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Canada Thistle

Cirsium arvense (L.) Scop.


Family: Asteraceae

Habit: rhizomatous perennial


Ecology

Management:

Because Canada thistle has a deep root system, the only approach for controlling this weed in an organic garden is to exhaust the storage roots (Exhaust perennial roots). Food reserves in the roots reach a minimum in June and then increase as food flows from the shoots to the storage roots. Consequently, shoots should be removed for the first time by early June. Persistent removal of the shoots before they attain several leaves will exhaust the storage roots within two years and eliminate the weed. One study found a 21 day weeding schedule was optimal. Sometimes, this species is eliminated from the core of the garden but continues to encroach from peripheral area. Such areas should be mowed regularly since cutting the shoots has a similar effect on the roots as hoeing or cultivating (see Weeds along fences).

The base of the stem where it meets the ground is free from spines, and the shoots are easiest to hand pull if grasped at that point or a little below-ground.

The root reserves are sufficient to push the shoot through any amount of organic mulch. Moreover, even if a tough synthetic mulch prevents shoots from appearing above ground, the storage roots proliferate below ground, leading to problems in future years.

Useful Management Techniques