This thistle looks much like the other thistles. It’s called the Canada thistle not because it originated in that country but that’s where it was introduced (scientists believe it was hiding in crop seed). The thistle grows three to five feet tall with stickers and little purple heads (there are some white ones).
The plant isn’t dangerous to animals. So, why bother? What’s the harm?
As far as invasive capabilities go, this is an overachiever. One plant can take over a 3 x 6 foot area in one to two years. The seeds can survive in the soil for 20 years. Mowing doesn’t help as the roots can survive at least 100 days without leaves and flowers. The root system is impressive–up to 15 feet horizontally and 6 to 15 feet deep). Breaking up the root system increases the number of plants.
A root system like this means the plant is a water and territory hog. It will get water before many native plants do and push them out. In late summer the plant will dry out and become a fire hazard. It’s like the relative who comes to visit and never leaves or helps around the house.
Mowing doesn’t help, fire won’t help–what can be done? Herbicides can manage the invasion. A solution of vinegar and salt can help. These solutions will also affect the soil and other plants, so reseeding an infested area is crucial. The best defense is prevention–make sure the native plants are healthy and get rid of invasives immediately.