-intensive method of control. The area should still be monitored over the next couple of years to be sure that none regenerate. Any that are found can simply be pulled by hand.

Disposal of the Plants Once Removed

Due to their ability to regenerate from fragments, all plants removed should be either burned or solarized. Burning is an easy and efficient way to get rid of plants, but if burning is not allowed in your area, then they should be polarized.

To solarize, put the shrubs under a thick black tarp or into thick black garbage bags and leave them in the full sun for at least 8 weeks to be sure that none of the vines are viable anymore.

Chemical Control of Periwinkle

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species.

Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used, species riches significantly increases.

Furthermore, there are no chemical control methods that effectively target only Periwinkle. And the starchy roots appear able to regenerate the plants not long after chemical control methods are used so additional control methods will still be required.

Chemical control is not recommended.

Biological Control of Periwinkle

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day, there are still rats in Hawaii, but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach.

In the case of Periwinkles, no biological control methods are currently being used. Most animals will not even graze on the plant. Due to its toxicity, it is not even recommended to use goats that eat anything, as it could potentially make them very ill.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form, this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. Integrated management is required because the area needs to be monitored for returning sprouts otherwise, all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting With Native Species is Crucial

In all cases of removal, the site should be replanted immediately because the bare soil will allow the Periwinkle or other potential invaders to claim the empty space left behind. A replanting program should already be planned and ready to implement immediately upon the removal of the Periwinkle. In most cases, these will be native understory plants, such as ferns, small shrubs, native vines, etc, that would have grown there before the Vinca species invaded. Find a local nursery that specializes in native species or ethically wildcraft your own from the local environment.

The only instances where replanting is not required are small isolated individuals that have not yet had a chance to form a clonal colony. In this case, simply remove the plant(s) and monitor them to make sure they do not return. Nearby native species should be able to rapidly recolonize the space they were removed from.

Ongoing Monitoring is Essential

In all cases of invasive Periwinkle removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. This is required whether the area is replanted or not.

If yearly monitoring is not put in place to remove young plants before they have a chance to become established, then all your hard work done in the removal process will be wasted if the patch is allowed to regrow.

References and Resources

CABI on Vinca major https://www.cabi.org/isc/datasheet/56402

Canadensys Plant Search https://data.canadensys.net/vascan/sea rch

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Eflora Plants of North America http://www.efloras.org/browse.aspx?flora_id=1

Fire Effects Information System on Periwinkle https://www.fs.fed.us/database/feis/plants/vine/vinspp/all.html

iNaturalist Plant Search https://www.inaturalist.org/home

USDA Plants Database https://plants.sc.egov.usda.gov/home

Willis, Lyrae (Unpublished).  Plant Families of North America.

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