How to Identify and Control Canada Thistle Weed?

What is Canada Thistle Weed?

Canada thistle (Cirsium arvense) is a stubborn perennial weed that causes substantial crop yield losses. This weed is native to Europe and Asia and has become widespread across North America.

European settlers brought this weed to North America in the 1600s, and it has since become a widespread and troublesome weed. It was originally brought over for its medicinal properties, as it was believed to have a variety of healing benefits, but it soon became obvious that this weed was extremely invasive and difficult to control.

This weed is also known by other names, such as creeping thistle, field thistle, and perennial thistle.

Canada thistle reproduces via seed and root regeneration, with the latter being the more successful. It saves food energy in its large root system to ensure survival over the winter and to power the plant’s reproductive drive the following season. Due to its contribution to the weed’s persistence, the extensive root system of Canada Thistle must be the focus of an efficient control program.

Canada Thistle Weed At A Glance

Scientific nameCirsium arvense
Common nameCanada thistle weed
Plant typePerennial herbaceous plant
NativeEurope and Asia
Invaded regionsNorth America, South America, Australia, New Zealand
HabitatCroplands, pastures, roadsides, gardens, waste areas
Height1 to 5 feet tall
LeavesLance-shaped, deeply lobed, with sharp spines along the margins
FlowersPink or purple in color, arranged in small clusters at the end of each stem
SeedsProduced in large quantities, can remain viable for up to 20 years
Growth rateRapid, can spread quickly through underground roots and shoots
CompetitivenessHighly competitive with crops and native vegetation for resources
Ecological impactCan reduce biodiversity and alter soil chemistry, and can also provide habitat for pests
Economic impactCan reduce crop yields and increase production costs
Control methodsMechanical removal, cultural control, chemical herbicides
ResistanceDevelops resistance to herbicides over time
Canada Thistle At A Glance

How to identify Canada thistle?

Canada thistle weed (Cirsium arvense) is an invasive perennial weed that can be difficult to control, so it is important to be able to identify it correctly.

Creeping thistle, also commonly known as Canada thistle, has clusters of tiny purple flower heads without spines on the bracts beneath the flower heads. This perennial weed can grow to a height of 5 feet. It spreads through seeds and extensive root systems. The roots are white and run horizontally just below the soil’s surface. These roots produce shoots that produce new plants, and Canada thistle can also propagate through root fragments that break off.

Characteristics Of Canada Thistle

Canada thistle plants have several branches and can grow to a height of 1 to 5 feet. The leaves of the plant are glossy and dark green. They are alternately arrayed on the stem and branches. The majority of the plants have thick, leathery leaves with deeply and irregularly serrated margins and long, sturdy spines; but certain biotypes have thin, flat leaves with short, fine spines, arranged sparsely around a slightly toothed edge. Stems rarely, if ever, have spines.

The average number of seeds produced by successful pollination is 1,500 per plant; however, this number can reach as high as 5,000. After eight to ten days of flower emergence, viable seeds may be found.

The seed has a feathery tuft of hair on top called a pappus, which helps the seed to travel away from the mother plant. However, in the case of the Canada thistle, this small “hang-glider” frequently drops its passenger near the parent plant, and many seeds never actually leave the seed head since their “glider” has already left without them.

Only a small fraction of seeds travel a long distance from their original location. When buried at standard tillage depth, most Canadian thistle seeds will survive for three to six years, although some seeds may survive for up to 21 years.

The optimal conditions for seed germination are warm, moist soils (20 to 30°C) and full light. According to research, this weed does not germinate well from seed in highly competitive environments, but once established, it can become a big problem due to its extensive root system.

Roots Of Canada Thistle

Once established, Canada Thistle is difficult to control due to its thick, fleshy roots. The plant reproduces extremely well through vegetative “cloning” from its root. After the aboveground parts are removed, new plants can regenerate within 19 days of emergence (two-leaf stage).

During its establishment phase, this weed produces a taproot that grows deep into the soil in search of consistent moisture. Once it finds moisture, it produces lateral roots, which can grow up to 20 feet in a single season and create a colony of genetically identical clones. It is common for roots to extend six to ten feet deep, and they can even go as far as 18 feet deep.

Lateral roots are not only crucial for colony expansion, but they also help the weed spread vegetatively with the help of humans. Root pieces as little as 1/8 to 1/4 inch (three to six mm) thick and 3/8 inch (eight mm) long can produce new plants. Tillage and sowing equipment have a significant role in the vegetative spread of perennial weeds.

Because one Canada thistle plant must cross with another to develop seed, plants that establish from seed have a high degree of variety. However, because vegetative reproduction from the roots is far more successful than seed reproduction, so a field typically contains only a few distinct individuals. The preponderance of plants has established themselves from root fragments.

Life Cycle Of Canada Thistle

Canada thistle is a perennial weed that grows back year after year. Understanding this cycle is crucial for good weed management because it is closely related to the seasonal cycle. It is particularly important to focus control efforts on the root system since it is the persistent roots that make this weed so difficult to eradicate.

As the soil temperature increases in the spring, buds on Canada Thistle roots are stimulated to develop into new shoots. When new shoots arise in late May and June, the long daylight hours initiate an urgent drive toward sexual reproduction. During this reproductive stage, the energy reserves in the roots provide the fuel for this process. During this stage, the Canadian (creeping) thistle plant produces flowers with a large number of seeds. Once the reproductive stage is over, the thistle plant enters survival mode.

Shortening day lengths, an indicator of impending winter, initiate the transfer of energy from the leaves to the roots for storage. Every winter, the above-ground parts of Canada Thistle completely die back, and in the spring, new shoots are produced from root buds. Old root tissue is continuously replaced by new, resulting in a two-year lifespan for a particular root.

How to Control Canada Thistle Weed?

There are several methods that can be used to control Canada thistle weed, such as:

Cultural Controls

Prior to the development of herbicides, Canada Thistle was completely controlled with tillage. In the 1930s, weed experts believed that intensive tilling was required to prevent new foliage from recharging root reserves.

New research revealed that this weed will continue to draw from root reserves during the flowering period. Therefore, experts now recommend removing the top growth just prior to flower bud break by tilling or mowing. This can be repeated until the plant enters the rosette stage, during which it transfers energy to the root, and then again tilling within one week of rosette emergence until the ground freezes. This strategy reduces the number of approaches required to deplete root reserves.

Chemical Control

Chemical control can also be used to manage Canada Thistle. The Bud Method of control is letting the thistle reach the bud stage, (which is when the plant is at a weak point in its life cycle) and then using a systemic herbicide. The abundance of foliage at this stage makes it easy for the plant to absorb nutrients, and the herbicide can be transported to the roots via sap circulation. Rates of herbicides used at this stage are often higher compared to the Rosette Method.

Combining fallow tillage with a systemic herbicide is known as the “Rosette Method” of controlling Canada thistle. Tillage or mowing in fallow can drain the root of more reserves than if the first blooming stem was left. When the day length is shorter than 15 hours, new shoot growth will form a rosette and continue to create carbohydrates for storage in the roots.

Herbicides For Canada Thistle

1) Clopyralid

Canada thistle is a highly invasive weed that can cause significant damage to agricultural crops. The use of herbicides is one of the most effective methods to control the spread of this weed. Among the herbicides that have shown excellent activity on Canada thistle is clopyralid, which is a plant hormone mimic (Group 4) that produces twisting symptoms.

Clopyralid is acceptable for use in wheat, flax, barley, and canola, but soil residues may persist for up to a year after application, and they may harm sensitive crops, especially legumes. Therefore, it is important to follow re-cropping recommendations to prevent harm to pulse crops and legume forages.

According to studies, Clopyralid will translocate more easily when this weed is transferring winter store materials to the roots, but it is also quite effective at the early bud stage of the crop. Control with clopyralid can be sustained throughout the following season when used in the autumn.

A combination of an in-crop application of clopyralid followed by a pre-harvest spray of glyphosate has been found to yield about 85% control, compared to the typical control of 70% for every single product, according to research. However, producers should weigh the cost of the additional herbicide and application against the potential return anticipated for the crop that will grow after.  

2) Aminopyralid

Another herbicide that is closely related to clopyralid is aminopyralid, which is only registered for use on rangeland, pasture land, and industrial land, but not on cropland. It works similarly to clopyralid and has a little longer soil residue than clopyralid.

3) Aminocyclopyrachlor

Aminocyclopyrachlor, an herbicide related to Clopyralid and Aminopyralid, is used mainly on pasture land, rangeland, and industrial land, but not on cropland. It has a longer soil residue than its related herbicides but functions similarly to clopyralid.

4) Picloram

Picloram is related to Clopyralid, Aminopyralid, and Aminocyclopyrachlor, but because of its soil persistence, the susceptibility of many broadleaf crops, and the potential for groundwater pollution and movement, it is less adaptable than those herbicides. It can be used on permanent pastures with medium to coarse soil, but its application on cropland is restricted to spot treatments for perennial weed control.

5) Glyphosate

Glyphosate, a nonselective herbicide belonging to Group 9, is used to eradicate various perennial plants, including Canada thistle. A yellowing of the plant’s new growth that extends throughout the plant is a sign of the symptoms’ gradual onset. It can be administered without worrying about re-cropping because it becomes inactive as it comes into contact with soil. It can be used to eradicate Canada thistle in glyphosate-tolerant crops for season-long control, as well as in fallow, after-harvest, and pre-harvest areas.


In conclusion, controlling Canada Thistle weed might be challenging, but it is possible with the right strategy and persistence. The first step in determining the most effective control method is identifying the species of Canada thistle present. Cultural practices like mowing and hand pulling can be useful for smaller infestations, but herbicides are a more dependable option.

Among the herbicides, Clopyralid, Aminopyralid, Aminocyclopyrachlor, and glyphosate are effective on Canada thistle. Each of these herbicides has its own strengths and limitations. For best results, it is crucial to carefully follow the label’s directions and apply the herbicide at the right time. In addition, keeping an eye on the area and swiftly removing any new growth can help stop the development of this weed. By using these methods, you may efficiently eradicate Canada Thistle weed and keep your landscape weed-free.

Remember that precise identification, consistent management, and patience are the keys to long-term success in controlling this weed. By using a combination of cultural practices and herbicides, and staying vigilant in removing new growth, it is possible to efficiently eradicate Canada thistle weed and keep your landscape weed-free.

Frequently Asked Questions

What does Canadian thistle weed look like?

The roots of the Canada thistle can reach a depth of up to 15 feet and spread out horizontally over several feet. The leaves are deeply lobed and spiny, with a distinctive waxy coating on the underside. The stems are erect and can grow up to 5 feet tall, with small, pink, or purple flowers that bloom in late summer.

How does Canada Thistle weed spread?

Canada thistle reproduces from seeds as well as from a deep root system that creates new shoots. Its root system can spread rapidly, making it difficult to control.

Is Canada Thistle toxic to humans or animals?

While Canadian thistle weed is not toxic, it can cause irritation if the spiny leaves come into contact with the skin. Its prickliness may deter animals from grazing on it, although it is not known to be toxic to them.

Can Canada Thistle spread through seeds?

Yes, seeds that are transported by wind, water, or animals can help spread Canada Thistle. This weed can generate a lot of seeds, which can remain viable in the soil for several years.

Can Canada Thistle be controlled without herbicides?

Yes, cultural practices like mowing, hand pulling, and smothering can help in the management of smaller infestations of Canada thistle or in areas where herbicides cannot be used. However, herbicides generally work better against larger infestations.

Can Canada thistle weed spread to other areas?

Yes, Canada Thistle weed can spread easily through underground roots or airborne seeds. It is important to control the weed before it spreads to other parts of your garden or neighboring properties.

Can Canada thistle be composted?

If the compost pile is not carefully managed to maintain a temperature of at least 140 degrees Fahrenheit for several days, it should not be composted. This is due to the fact that this weed can still grow from stem fragments and root pieces, even after being cut up and put into a compost pile.

Can Canada thistle weed be eradicated completely?

Complete eradication of this weed can be difficult, especially in large infestations. However, with consistent management and persistence, it is possible to keep this weed under control and prevent it from spreading further.

Can Canada Thistle be used for any useful purpose?

Although Canada Thistle is considered a noxious weed, some people have used it as a medicinal herb or a food source for animals. However, without proper research and consultation with experts, it is not advised to use this weed for these purposes.

What are the key characteristics of Canada Thistle weed?

When trying to identify Canada thistle weed, it is important to look for the following key characteristics:

  • Leaves with sharp spines along the margins.
  • Pink or purple flowers are arranged in small clusters at the end of each stem.
  • Perennial growth habit, with the ability to spread through underground roots.
  • Green or bluish leaves that are alternate on the stem.

How can I prevent Canada Thistle from growing in my garden?

The best way to prevent Canada thistle from growing in your garden is to maintain healthy soil and plant dense, competitive vegetation that can outcompete this weed. You should also regularly inspect your garden and promptly remove any Canada thistle seedlings or plants that may have grown.

Can Canada Thistle be used for medicinal purposes?

This weed has been used traditionally in herbal therapy to treat digestive problems and as a diuretic. However, there is limited scientific research on its effectiveness and safety for medicinal purposes.

Is Canada thistle native to North America?

No, it is not native to North America. It was introduced from Eurasia and has become naturalized in many parts of the continent.

How can Canada Thistle be prevented from spreading?

It’s crucial to eradicate fresh infestations of Canada thistle as soon as they are found in order to prevent this weed from spreading. Furthermore, planting competitive vegetation and maintaining healthy soil can help reduce the likelihood of Canada thistle establishing in the first place. Also, regular mowing might stop the plant from producing seed heads and spreading further.

Why is Canada Thistle weed considered a problem?

Canada Thistle is considered a problem because it is highly invasive and can quickly take over an area, out-competing native plant species and reducing biodiversity. It also has a deep and extensive root system that can make it difficult to control, and it can result in crop losses in agricultural fields.

Is Canada Thistle harmful to the environment?

Canada thistle can be harmful to the environment if it spreads to natural areas and competes with native plant species. It can also affect animal habitats and disrupt ecosystems. Additionally, some herbicides used to control Canada thistle can be harmful to non-target species, so it’s crucial to carefully follow the label’s instructions while using herbicides.

How can Canada Thistle be managed without the use of chemicals?

Cultural methods including hand-pulling, mowing, and smothering are excellent alternatives to chemical control for Canada thistle. However, compared to herbicide treatments, these methods may require more effort and persistence. In order to stop the spread of this weed, you should also make sure to properly dispose of any Canada thistle plants or seeds you may have.



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