All thistle plant species are invasive and can ruin the appearance of your lawn. Most thistles are perennial or biennial and considered noxious weeds. Musk, bull and plumeless thistles are among the most common biennial types, while the most common perennial species are the wavyleaf and flodman thistles. In home gardens and grazing pastures, chemical control of thistles isn't always 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. 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 regrowing.
Cultivate the thistle-infested area using a tiller before the thistles are 7.5 cm (3 inches) tall. Till the area repeatedly until the first winter freeze each time the thistle plants regrow, but haven't yet reached 7.5 cm (3 inches) in height.
Prevent the recurrence 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.
Thoroughly clean any gardening or farming equipment after using it in a thistle-infested area to prevent spreading thistle seeds to other areas and pastures.
Thistle seeds can also spread by travelling in hay, so don't transport contaminated hay to fields or areas without thistle infestations.
Be aware that mechanical control of thistle weeds with mowing can take several years to completely eradicate thistle weeds, particularly the perennial thistle species. Cultivation with tilling, however, can completely eradicate biennial thistles in one growing season and effectively control populations of perennial thistles.