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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