Things You'll Need
Several varieties of thistle invade home lawns. All of them are difficult to kill. Thistle plants have two means of entering and spreading throughout your lawns: seeds and regenerative rhizomes. A single large thistle plant can produce tens of thousands of seeds, and deep roots that store lots of energy to resume growth right after you cut back. To control these pernicious weeds for good, you must employ chemical or physical control methods consistently over several seasons.
Mechanical Thistle Control
Cut the thistle plant to grass level with a pair of sharp lopping shears.
Bag the cut plant material and throw it away.
Mow the lawn as usual, to keep the thistle short, stunted and unable to produce flowers. Do not let the thistle grow more than 3 inches tall. Eventually, in one or two seasons, the thistle's roots will starve and it will die.
Chemical Thistle Control
Cover any nearby desirable broad-leaf plants with plastic. The broad leaf herbicide used to kill thistle will not harm grass plants. However, accidental overspray that lands on garden plants will burn or even kill some of the leaves.
Spray the thistle plants with a broad leaf herbicide according to the manufacturer's instructions. Early in the morning or late in the evening are the best times to spray. Coat all of the thistle plant's foliage thoroughly, until just before the point of run-off.
Respray the thistle plants and any regrowth every 10 to 14 days until they are dead.
Look for herbicides that include a sticker-spreader. This additive helps the herbicide cling to the thistle's hairy stems and leaves and makes the systemic herbicide more effective.
The best time to chemically control thistle is during warm months when it is actively growing. Broad-leaf herbicides that work well on thistle contain Metsulfuron-methyl, 2,4-D, dicamba, triclopyr or aminopyralid as their active ingredient. Chemical control methods are most effective on young, small thistle plants.