Canada Thistle

Canada Thistle

Cirsium arvense

Information Sheet (PDF)

Colorado List B - Control required in Jefferson County

General

• Colony-forming unisexual plants
• Elevation: 4,000 to 9,500 feet
• Family: Sunflower (Asteraceae)
• Herbaceous
• Perennial

Habitat

• Infestations are found in cultivated fields, riparian areas, pastures,  forests, lawns and gardens, roadsides and in waste areas; due to its  seeding habits, vigorous growth and extensive underground root system,  control or eradication is difficult; Canada thistle is one of the most  widespread and economically damaging noxious weeds in Colorado

Plant

Vegetation

• Leaves are 4 to 8 inches long, usually dark green with spiny, serrated edges
• Mature plants are 2 to 5 feet tall

Roots

• Horizontal growth is quick, roots can regenerate from small pieces
• Horizontal roots may extend 15 feet or more, vertical roots may grow 6 to 15 feet deep

Flower

• Buds are 3/4 inch or less in diameter
• Color: purple, pink or white
• Season: June to August

Seed

• Can remain dormant in soil up to 20 years
• Seed can be viable 8 to 10 days after flowering
• Seeds germinate from late May through early fall

Seedling

• Seeds germinate in spring or fall

Reproduction

• Root propagation is main source for new plants
• Seed and horizontal roots

Control

Biological

• Not effective in our area

Chemical

• View Information Sheet (PDF)

Cultural

• Prevention - maintain health of site

Mechanical

• Burning - N/A
• Grazing - N/A
• Mowing - Mowing is best before seed set; mow every 3 to 4 weeks June through  September to weaken plants; works best when combined with fall herbicide  treatments. 

More Information

• Colorado State University Extension

Use all chemicals according to the manufacturer's label. No specific recommendation or endorsement is made or implied by listing methods or products.