Introduction
Canada thistle is native to Europe and northern Asia and was brought to the United States in the early 1600’s as a contaminant in grain. By the 1950’s, Canada thistle was declared a noxious weed in 43 states. Canada thistle was partially responsible for the existence of noxious weed laws.  In the 1900’s, the Canada thistle law was enacted.  It gave a person the right to trespass onto someone else’s property in the name of Canada thistle eradication.
Concerns
Canada thistle is an aggressive competitor.  This weed has a long creeping root system that will steal precious nutrients and water from native vegetation. The height of this weed also shades the ground below making it very difficult for grasses and forbs to grow. Canada thistle will form colonies in areas that have been recently disturbed, abandoned fields, ditches, and prairies.
The presence of this weed will change the plant structure of communities and decrease biodiversity. Canada thistle can be a huge problem on agricultural land due to the fact that it is an alternate host for some insects and pathogens that are known to attack certain crops.  It is also difficult and expensive to control.
Identification
This thistle is unique because it is a perennial, whereas most thistles are biennial, this also makes it harder to control.  Canada thistle can grow from 1 to 4 feet tall. The leaves of this thistle are attached alternately along the stem; they are lance-shaped, lobed and spine-tipped. The leaves get gradually smaller as they progress up the stem. The stem is branched and sometimes hairy, but it lacks spines.
The flowers of the Canada thistle are usually purple and sometimes white. They are ½ to ¾ inch in diameter and grow in clusters at the end of stems. Each plant can produce up to 5,300 seeds that are easily distributed by the wind due to the tufts of hair attached to them. The seeds remain viable in the soil for up to 20 years.
What can you do?
Stop small infestations before they take over.  Burning and mowing are somewhat effective because they allow native and desirable plant to compete with this weed. These two techniques must be repeated for many years to achieve real success because of the longevity of the seeds in the soil and the weeds ability to sprout from broken roots. Hand pulling and cultivation generally are ineffective and time consuming. These two methods will not remove all of the roots and plant will re-sprout from the remaining roots. There are herbicides that are effective. If you have any further questions regarding this weed or other weeds on your property call your local weed district.
Visit the MWCA Weed ID pages for additional information and pictures of Canada Thistle.
This series of articles was developed by Ravalli County. If you would like to use these articles please contact Ravalli County Weed District Weed Coordinator at (406) 777-5842.