Canada thistle (Cirsium arvense)

Several flowering Canada thistle plants

 
 

Canada thistle is a widely distributed perennial plant that can grow up to five feet tall. Its leaves have sharp spines along the edges. Canada thistle grows in a variety of habitats with full or partial sun and is often found in disturbed areas such as roadsides, trails, pastures, and recently flooded areas.

Caution: Plants have spines along leaves.

Description

Appearance

Canada thistle grows up to 5 feet tall, has leaves with sharp spines and flower heads made up of many small purple/pink flowers.

Leaves and stem

Leaves are alternate (come off the stem one at time at each leaf node). Leaves are deeply divided and have spines along the edges. Stems generally do not have spines. It has slender, grooved stems.

Flowers

Clusters of tiny pink/purple flowers make up the flower heads. There are smooth green bracts below the flowers that look like small green leaves or scales. The bracts do not have spines on their tips. The flower heads are about ¾ inch long. Canada thistle has male and female flowers on separate plants, but the flowers look similar to each other. Male flowers will not produce seeds. Plants bloom between June and September.

Seeds

Canada thistle has small seeds with feathery, white tufts to help with wind dispersal, although research has found that most seeds land near the parent plant. Seeds can separate from the white tufts. Seeds remain viable in the soil for over 20 years.

Roots

Each plant has a fibrous taproot with wide spreading horizontal roots. Each small section of root can form a new plant, enabling the plant to spread vegetatively.

Biology

Canada thistle invades natural areas such as prairies, savannas, open areas in forests, and dunes if some degree of disturbance already exists. It also invades wet areas with fluctuating water levels such as streambanks, sedge meadows, and wet prairies. Canada thistle can reproduce by seed and has male and female flowers on separate plants. Additionally, plants can spread by growing in circular patches spreading vegetatively through horizontal roots, which can spread 10 - 12 feet in one season.

Origin and spread

A native of southeast Europe and Asia, it is suspected that Canada thistle was introduced in contaminated imported crop seed in the 1700s. Today Canada thistle is found throughout the Midwest and in all Minnesota counties.

Refer to EDDMapS Distribution Maps for current distribution.

Don't be fooled by these look-alikes

  • Spotted knapweed, Centaurea stoebe spp. micranthos (invasive) – does not have spines on leaves while Canada thistle does.
  • European marsh thistle, Cirsium palustre (invasive) - has spiny stems while Canada thistle does not have spines on its stems.
  • Bull thistle, Cirsium vulgare (invasive); musk thistle, Carduus nutans (invasive); and plumeless thistle Carduus acanthoides (invasive) - have much larger flowers than Canada thistle and have more spines along stems.
  • Swamp thistle, Cirsium muticum (native) – the bracts underneath the purple flowers have a light, white, woolly covering. The spines on the leaves are softer than those on Canada thistle leaves. Minnesota has several native thistle species. View the Minnesota thistle comparison webpage by the Friends of the Eloise Butler Wildflower Garden for descriptions and photographs of the species.
 

Regulatory classification

This species is a Minnesota Department of Agriculture Prohibited Noxious Weed on the Control List meaning that efforts must be made to prevent the spread of seeds or other propagating parts. Additionally no transportation, propagation, or sale is allowed.

Threat to Minnesota

  • Once Canada thistle has been established, it spreads quickly, replacing native plants and decreasing diversity.
  • Canada thistle can reduce the amount of desirable forage for grazing animals in pastures.
  • Canada thistle grows quickly in disturbed areas making it a challenge in landscape restoration projects.

What you should do

One way that invasive plant seeds and fragments can spread is in soil. Sometimes plants are planted purposefully. You can prevent the spread of invasive plants.

PlayCleanGo: Stop Invasive Species in Your Tracks

  • REMOVE plants, animals and mud from boots, gear, pets and vehicles.
  • CLEAN your gear before entering and leaving the recreation site.
  • STAY on designated roads and trails.
  • PLANT non-invasive species.

Native substitutes

 

Control methods

Mechanical control can be done by repeated hand pulling or using a tool such as shovel. Take care to remove as much of the root as possible. If pulling plants, leave plants on-site to decompose or follow Minnesota Department of Agriculture guidance for plant disposal. Mowing will weaken roots and is most effective when flower buds are just about to open. Do not mow when seed is present, as that will spread the seeds. For prescribed burns, the goal is to favor native grasses over Canada thistle. Spring burns in April through June are most detrimental to Canada thistle, but also stimulate Canada thistle seed germination so follow up monitoring is needed. Repeated burns over three consecutive years are recommended.

Herbicide control can be done using a spot application with glyphosate or with the selective herbicides clopyralid or metsulfuron. Herbicide control of Canada thistle is most effective when you apply the herbicide in spring (April, May or June) or fall (September or October).

Biological control insects for Canada thistle are commercially available, but Canada thistle control results have been variable and the insects' impacts on native thistles have not been thoroughly tested. Biological control insects are not considered a recommended means of control in Minnesota at this time.

Reporting

Report new occurrences by submitting a report through EDDMapS, emailing Report a Pest, calling Report a Pest (1-888-545-6684), or contacting your local county agricultural inspector.

Resources

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