Bill Johnson

Canada Thistle

Cirsium arvense (L.) Scop.
Aster family (Asteraceae)

Origin: Europe and Asia

Background
Canada thistle was accidentally introduced to North America in the 1600s and is designated as a noxious weed in 43 states.

Distribution and Habitat
Canada thistle is an extremely widespread weed of agricultural and ecological areas in the U.S, occurring throughout the northern states and Southwest but is largely absent in the South from Texas to Georgia. Twenty large national parks across the country report it as a serious invasive plant affecting natural resources. It invades a variety of dry to moist open habitats including barrens, fields, glades, grasslands, pastures, stream banks, wet meadows, wet prairies, and open forests. It is not very tolerant of shade.

Ecological Threat
Once established, if conditions are suitable, Canada thistle can form dense stands that shade out and displace native plants, changing the plant community structure and species composition and reducing biodiversity. It spreads rapidly and is very difficult to remove.

Description and Biology

• Plant: erect, perennial herbaceous plant, to 4 ft. tall with an extensive creeping rootstock and ridged and hairy stems.
• Leaves: lance-shaped, irregularly lobed, 2-6 in. long with weakly- to strongly-prickled margins.
• Flowers, fruits and seeds: flowering occurs in late June to August; flowers are purple to white and about 1 in. long by ½ in. across; seeds are called achenes, are 1-1½ in. long and have a feathery pappus.
• Spreads: by wind-dispersed seed; expands locally by vegetative means through lateral roots and root fragments.
• Look-alikes: a number of native and exotic thistle species, some which are very rare. Numerous species of thistle occur in North America. Some are invasive, some are native, and most are dependably difficult to distinguish without assistance.

Prevention and Control
Management of Canada thistle is very difficult and requires repeated applications of systemic herbicides including products not covered in this guide. Glyphosate is not very effective against it. Other sources will likely need to be contacted for more effective herbicides (see Control Options).

Native Alternatives
After eradicating, plant area with native vegetation appropriate to site conditions. Refer to References.