The Enemy - Canada thistle ( Cirsium arvense) is a perennial weed that can grow up 1 to 4 foot tall and came to us from Eurasia, probably from a crop seed prior to 1800. This is the most widely spread weed we have in Idaho. This thistle usually has a pink flower but has also been seen with white ones. In the spring the rosettes are normally small and hairless and are not normally found alone, as many of the plant are connected by the roots. Although this plant, and other thistles, are completely edible by livestock and humans, they usually stay away from them due to the spines that grow along the leaf margins.
The Strategy - It has been seen it in our lowest agricultural fields as well as in meadows at our highest elevations thus it has become one of our most damaging weeds. Once the seeds mature they develop a plume (like dandelions) that allow the seed to be blow for miles by the wind. But interesting enough, I would challenge anyone to see if any of the seeds actually grow into a plant. Intead, the plant has a vast horizontal root system (rhizomes) that allows the plant to ‘creep’ into adjacent properties and even through asphalt. Many of the islands on the South Fork of the Snake River are totally covered by this thistle.
The Defense - Like many of our perennial plants Canada thistle can be controlled (fairly easily) with herbicides at three times of the year. Early spring (while in the rosette stage); mid summer (during the early bud stage); or fall (after the first killing frost). Herbicides such as Tordon 22K, Curtail, Redeem, Milestone, and Transline (Lontrel in horticulture) are all great choices. 2,4-D and Roundup are not products of choice as perennial plants will regrow after treatment (but may be your only option in a garden or other horticulture setting). Many biological control insects are available. As with any plant with rhizomes, mechanical control is useless (in fact it encourages more growth). As always call your local County Weed Superintendent for more details.
U of I Idaho's Noxious Weeds Control Guidelines (183 KB PDF download)