There are many different species of thistle plants, all of which are invasive. Most thistles are perennial or biennial and considered noxious weeds in many parts of the U.S. Musk, bull and plumeless thistles are among the most common biennial types, while the most common perennial species are the Canada, wavyleaf and Flodman thistles. In home landscapes and grazing pastures, chemical control of thistles isn’t always desired or feasible. If you want to kill thistle weeds without using herbicides, you have several control options.
Employ biological controls of biennial thistle weeds by using insects that naturally feed on the plants. Introduce to the thistle-infested area insects like the adult thistle-head weevil or Rhinocyllus conicus and the thistle rosette weevil or Trichosirocalus horridus.
Introduce insects like the thistle-stem gall fly or Urophora cardui, the flower weevil species Larinus planus or the stem-mining weevil species Ceutorhynchus litura to control perennial thistle species like the Canada thistle. Introduce the starthistle bud weevil species Bangasternus orientalis to control yellow starthistle weeds.
Remove thistle weeds mechanically by mowing repeatedly, at any time the thistle plants are budding but not yet seeding. Mowing repeatedly during the early-bud stage can prevent the thistles from producing seeds and spreading. Mow as low to the ground as you can, aiming to cut the thistle plants below the terminal bud to prevent them from re-growing.
Cultivate the thistle-infested area using a tiller before the thistles are 3 inches tall. Till the area repeatedly until the first winter freeze each time the thistle plants re-grow, but haven’t yet reached 3 inches in height.
Prevent recurrences of thistle weed populations by keeping the area planted thickly with cover plants and reseeding any disturbed or bare-soil areas promptly. You can control and prevent thistle growth in pastures by using proper rotational-grazing techniques. Avoid overgrazing in the pasture, which can make the pasture more at-risk for thistle weed growth.