Let’s face it; some gardeners love to tempt fate. The flower that blooms once a decade, the plant that eats hamburger and the plant that stinks up the neighborhood all have their place in the environment. The thistle, however, is an invasive, noxious weed. Should you decide, like the 13th century Scots who staved off an invasion by bare-footed Norsemen, that there’s a place for this plant in your garden, be prepared for the consequences. Even in Scotland where it is a national symbol, thistles are considered unwanted weeds.
Buy cheap seed to sow your lawn—anything with more than 15 percent weed seed is sure to contain thistle seed. Sow seed at less than the recommended rate—thistles flourish in thin lawns and bare spaces in gardens. Never pull a thistle or dig it out—they will thin themselves.
Propagate thistle in cool, wet spring and fall weather. Biennial thistles, like musk and plumeless thistle, can produce thousands of seeds per plant but perennial thistles, like the Canada thistle, may produce only 40 to 80 seeds per plant. Plants need not be pinched--the seed heads will form, mature and fly off from July through frost.
Apply fertilizer to adjoining areas but never to thistles or plants that surround them—the less fertile the soil, the better. When surrounding grasses become dormant or die, thistles thrive. Under no circumstances should herbicides that include glyphosate, 2,4-D, dicamba or combinations of these be used to eliminate grasses; thistle is sensitive to these chemicals and can strangle grass without any help.
Weed around rosettes growing in beds and lawns in the fall. These are baby thistles that will bloom the following season. Canada thistle, a perennial, is especially vulnerable and digging rosettes every fall will result in plants that never mature.
Delay mowing lawns until the grass is so long that you must remove more than a third of the leaf to achieve the proper height. This practice weakens the lawngrass and gives perennial thistles, like Canada, Flodman and wavyleaf, more time to develop rosettes and spread roots, which can put up more rosettes, between mowings.