Canada thistle (Cirsium arvense) is a creeping perennial, 2 to 5 feet tall, with alternate, dark green leaves. The leaves are oblong, deeply lobed, with spiny toothed edges. Stems are branched, often slightly hairy, and ridged. The flowers are purple to light lavender or even white, in clusters of 1 to 5 per branch at the tops of branches.
Native to Eurasia and found from 4,000 to 10,000 feet elevation. It can survive dry conditions once established and is somewhat shade intolerant. It is found along edges of forested areas but not within forests.
Biology and spread
Canada thistle produces 40 to 60 seeds per flower head; seed is long-lived (20 years or more) and dispersed by wind or in contaminated seed or forage. It has an extensive underground root system and regenerates from root fragments.
Colorado Noxious List B. One plant can colonize an area 3 to 6 feet in diameter in 1 to 2 years. May form monocultures and prevent the coexistence of other plant species.
Cultural, physical and mechanical control: Repeatedly cut or remove the plant.
Biocontrol: The weevil, Ceutorhynchus litura, is available from the Colorado Department of Agriculture, but may not be effective as a sole control method.
Chemical control: Products such as Transline and Milestone can be used in rangeland or permanent grass pastures, but are NOT labeled in turf.