canada thistle

Canada thistle. Photo: Caleb Slemmons, National Ecological Observatory Network, Bugwood.org

Updated: June 3, 2021

About Canada thistle

Life cycle

Canada thistle (Cirsium arvense) is an invasive perennial broadleaf weed with a deep root system.

Growth habit

Clump-forming; grows to 2 - 5 ft. high; leaves long and narrow, alternate, irregularly lobed with spiny margins; spineless stem.

canada thistle seedling
Photo: Phil Westra, Colorado State University, Bugwood.org

Reproduction

Spreads by fleshy, creeping rhizomes; lavender disk flowers followed by fluffy pappus capable of floating long distances in the wind; 680 seeds produced per stem; seeds remain viable in soil from 10 - 20 years.

Conditions that favor growth

Infertile areas without plants covering soil.

infestation of canada thistle
Photo: UAF Cooperative Extension, Bugwood.org

Cultural control

Keep vacant areas covered with turf or ground cover; maintain soil fertility appropriate for the growing crop.