This is Cirsium arvense, more commonly known as Canada Thistle. The Canada thistle is a 2 to 5 foot (0.6 to 1.5 meters) tall forb with deep, wide spreading, horizontal roots. It is distinguished from other thistles (Cirsium spp.) by its deep-running perennial rootstocks, more slender stems, and small compact heads. Plants are tall and lax, with few flowers, on sites that are shaded most of the day.
In North America the Canada thistle occurs approximately from latitudes 37degrees North to 59 degrees North. It does not survive in the southern United States. It is thought to be native to southeastern Europe and the eastern Mediterranean area. It now occurs throughout Europe, North Africa, Asia Minor, and across central Asia to Japan. It is also naturalized in South Africa, New Zealand, and southeastern Australia.
The survival and spread of this species is due to the highly successful vegetative propagation carried on by the creeping horizontal roots which survive winters (yes, even in Minnesota), and continue to give rise to numerous aerial shoots year after year. The plants can survive indefinitely through the root system. The shoots emerge in the spring when the mean weekly temperature reached five to eight degrees Centigrade.
Why the horticulture lesson today on the Canada thistle? Or is it botany? Well, whatever it is, the Canada thistle is a nasty, noxious weed, according to Minnesota state law. The City has an obligation under State law to remove it, or cause it to be removed, when we acknowledge its presence. That might sound easy enough, but when you consider the hundreds of individual acres that are owned by the City, you can get an idea of the challenge and cost of this obligation.
We handle the removal of Canada thistle through what we call “passive enforcement”. If we receive a report of it, we will investigate it and enforce the law accordingly. We do not, however, go out and patrol for it. Maybe we ought to, but we are not staffed or budgeted to do that on private property, let alone public property in Eden Prairie.
If you want to get rid of your Canada thistle, here’s what you can do:
Response to Herbicides: Effective control can be achieved by using several broad-leaved herbicides that do not harm grasses. For more site specific control recommendations, please refer to the latest edition of the Pacific Northwest Weed Control Handbook.
Response to Cultural Methods: Planting competitive crops, such as alfalfa and forage grasses can be very effective in controlling an infestation of Canada thistle.
Response to Mechanical Methods: Repeated tillage at 21-day intervals for about four months can be effective on minor infestations of Canada thistle. Repeated mowing to weaken stems and prevent seeding is also effective in low level infestations.
For more information on the Canada thistle, check out this link. Most of my science info in this post was lifted right out of this guy’s article.
This isn’t the best time of the year to kill Canada thistle. It’s probably already propagated. But is there really a bad time of year to kill a noxious weed? I think not. Have at it. Get a jump on them for next year.
