Weeds: Canada thistle – Cirsium arvense

• Family: Asteraceae (Compositae)
• Cycle: Perennial
• Plant type: Broadleaf

Biology

Canada thistle is an aggressive, spreading weed. The aboveground portions of the plants die back in the winter, but resprout from perennial roots in the spring. The alternate leaves are variously shaped, but are typically elongate and variously toothed or lobed, with spines on the margins and at the tips. Leaves may be hairy beneath. The erect, rigid, branching stems may reach up to four feet in height. Flowers (usually purple, occasionally white) are about 3/4″ across. They are borne at the top of the plant in clusters. The base of the flower head lacks spines. Male and female flowers occur on different plants, with both male and female plants required for seed production. Canada thistle spreads aggressively from a deep, extensive system of roots and rhizomes. Uncontrolled patches may reach several yards across. SPECIAL INFORMATION: Cultivation is seldom recommended as a control, as breaking up the roots produces new plants. Canada thistle is designated as a Class ‘C’ noxious weed in WASHINGTON and a Class ‘B’ noxious weed in OREGON. Management may be required by law in your county. In addition, it is on the Oregon noxious weed quarantine list, which prohibits sale, purchase, and transport of plants, seeds, and plant parts. Consult your local Noxious Weed Control Board for more information.

Habitat

Canada thistle is found in fields, waste and cultivated areas, and pastures, commonly on heavy or rich soils.

Management Options

Use Integrated Pest Management (IPM) for successful plant problem management.

Non-chemical Management

Select non-chemical management options as your first choice!

• Inorganic mulches, such as plastic, commercial “weed barrier” fabrics and other materials such as roofing paper, is an effective weed management option.
• Cover inorganic mulches with a thin layer of soil or organic mulch.

Chemical Management

IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides.

• Apply according to label directions.
• Glyphosate products should be applied as spot treatments only!
• NOTE: Some ingredients listed here are only available in combination.
• Read the label carefully on combination products to make sure the product is suitable for your specific situation.

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