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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Introduction

Castor Bean, or Castor Oil Plant, Ricinus communis, is a member of the monotypic genus Ricinus of the Euphorbiaceae family. It is an aggressive invasive perennial around the world, and the subtropical and tropical Americas are no exception. Castor Bean is an extremely widespread weed throughout Mexico and is also quite common throughout the southern USA. It also grows into the temperate northeastern corner of the USA, showing that it is more than capable of one day invading the Pacific Northwest and southern Canada, even though so far, it has not been reported there. The seeds of the Castor Oil plant are extremely toxic, so much so that the ingestion of even 3 or 4 seeds can cause severe gastrointestinal distress and, in some cases, even death.

Description of Ricinus communis

Ricinus communis Leaves & Stems

Castor Oil Plant grows from 1 – 5 m tall from a large taproot with several lateral roots.

It is a somewhat woody perennial shrub in tropical and subtropical climates but can behave as a herbaceous annual in temperate climates. It has a large woody hollow stem that is often purplish and covered with white powdery wax, along with the petioles and often the leaves. The stems and branches have conspicuous nodes and ring-like scars.

Leaves are arranged alternately on the stems, and at their bases are stipules 1 – 3 cm long that unite into a sheathing bud. The round leaf stalks (petioles) are 3 – 50 cm long and support very large palmate leaves with 5 – 9 lobes that are united at the bottom, with the petiole being placed just off-center (peltate) on the backside of the leaf. Leaf blades are irregularly toothed (serrated) and are very large, from 10 – 70 cm across.

Ricinus communis Flowers & Fruits

It is a monoecious plant with separate male and female flowers on the same plant.

Flowers appear in erect terminal panicles up to 40 cm long. The male flowers are located on the base of the panicle, with the female flowers located at the top of the panicle. The male flowers lack petals and sepals and are made of clusters of many stamens in branched bundles.

The female flowers lack petals but possess sepals, but they fall off early (caducous). They have a superior soft spiny ovary with 3 styles that are red or green and are 2 cleft (divided).

Fruits are ellipsoid or sub-globose and 15 – 25 mm long. They are green when young and turn brown when mature. They are usually covered in conspicuous spines but occasionally are smooth.

The fruits hold ellipsoid seeds 9 – 17 mm long that are brittle and mottled brown with a caruncle located at the base. The seeds are extremely toxic and can be lethal, even if ingested in small quantities.

Ricinus communis Toxicity

The seeds of Ricinus communis are extremely toxic. They are one of the most potent natural biological toxins known to man. This is due to the presence of an albumin called ricin. The ingestion of just a few seeds can cause severe gastroenteritis, dehydration, liver or kidney damage, and even death.

Similar Species Frequently Confused With

There are no other species in the Ricinus genus, but there are several unrelated plants that, at first glance, are often confused with Ricinus communis. However, as none are related, they can easily be differentiated by their flowers and fruits but also as follows:

• Kalopanax septemlobus is a member of the Araliaceae family. While it is native to Asia, it has been introduced to the northeastern USA. It has similar-looking leaves but its stems are covered with very long spines, and the plant reaches heights of 30 m. Its flowers have 4 or 5 petals, and they grow in umbels.
• Tetrapanax papyrifer is native to Taiwan but has been introduced along the east and west coasts of North America and central Mexico. It grows to similar heights and has large palmate leaves, but its palmate leaves’ lobes are also divided, unlike Ricinus communis. Its flowers are produced in large umbels at the top of the plant, and the flowers have 4 or 5 petals.
• Carica papaya Papaya is native to Mesoamerica but has been widely introduced into northern Mexico and the southern USA for its edible fruit. When not in fruit, it can be differentiated by its more succulent than woody and usually unbranched stem. Its large palmate leaves are also clustered near the top of the plant, and the base is deeply cordate, not united with a peltate attachment as in Castor.
• Fatsia japonica Japanese Aralia has been introduced along the east and west coasts of North America and possibly into Mexico. It grows to similar heights with palmate leaves, but its bases are not united and do not have a peltate attachment to its petioles. They produce umbels of bisexual flowers that possess both petals and sepals.
• Cnidoscolus aconitifolius or Mala Mujer (Bad Woman) is native to Mesoamerica but has also been introduced into northern Mexico, Florida, and sporadically throughout the southern USA. It can be distinguished by its large palmate leaves that only have 3-5 lobes. It is also monecious, but its flowers are small and white.
• Jatropha gossypifolia is native from Mexico to South America but has been introduced to the southern USA. It is a smaller plant with smaller leaves 7-15 cm wide with a cordate base. It also produces flowers with purple to reddish petals.
• Manihot esculenta Cassava is native to South America but has been widely introduced throughout the sub and tropical Americas as a food plant. It usually grows to only 2 m, and its palmate leaves have only 3-5 lobes that usually have smooth edges or minutely toothed at most. Its fruit is a six-angled globose capsule.

Native Distribution of Ricinus communis

The castor plant is native to northeastern Africa but has naturalized throughout Africa, some of which may have been naturally spread, and others were aided by humans as far back as the stone age.

Habitat Types Where Ricinus communis is Found

In its native environment, Ricinus communis grows naturally in any disturbed habitat. It has a wide ecological tolerance allowing it to adapt to a multitude of conditions.

Castor Bean tolerates a wide range of soil conditions from acidic to alkaline, heavy to light, including infertile soils and shallow soils.

Its moisture requirements are moderate, and it can easily tolerate extended drought and even some inundation as long as the soil drains readily and is not submerged for extended periods of time.

Ricinus communis also tolerates a wide range of temperatures. It will survive -15 C in the winter and tolerates temperatures of 35 C in the summer.

Human Uses of Castor Bean or Castor Oil Plant

Ricinus communis has a long history of use by people. It has been cultivated for its oil for at least 6000 years, with the earliest recorded use in ancient Egypt.

The plant is used as an ornamental and botanical curiosity due to its enormous size. The oil was used both medicinally and for illumination before the use of other fuels, candles, and electricity became widespread.

Medicinally, Castor Oil is used as a purge internally as well as a cure-all in smaller doses for numerous internal ailments. Externally it is used for a variety of sores and skin conditions.

Castor Oil is also used in cosmetics, soap making, foods, as a lubricant, in paints, plastic, and linoleum. The press cake is poisonous and cannot be fed to animals, but it is often used as a fertilizer or fuel.

Distribution of Ricinus communis in North America

The species was first brought to North America in the 1700s. By 1760 it was documented as naturalized in Florida.

In Canada, Ricinus communis has not yet been recorded.

In the USA, Castor Bean has been recorded throughout many southern states, including California, Arizona, Utah, Kansas, Texas, Missouri, Louisiana, Alabama, Mississippi, Georgia, Florida, and North and South Carolina. It is also found in the northeastern states of Illinois, Michigan, Ohio, Virginia, Delaware, Maryland, New York, New Jersey, Connecticut, Massachusetts, and New Hampshire. It is also found in Hawaii.

In Mexico, Ricinus communis has been reported in every state. It is particularly abundant on the east and west coasts and in south-central Mexico. It is less common in the northern desert states but has still been reported in those locations.

Castor Bean has been introduced on every continent except Antarctica and is even widespread throughout the Pacific Islands. It is a global problem.

How Castor Bean Spreads

It is primarily spread through long distances by deliberate human introductions as an ornamental or for its industrial uses.

Short-distance dispersal occurs through the explosive release of mature seeds from the fruits.

Humans also aid in short-distance dispersal by spreading seeds in garden waste, soils, and vehicles.

Habitats at Risk of Invasion in North America

Due to its wide range of ecological tolerances, many habitats are at risk of invasion, particularly any habitat with disturbance. This means that anywhere humans develop, the land is at risk of invasion.

In addition to developed areas, it also invades grasslands, heathlands, riparian communities, and farmland. It is also commonly found on roadsides and in waste areas.

It is often a primary invader of recently burned lands.

In Spain, it has even invaded sand dunes, whereas it was previously thought unable to survive in permanently arid areas.

The only habitats not thought to currently be at risk are permanent wetlands, dense old-growth forests, and alpine habitats.

Potential Benefits of Invasion

Since Ricinus communis is extremely toxic, it provides no real wildlife value.

It also reduces biodiversity where it invades.

There are no known benefits of its invasion other than the occasional native bee that may visit its flowers.

Methods to Remove Ricinus communis

As always, prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in some local garden stores. Do not buy or transport any, and do not plant them 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.

Physical Control of Castor Bean

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 Ricinus communis is while the plants are still small enough to pull out by hand. Due to the plant’s toxicity, gloves should be worn even when the plant is young. Pulling young plants when the soil is moist, and the roots cling less tightly to the soil is ideal. Sometimes even medium-sized plants can be pulled by hand from moist soil.

If the soil is dry or the plants are larger, weed pullers can be useful in removing more stubborn plants. Try to get as much of the root and crown as possible to help prevent resprouting. This is especially important when the soil is dry, as the root system will often break rather than give to the pulling.

With larger plants or colonies, you will need to cut the above-ground growth down, then dig out the individual crowns or cultivate the soil repeatedly with a machine for large colonies. This will help prevent any regeneration.

Burning is not recommended as a means of control as Castor Bean is a quick regenerator on burned sites, and burning may encourage rather than discourage its growth.

Disposal of the Shrubs Once Removed

If you have plants that have seeds on them, they must either be burned or solarized. Solarizing is usually best with Ricinus communis.

However, sometimes with large patches burning is more efficient for disposing of large amounts of plant matter rather than solarizing. If burning, be aware of their ability to regenerate rapidly from seed after a fire. If the fire does not attain sufficient heat, there will be a surge in seedlings not long after the fire. The area will need ongoing monitoring to deal with seedlings as they emerge.

Otherwise, to solarize, put the shrubs under a thick black tarp or into thick black garbage bags. Then leave them in the full sun for a good 8 – 10 weeks at least to be sure that all seeds are no longer viable. Many sources recommend shorter solarization periods. However, success depends on latitude, sun exposure, daytime high temperatures, and other factors that make shorter time periods prone to failure. Leaving invasive species to solarize as long as possible is always the best. Then they can be disposed of in a landfill, but be sure to inform them of their invasive status so they can be dealt with accordingly.

Chemical Control of Castor Oil Plant

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 Ricinus communis.

Chemical control is not recommended.

Biological Control of Ricinus communis

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 usually 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.

Because Ricinus communis is cultivated as a crop, no biological control methods are being developed for it. It is, however, prone to several pests and diseases, including mung moth, pink bollworm, seedling blight, rust spot, leaf spot, gray mold, stem canker, leaf blight, and bacterial wilt. However, these all affect other crops as well, so none are being developed to control Castor Bean.

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 many cases of removal, the site will need to be replanted immediately. This is because the bare soil will allow the seed bank to germinate and reinvade the patch they were removed from. Removal of single isolated individuals does not require replanting but the removal of a patch will. A replanting program should already be planned and ready to implement immediately upon the removal of the Castor Bean.

Ongoing Monitoring is Essential

In all cases of invasive Ricinus communis 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 seedlings are allowed to emerge unchecked, the invasive Castor Bean will simply re-invade and take over before the native species are able to grow back in their place. Removal of young plants by physical means is always the easiest and most effective means of control.

References and Resources

CABI on Ricinus communis https://www.cabi.org/isc/datasheet/47618

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

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

Ilex aquifolium goes by several common names, including English Holly, European Holly, Common Holly, and Christmas Holly. It is a highly invasive plant in the moist climates of North America, particularly on the west coast, where it is beginning to dominate the understory of some forest ecosystems. Despite this, it is still widely sold in stores throughout this region.

I grew up on the coast of BC, Canada, and remember finding it all the time under the canopy of the forest. I recently returned to the coast and spent some time there and was horrified by the amount it appears to be spreading and replacing our native understory species. In a single forest walk, I managed to pull out about 40 immature plants from the forest canopy. There were several larger ones I could not pull out by hand due to their deep and aggressive root system.

Next time I return to the coast and go for a forest walk, I will be sure to bring a trowel or a small shovel. Do your part on your forest walks and pull out any small holly shrubs you find growing before they get large and too difficult to remove. Just make sure you are not pulling Oregon Grape or native hollies if you live on the east coast; see Similar Species below for more information.

Description of Ilex aquifolium

Leaves & Stems of Ilex aquifolium

Common Holly is a slow-growing evergreen perennial shrub or small tree that grows from 5-25 m tall. However, it usually only grows to the shorter 5-10 m shrub height. It may grow from a single trunk, or it may be multi-stemmed. Their trunks are usually less than 40 cm in diameter. Larger trunks can be from 40-80 cm in diameter, but they rarely exceed 100 cm. The bark is green when young but turns grey when it matures. While specimens as old as 500 years have been reported, it seldom lives longer than 100 years.

The leaves of English Holly are arranged alternately on the branches. They are tough, thick, and glossy green and measure 3-12 cm long and 2-6 cm wide. The margins are often characteristically undulate (wavy) and toothed with spiny tipped teeth. Sometimes the leaf margins are smooth, particularly on older specimens.

Flowers & Fruits of Ilex aquifolium

English Holly is a dioecious plant meaning that there are separate male and female plants. The flowers are whitish and have 4 petals and 4 sepals that are both fused near their base. In males, the flowers are usually more yellowish and appear in axillary groups (in clusters in the leaf axils). Female specimens have single flowers or in groups of three, and the flowers or white or somewhat pink. The male and female shrubs are impossible to tell apart until they begin to flower from 4 – 12 years of age.

Fruits are small red or yellow drupes, berry-like in appearance, and only grow on female plants. They are 6-10 mm in diameter. Each drupe contains 3-4 small stony seeds.

Toxicity of Ilex aquifolium

The drupes are toxic to humans and pets, causing vomiting and diarrhea, but they are generally not lethal. Birds typically eat them later in the winter after the frosts have softened them and made them less bitter; they are immune to their toxic effects.

Similar Species Ilex aquifolium is Frequently Confused With

There are a few other plant genera that have similar leaves to Ilex aquifolium. The Oregon-Grape genus Berberis of which we have a few native species growing in North America have evergreen spiny-toothed leaves, but their leaves are opposite rather than alternate, and they have blue-colored berries rather than red or yellow drupes. Osmanthus heterophyllus or Holly Olive is a member of the olive family native to Europe that also has similar leaves and has been introduced in the USA. Holly Olive differs from true hollies by its opposite rather than alternate leaves and bark that turns grayish to blackish with age and has a characteristic cracking pattern to it.

The other Ilex species can be distinguished as follows:

• Ilex opaca American Holly – native to the eastern and south-central USA. It has matte yellowish-green leaves that are seldom glossy, as in Ilex aquifolium. Its trunk is grey with wart-like bumps, and its mature branches turn brown rather than grey. It also grows more of a tree than a shrub, with trunk diameters commonly of 50 cm and up to 120 cm in diameter.
• Ilex cornuta Chinese Holly – native to China but widely introduced throughout the eastern and south-central USA. It has more rectangular-shaped leaves with only 5 (4) spines, but they are glossy green like Ilex aquifolium. Chinese Holly never grows to tree size, it rarely exceeds 3 m in height, and its drupes are larger than European Holly. The bark is smooth and grey but becomes flaky with age.
• Ilex vomitoria Yaupon Holly – native to southeastern North America. It is a small tree or shrub with glossy green leaves, but the leaves are much smaller and are not spiny. Instead, the leaves have crenate or coarsely serrated margins.

Native Distribution of Ilex aquifolium

European or Common Holly is native to western and southern Europe, northwest Africa, and southwest Asia.

Habitat Types Where European Holly is Found

European or Common Holly is typically found growing under forest canopies, oak, and beech, especially in its native range. It typically requires moist and shady environments, making it particularly suitable as an undergrowth species in any moist forest. Also, it is common on moist and shady slopes, cliffs, and mountain gorges. Sometimes, however, it can also form dense thickets in more open fields and meadows. It is a rugged pioneer species that can tolerate both frosts and droughts.

Human Uses of Common Holly

European or Common Holly is widely used as an ornamental shrub or small tree in gardens. It is also extremely popular in Christmas decorations, where it is put in wreaths, garlands, and various other displays.

In traditional European herbal medicine, the leaves of Ilex aquifolium were used as a diuretic, to treat fevers, and as a laxative. It is seldom used in modern herbal medicine, however, and its berries are considered toxic, so the plant should be used with caution.

Distribution of Ilex aquifolium in North America

In Canada, Ilex aquifolium has been recorded as introduced in BC and possibly in Ontario. It is becoming particularly invasive in coastal BC.

In the USA, English Holly is found in Washington, Oregon, and California in the west and Virginia and New Jersey in the east. It has also been introduced in Hawaii. According to iNaturalist, it is located in many more states, but many may still be in cultivation, and some could be native Ilex spp that have been misidentified.

In Mexico, Ilex aquifolium has not yet been reported so far.

English Holly has been introduced on every continent. It seems particularly bad in New Zealand, Tasmania, and North America.

How English Holly Spreads

It is primarily spread long-distance by deliberate human introductions as an ornamental species in gardens.

Short-distance dispersal occurs through birds that eat the abundant drupes later in winter after they have softened and their taste improves. They spread the seeds through their feces, where the seeds then germinate in any suitable habitat in both shade and sun.

English Holly also spreads readily vegetatively through suckering. This is how most of the thickets of Common Holly form and how it becomes a dominant understory species of the forests.

Habitats at Risk of Invasion in North America

The way that English Holly spreads through the feces of birds puts all habitats at risk providing there is enough moisture. They will not grow in permanently wet wetlands, and they also will not grow in arid or semi-arid areas. Any moderately moist to the wet climate, however, is at risk of invasion.

Their ability to grow in both sun and shade makes them a common invader of forests in particular but also roadsides, fields, meadows, cliffs, gorges, and anywhere else as long as there is ample moisture.

European Holly has proven to be particularly invasive in the temperate rainforests of northwestern North America, where it easily invades the mixed forest canopies and thrives in the moist environment there.

Impacts of Invasion

Ilex aquifolium has deep and aggressive roots that tend to out-compete the native species for nutrients and water. There is also evidence that English Holly alters the soil conditions by adding sulfur and large amounts of organic matter to the soil, which makes the conditions less hospitable for native species.

English Holly forms dense thickets that can dominate the understory of the forest and out-shade native shrubs and other native plants, reducing the biodiversity of the forests it invades.

Potential Benefits of Invasion

Thickets of Ilex aquifolium are known to be used as a refuge by small birds and deer. The flowers are attractive to bees, small butterflies, wasps, and flies, and the berries provide an important late-winter food source. However, as with all invasive plants, the native wildlife would have depended on the native plants that grew there before the Common Holly replaced them.

Methods to Remove English Holly

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 European Holly, and never 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 stop selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

If you live in the eastern part of North America, why not grow one of the two native hollies instead, Ilex opaca or Ilex vomitoria? If you live in western North America and want some spiny glossy green foliage, try growing native Berberis species instead. They have lovely foliage and beautiful blue-colored berries. The native Ilex and Berberis species can even be substituted for Common Holly in Christmas decorations.

Physical Control of European Holly

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

Physical methods to remove European Holly generally involve uprooting the plants. This is best done when the plants are young before their aggressive roots have had a chance to establish themselves. Fortunately, they are a slow-growing species, so on a single walk through the forest, you can single-handedly destroy dozens of young plants with very little effort. Larger plants will need to be dug out with a shovel to get their roots. If the roots are not dug out, they can re-grow from the stump.

For larger shrubby plants, use pruners to remove the upper growth before using a shovel to dig out the root mass. For tree forms of Ilex aquifolium you can try girdling it. This involves removing a strip of bark from all the way around the circumference of the tree. As with all trees and woody shrubs, the only part of the tree’s trunk that is alive and transporting nutrients is the layer directly under the bark. If you cut down to the heartwood and remove the bark in a strip all around the tree, it can no longer transport water and nutrients to the leaves. The tree will then die and can be left to rot or be removed.

The best time to remove European Holly is before it is in berry, in the spring or early summer. If you need to remove them when in berry, be sure to clip off the berries before removing the shrub or tree and place them in the bag for proper disposal.

Disposal of the Shrubs Once Removed

Trees or shrubs without berries on them can be burned, cut into firewood, or left to rot. If you have plants that have mature berries on them, they must either be burned or solarized. If you are unable to burn in your area, then solarizing is a viable option. To solarize, put the shrubs under a thick black tarp or into thick black garbage bags. Then leave them in the full sun for a good 8 weeks at least to be sure that all seeds are no longer viable. The plants can then be disposed of. They can be brought to a garbage dump where you can inform them they are an invasive species so they can deal with them accordingly.

Chemical Control of European Holly

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.

Furthermore, there are no chemical control methods that effectively target only European Holly. If you do use chemical control, it must be stem injected. The tough evergreen leaves readily resist the absorption of pesticides making foliar applications ineffective.

Chemical control is generally not recommended.

Biological Control of Ilex aquifolium

Biological control involves the use of a predator, herbivore, disease, or some other biological agent to control an invasive species once it is fully 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. 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 for some invasive species.

Unfortunately, there are no current methods of biological control for English Holly in North America other than goats. Domestic goats are aggressive and non-selective browsers. They will literally eat anything. This will work best on shrubs and thickets. Often goats will eat the bark off of trees so that they can even kill off the trees if they manage to eat all the way around the tree.

To use goats, the patch should be penned in, and then the goats left there to graze for at least 2 years. They will eventually destroy all the above-ground growth and then eat the new sprouts as they come up. This can then be followed with rootstock removal for any stubborn plants that may remain. Alternatively, simply leave the goats in the area for another year until you see no more sprouts. At this time, they can be moved to another patch, and that patch can be replanted with native vegetation. An ongoing monitoring program should then be implemented to be sure they do not sprout again.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest 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 Sometimes Crucial

Removal of single plants from the understory of the forest will not require replanting with native species. Simply remove and monitor, and other plants will grow in their place. However, in cases of the removal of thickets that dominate the area, the site should be replanted with native species. A replanting program should already be planned and ready to implement once the English Holly has been removed. You will need to research species that are native to your area. Ideally, use ones from your local environment to ensure local ecotypes are represented. Alternatively, you can purchase suitable plants from a nursery in your area that specializes in native species.

Ongoing Monitoring is Essential

In all cases of invasive European Holly 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. Sometimes roots left behind will re-sprout into new plants. Furthermore, birds will keep bringing in new seedlings if there are still plants in the wider surrounding environment. For these reasons, yearly monitoring should be put in place to remove young plants before they have a chance to become established, especially since that is the easiest time to remove them. With ongoing monitoring, we can keep invasive species in check and promote local biodiversity in the process.

References and Resources

Best Management Practices for English Holly in the Metro Vancouver Region http://www.metrovancouver.org/services/regional-planning/PlanningPublications/EnglishHollyBMP.pdf

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

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

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

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

Wikipedia on Ilex aquifolium https://en.wikipedia.org/wiki/Ilex_aquifolium

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

Himalayan Blackberry or Rubus armeniacus or Rubus bifrons (confusingly, both names seem to be currently accepted) is a well-known invasive species in some areas. Where I grew up on the Sunshine Coast of British Columbia, Canada, it was everywhere. It does less well in the colder interior, but it still does grow there, just much less invasively. As a child, I did not understand that it was invasive and not native. I just loved eating handfuls of the huge juicy, and sweet berries. I would go out picking bucketfuls of berries every summer with my mother and my three brothers. Now I know how invasive it truly is, and somehow the fruits seem a little less sweet to me now. Once established, the removal of Himalayan Blackberry can be very challenging.

Description of Rubus bifrons or Rubus armeniacus

Leaves & Stems of Rubus bifrons or Rubus armeniacus

Rubus bifrons and Rubus armeniacus are members of the Rosaceae family of the Rosales Order of flowering dicots. They are a perennial vine-like trailing shrub that grows 1 – 7 m in length and 0.4 – 1 cm in diameter with arching or creeping stems that are sparse to densely hairy and armed with numerous prickles. They have a large root crown of roots and rhizomes that grow to 0.5 m and sometimes up to 2 m deep.

The leaves are deciduous or sometimes somewhat evergreen and palmately compound with 3 – 5 spreading leaflets. Leaflets are elliptic or ovate to sub-orbiculate and 6 -15 cm long and 4 – 9 cm wide with a rounded or somewhat cordate base. Margins are moderately to coarsely serrated, and the apex is acute to acuminate. The lower surface is whitish to grayish green, while the upper surface is glossy bright green or dark green. There are filiform or linear stipules that are 7 – 15 mm in length. The largest veins have hooked prickles on them.

Flowers & Fruits of Rubus bifrons or Rubus armeniacus

Himalayan Blackberries usually have loose terminal inflorescences that generally extend past their subtending leaves. Sometimes the inflorescences may grow in the leaf axils as well. They are in a thryse with 10-60(-100) flowers in them. Pedicels are covered with prickles, are densely hairy, and may or may not be glandular. Flowers are bisexual with 5 white or pink petals. Petals are not connate (free), are obovate to elliptical and shape, and are 10 – 15 mm long each. It has filiform stamen filaments, and its ovaries are apically hairy.

They produce abundant black fruits that are juicy drupecetums, an aggregation of drupes on a fleshy accessory peduncle. Most people refer to them as ‘berries’. Fruits are globose or almost cylindrical and 1 – 2 cm long with 15 – 50 individual drupelets that strongly adhere to the accessory peduncle. Abundant fertile seeds are produced in their fruits, from 7000 – 13000 seeds per square meter of stems. Seeds can remain viable in the soil for several years.

Similar Species Frequently Confused With

There are native Rubus species throughout North America, but many are smaller plants with smaller or differently shaped and/or colored berries. Some native and introduced Rubus, however, grow to similar sizes but can be differentiated as follows:

• Rubus laciniatus, Cutleaf or Evergreen blackberry, is native to Eurasia and has been fairly widely introduced in North America. It grows in similar habitats to similar sizes with similar fruits but can easily be distinguished by its lacinated or deeply cut leaflets as opposed to the whole serrated leaflets of Himalayan Blackberries.
• Rubus pascuus Chesapeake Blackberry is a native shrub of Kentucky, Virginia, Maryland, Delaware, North Carolina, and South Carolina. It has tighter and shorter inflorescences, is more densely glandular, and its terminal leaflets are erect rather than spreading in Rubus bifrons.
• Rubus pensilvanicus Pennsylvania Blackberry is native throughout eastern Canada and the USA. It usually is a bit smaller than the Himalayan Blackberries, and its very red stems are densely covered in red prickles. It also only ever has white petals on its flowers, and its abaxial leaf sides are hairy but never whitish or grayish-green.
• Rubus ulmifolius Elmleaf or Thornless Blackberry is another European species that has been introduced that superficially looks very similar, but it is only found in California, Oregon, and Nevada. If prickles are not present, they can easily be distinguished by this. However, sometimes it does have prickles, in which case you can differentiate it by its strongly pruinose stems that Himalayan blackberries lack.
• Rubus vestitus European Blackberry is found on the Pacific coast of the US and Canada. It is an armed shrub that is generally smaller than 2 m. It also has narrower inflorescences, nearly round terminal leaflets, and stipitate-glandular hairs lacking in Himalayan Blackberries.

Native Distribution of Rubus bifrons or Rubus armeniacus

There is much debate as to the taxonomic description of this species. Some sources call them all Rubus armeniacus and say these are from Europe. Other sources say that the species are only native to Armenia and Iran. Other sources call this plant Rubus bifrons and say it is widespread in Europe and includes Rubus armeniacus. In either case, whether the same or separate species, they are virtually identical, and both are native to Europe, one more widespread and the other less so.

Habitat Types Where Himalayan Blackberry is Found

Himalayan Blackberry grows in both wetland and upland environments. While it grows abundantly in riparian areas, it does not invade the permanently wet soils of the wetlands though it will tolerate temporary flooding of up to 40 days. It will even tolerate occasional flooding of brackish water. It grows best at low elevations but can grow up to 1200 and occasionally 1800 m above sea level.

Himalayan Blackberries prefer moist, nutrient-rich loam but do not require it. They can also be found growing in soils with a wide range of textures, fertilities, and pH from acid to alkaline. It prefers sunny sites but easily grows in part shade as well.

Human Uses of Himalayan Blackberry

Himalayan Blackberries are widely used as food. They are eaten fresh as berries or frozen or canned for winter use in pies, cakes, jams, jellies, and wines.

Blackberry roots and berries are often used to treat diarrhea, fluid retention, gout, diabetes, gout, and inflammation. It is sometimes used to prevent heart disease and cancer. It makes a good rinse for irritation of the mouth and throat.

They are planted along fences and trellises to create impenetrable barriers to keep people and animals out of an area.

Distribution of Rubus armeniacus, Rubus bifrons in North America

The Himalayan Blackberry species were first brought to North America as a food crop in 1885 for its abundant berries.

In Canada, Rubus bifrons has been recorded in British Columbia, Ontario, Quebec, and Nova Scotia. Canadensys does not distinguish between the two species and uses only Rubus bifrons stating that Rubus armeniacus is a synonym.

In the USA, the Himalayan Blackberry is reported as two separate species. USDA states that Rubus bifrons is only found in the eastern part of the country in Oklahoma, Texas, Missouri, Arkansas, Louisiana, Kentucky, Tennessee, Mississippi, Alabama, Georgia, South Carolina, North Carolina, Maryland, Virginia, Washington DC, Pennsylvania, New Jersey, New York, Connecticut, Rhode Island, and Massachusetts.

USDA states that Rubus armeniacus is more widespread in the west and scattered in the east. It is found in Washington, Oregon, California, Idaho, Montana, Nevada, Arizona, Colorado, New Mexico, Missouri, Arkansas, Illinois, Kentucky, Tennessee, Alabama, Ohio, Virginia, Delaware, Pennsylvania, New Jersey, and Massachusetts.

In Mexico, Rubus bifrons so far has been reported in Veracruz and Oaxaca. Rubus armeniacus has been reported in Baja California (norte), Puebla, Mexico State, Jalisco, and Chiapas.

Himalayan Blackberries have now been introduced on every continent except Antarctica.

How Himalayan Blackberry Spreads

It is primarily spread long-distance by deliberate human introductions planting it in their gardens. It quickly escapes from cultivation due to its aggressive growth.

Short-distance dispersal occurs through birds and animals which eat the abundant fruit and then spread the seeds around in their feces. Humans also spread the fruits by picking them, eating them and dropping them, etc. They also spread vegetatively through their root crown and rhizomes and by rooting at nodes on their stems that come into contact with soil.

Habitats at Risk of Invasion in North America

Himalayan Blackberries are considered particularly invasive on the west coast of North America in low-elevation riparian, deciduous, and conifer forests as well as grasslands. It is heavily invasive in riparian communities in that area. It invades the same types of areas in the east but may be less invasive there due to the other aggressive native and non-native vines that are abundant in that region.

Himalayan Blackberry will not invade permanently wet soils or high-elevation montane forests. It will grow in middle-elevation montane forests but not very well. It will also not grow in most arid areas due to lack of moisture unless it is in a riparian zone with perennial access to water.

Impacts of Invasion

In North America, the Himalayan Blackberry is considered the most invasive nonnative shrub on the western coast from British Columbia, Canada, south to California, USA. They are aggressively spreading shrubs that quickly scramble over and smother all other vegetation in their path, excluding tall trees, as they cannot climb as high as English Ivy, for example. However, it can quickly smother and kill all young trees in its path. It is highly competitive for light, space, and nutrients. Due to its aggressive growth, it quickly out-competes native vegetation.

It replaces the more bio-diverse native shrubs and herbs that would have otherwise grown there, creating complete monocultures if the conditions are suitable for them. This out-competes our native vegetation, reducing biodiversity and even causing local extirpation of species. It also forms dense thickets in riparian areas that impede movement and access to water by humans and animals alike.

Himalayan Blackberry is also an agricultural and range land weed that replaces crop and forage lands quickly if left unchecked. Himalayan Blackberry and Scotch Broom are the two most abundant agricultural and range land weeds, causing an estimated $85 million in losses annually.

The abundant dead canes they produce are also a significant fire risk when they die and become brittle as they burn easily.

Potential Benefits of Invasion

Himalayan blackberries are widely eaten by bears, foxes, coyotes, squirrels, raccoons, deer, birds, slugs, and countless other native species. Most native ungulates prefer to browse the leaves, but some (white-tailed deer, for example) have been observed browsing the fruits. Leafcutter ants and leafhopper butterfly larvae also feed on the leaves. It is the preferred larval host plant for leafhopper butterflies. Pollen and nectar are also used by native bees, honey bees, and hummingbirds.

In addition, a multitude of small animals and birds use the blackberry thickets for cover, nesting, roosting, etc. The impenetrable thickets of thorns provide them protection from predators.

All of these animals, however, would have fed on the native Salmonberries, Thimbleberries, or other native species displaced by the invasive Rubus bifrons, however. The dependence of wildlife on the berries creates an additional challenge to the control or removal of the Himalayan Blackberries. Check out the control methods below for more information.

Methods to Remove Himalayan Blackberry

As always, prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in many local garden stores. Do not buy or transport any Himalayan Blackberry. 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.

Instead, buy suitable native Rubus species. There are multiple native blackberries and raspberries that are even more delicious and are just as easy to grow as invasive blackberries. Native Rubus species provide additional biodiversity and wildlife values as well.

In fact, due to wildlife values, this makes the removal of Himalayan Blackberry more challenging. Because native wildlife now depends on this non-native species, when you remove a large patch, it should be done in stages. Clear one-third or one-quarter of the patch (depending on the size).

Once cleared and few sprouts keep coming back, then it should be replanted with a variety of native salmonberries, thimbleberries, or other native shrubs that would have originally fed the wildlife in your area.

Research what species are native to your area to find suitable replacements. Then tackle the next fraction of the patch until you have replaced the invasive blackberry with a more biodiverse native selection.

Ongoing monitoring will, of course, be needed to ensure it does not return and overtake your planted native species. See the Integrated Management section below.

In public and natural areas minimizing disturbance and replanting disturbed areas as quickly as possible are the best prevention strategies for reducing the chance of invasion by Himalayan Blackberry or any other invasive species.

Physical Control of Himalayan Blackberry

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 Himalayan Blackberry is early in the spring, before or later during flowering but before the fruit has set to avoid further spreading the seeds.

Physical methods to remove Himalayan blackberries generally involve cutting, hoeing, digging, and burning. Often a combination of methods is used. Always wear gloves when handling the canes, as the prickles are very tough and sharp and will easily pierce the skin.

Young plants can be pulled by hand.

Mature plants require the cutting of the canes with cutters or loppers and then removing the rootstock.

Burning is also an effective method to remove the above-ground growth, but as with cutting, the rootstock will need to be dealt with as well. A single cutting back of the above-ground growth, however, if not followed with additional treatments, will actually increase the density of the thicket when it regrows. It must be followed with additional treatments and ongoing monitoring.

If the rootstock is not removed, it will grow back quite vigorously. One method is to repeatedly cut it back or burn it multiple times over multiple years until the roots are starved by the lack of above-ground growth.

Rootstocks can also be dug out with a shovel or a weed puller, but they are very tough, and this is only suitable in small patches.

If you have only a small patch to deal with and root digging is too labor-intensive, you could also cover the entire area with a black tarp for at least two growing seasons to solarize it. This is much less labor-intensive than returning repeatedly, but again this only works on small patches.

Note that it is virtually impossible to remove all of the rootstocks due to the size and depth that the roots spread to. Removing as much as you can is critical to success. However, it must be followed by ongoing monitoring over several years to ensure all new sprouts are destroyed.

Disposal of the Shrubs Once Removed

If you have live stems or plants that have seeds on them, they must either be burned in a hot enough fire or solarized. The fire must be hot enough to destroy the seeds. If you are not allowed fires in your area, then you will have to solarize them.

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 – 10 weeks at least to be sure that all seeds are no longer viable. Some sources suggest less time for polarization, but in my experience, differential heat throughout the bag/tarped area as well as variations in sunlight intensity, can result in the survival of at least some seeds if done for a shorter duration. Take them to your local dump once done and inform them that they are invasive species so that they are disposed of appropriately.

Chemical Control of Himalayan Blackberry

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.

Furthermore, there are no chemical control methods that effectively target only Rubus bifrons or Rubus armeniacus. And due to the nature of Himalayan Blackberry, multiple applications are always needed over multiple years, further degrading the local environment.

Chemical control is not recommended.

Biological Control of Himalayan Blackberry

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. This is because the control agent would need to effectively destroy over 99% of plants, roots, and seeds to actually control the Himalayan Blackberry on their own. Results this high have never been achieved in the field.

However, using biological control in conjunction with physical control and ongoing monitoring can be very effective at times. However, in North America, so far, there are no biological control methods in terms of insects, fungi, or other pathogens that have been approved. This is because they all put our abundant list of native Rubus species at risk as well.

Using Goats to Remove Himalayan Blackberries

The only recommended biological control method at this time is grazing by domestic goats. Domestic goats are aggressive and non-selective browsers. They will literally eat anything. To use goats, the patch should be penned in, and then the goats left there to graze for 3 years. They will eventually destroy all the above-ground growth and then eat the new sprouts as they come up.

This can be sped up by an entire year by first removing the above-ground growth and then allowing the goats to eat the sprouts as they come up.

This can then be followed with rootstock removal for any stubborn plants that may remain, or simply leave the goats in the area for another year until you see no more sprouts.

At this time, they can be moved to another patch, and that patch can be replanted with native vegetation and a monitoring program implemented to be sure they do not sprout again.

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 Himalayan Blackberry removal, the site will need to be replanted once the majority of the infestation has been dealt with. This is because the bare soil will allow the seed bank of Himalayan Blackberry in the soil to germinate and re-invade the patch they were removed from. Or it will allow the invasion by other nonnative species, which all favor disturbed sites.

A replanting program should already be planned and ready to implement in the 2nd to 4th year upon removal of the Himalayan Blackberry, depending on the speed and success of the removal method.

A variety of native species that are ideally seeds from the local environment to ensure local ecotypes should be planted. If local seeds cannot be gathered or started in time, then purchasing native plants from a reputable nursery specializing in local native species is a good alternative. This must then be followed by ongoing monitoring.

Ongoing Monitoring is Essential

In all cases of invasive Himalayan Blackberry removal, ongoing monitoring is absolutely essential.

Multiple visits should be made in the first two years to ensure that any surviving individuals, their sprouts, and their seedlings are removed so that the population is not able to recover.

Then yearly monitoring after that for at least five years to ensure that none return. This is required whether the area is replanted or not. Himalayan Blackberry is aggressive and prolific and will out-compete planted vegetation if yearly monitoring is not put in place to remove young plants before they have a chance to become established.

References and Resources

CABI on Rubus armeniacus https://www.cabi.org/isc/datasheet/116780

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

Fire Effects Information System on Himalayan Blackberry https://www.fs.fed.us/database/feis/plants/shrub/rubspp/all.html

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

Rubus armeniacus fruits picture from Wikipedia By Daderot – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=21796138

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

Wikipedia Himalayan Blackberry https://en.wikipedia.org/wiki/Rubus_armeniacus

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

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