Canada Thistle

Common Name: Canada Thistle
Scientific Name: Cirsium arvense (L.) Scop.
Alternate name: Creeping thistle

Legal Status

Prohibited Control

Efforts must be made to prevent seed maturation and dispersal of plants into new areas. Additionally, no transportation, propagation, or sale of these plants is allowed. Failure to comply may result in enforcement action by the county or local municipality. 

Background

Canada thistle is native to Europe. It was introduced to North America in the 1600s, probably in agricultural seed shipments. It is now widespread throughout the United States and Canada.

Description

Canada thistle is an aggressive perennial with a vigorous root system that continually produces new shoots, allowing it to invade new areas and outcompete other vegetation.

• Grows 2-5 feet tall.
• Leaves are alternate, lance shaped, irregularly lobed, and have wavy spiny/toothed margins.
• Stems are usually smooth but may have short hairs and are slightly grooved.
• Flowers are purple and pink, occasionally white, and grow in clusters at the end of the stems. Buds are ½ inch wide and ¾ -1 inch long, tear-drop shaped, and lack spines.
• This plant produces large quantities of seed and spreads by roots (rhizomes).
• Seedlings emerge as small rosettes in the fall or early spring and eventually bolt into erect branched flowering stems. Flowers begin developing in late June, blooming between July and August.
• This plant is most recognizable in mid-July when flowers change to seedheads with distinctive white fluffy tops. Seeds are attached to the "fluff" and can be carried by wind to new areas.

Habitat

Canada thistle grows in a wide range of habitats but is most common in disturbed areas, including along roadsides, trails, natural areas, pastures, forest and field margins, mining locations, waste areas and unmaintained gravel pits. This plant establishes quickly after new road construction, housing and development projects, overgrazing of pastures, forestry clear-cuts, and destructive flooding events.

Means of spread and distribution

Spreads primarily through rhizomes (roots) and seeds. It is common throughout Minnesota.

Impact

This plant is highly invasive and can:

• Severely reduce pasture capacity and desirable forages
• Degrade wildlife habitat
• Hinder reforestation and landscape restoration efforts

Once established, a population can quickly displace native vegetation, including desirable pollinator habitat, creating large stands with little biological diversity and low habitat value.

Prevention and management

• A sound management plan is necessary to manage this species and will take a commitment of several years to ensure that the population has decreased significantly and is not a serious problem. Because this is a perennial species that produces a lot of viable seeds, in addition to a vast underground network of spreading roots, management efforts must focus on both the prevention of seeds and eliminating root growth. Management plans that combine fall and early spring applications of specific foliar herbicides and pre-flower mowing throughout the growing season have shown to produce excellent results, especially when follow-up management occurs for several years until the population is significantly reduced. Restoration of treated sites by fostering existing or newly planted site-specific native plant species can also sustain management well into the future and prevent new populations from developing.
• Mowing or cutting of stands prior to flowering in late June can significantly reduce the overall seed production of a population. However, Canada thistle will continue to flower after mowing, so repeated attempts must be made throughout mid and late summer to prevent overall seed production. Mowing in a single year will not be sufficient to control large stands. Multiple consecutive years of mowing will be required. Any equipment used during the process should be inspected and thoroughly cleaned to prevent spreading seeds to new areas.
• Fall and spring applications of foliar herbicides move into the plant's root system have been shown to provide the most effective management. Infestations need to be monitored for several years after initial treatments. Follow-up applications should be administered for surviving plants and new seedlings. If using herbicide treatments, check with your local University of Minnesota Extension Agent, co-op, or certified landscape care expert for assistance and recommendations. There are several businesses throughout the state with certified herbicide applicators that can be hired to perform chemical applications.
• Several biological control agents are commercially available. However, results have been variable and non-target impacts are not thoroughly tested. More information is needed before these agents can be considered a viable means of control.
• Canada thistle lifecycle and treatment timing graphic