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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 field of Canada thistle in bloom

An entire field of Canada thistle. Photo by Steve Dewey, Utah State University, Bugwood.org.

Canada thistle leaves

A close up of Canada thistle leaves. Photo by Jan Samanek, Phytosanitary Administration, Bugwood.org.

Canada thistle seeds

The many seeds of Canada thistle. Photo by B. S. Walters, michiganflora.net.

Canada thistle seeds close up

A close up of Canada thistle seeds. Photo by Jan Samanek, Phytosanitary Administration, Bugwood.org.

Control methods: Pull new seedlings within 2 ½ weeks of germination before they perennialize. Pulling entire plants when in full flower–even without getting all of the root fragments–will also help deplete root reserves, and keep it from going to seed. Check for resprouts, and keep digging or pulling those out. Eventually the stored resources will be used up. Alternately, using a small amount of herbicide to spray 3-6” resprouts is more effective than spraying entire plants, as the plant has utilized root reserve to grow to flowering size. For large patches, they can be cut down, and covered in cardboard and mulch to smother. Most control methods likely need to be repeated over multiple years. 
For information about using herbicide to control Canada thistle, check this link from Penn State. Due to how difficult it is to eradicate Canada thistle, and depending on each site’s specific dynamics, multiple management methods are usually necessary. 
Created by: Emily Lilla, former Matthaei-Nichols Natural Areas Stewardship Technician.
Photo credits:
Cirsium arvense Images, Bugwood.org
Sources:
Cirsium arvense
Species Details
FACTSHEET CANADA THISTLE 
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