This is an extremely common weed in Montana and one that can be hard to eradicate. The plant has an extensive root system descending over 15 feet deep in the soil. It puts out horizontal root runners which are the number one method of spreading. Each plant also produces over 1500 seeds per flower which are light and easily borne by the wind. A Canadian thistle can regenerate from a piece of root just .025 inches long and .0125 in diameter. Its seed base typically lasts 5-7 years. Due to the extensive and hardy root system, the effective management of the plant is either repeated mowing during active growing periods for several years to prevent flowering and re-seeding, or herbicide applications in spring and early summer prior to flowering. It is critical to use an appropriate herbicide which will kill not only the vegetation but the root system. Herbicides, such as Opensight and Milestone which leave a residual in the soil for seed control improve the effectiveness of the herbicide and accelerate control.
New rosettes will appear and be actively growing in the fall if the plant receives adequate moisture and prior to dormancy due to freezing. Applications during this window can be effective as well.