Back by popular demand and with additional details! Canada thistle, Cirsium arvense, is a common prickly invasive that’s extremely difficult to eradicate–a prohibited, noxious weed in Michigan. Native to Eurasia, not Canada, it’s also known as creeping thistle, field thistle, and Californian thistle. According to the University of Michigan Herbarium, it was present in North America by 1800. Fun fact: Canada thistle is from the sunflower (Asteraceae) family.
Found along roadsides, ditches, fields, gardens and other disturbed sites, Canada thistle grows 1-4 feet tall with those detested prickly stems and leaves, and branches near the top. Leaves are alternate and spiny; a lookalike plant is bull thistle, Cirsium vulgare, which has larger, more spiny leaves. Flowers are purple, fragrant, and numerous, producing small, light brown seeds with hair-like tufts that aid in dispersing 1000-1500 seeds per plant. Seeds develop quickly, 8 to 10 days after flowering begins. And they can remain viable in the soil for up to 20 years!
Reproducing through seeds and extensive creeping rhizomes that form dense patches–since any root fragments can sprout–Canada thistle can negatively impact habitat and species diversity. On farms, it presents an economic threat as it competes with crops and can reduce yields.
A close up of Canada thistle leaves. Photo by Jan Samanek, Phytosanitary Administration, Bugwood.org.