Thistles can be an invasive and stubborn weed, threatening plants you desire in a landscape. Prevent these weeds from growing and spreading by using hay to smother thistle growth. When you spread hay in a thick layer over the unwanted thistles, you smother the growth and kill the thistles. Watch how the thistles react to see if you'll need other methods to eradicate thistles completely.
Mow down the thistles in early spring so they are as short as possible before laying down the hay.
Apply at least 4 inches of hay to the area where the thistles are growing to smother them. Replenish the hay when needed so it stays at least 4 inches thick over the thistles.
Check the thistle growth beneath the hay every few weeks to assess how they are responding to the smothering. You should see yellowing and withering foliage as the hay smothers the thistles and deprives them of sunlight and nutrients.
Apply an herbicide in the autumn if the thistles are still alive beneath the hay. Often after you smother thistles during the growing season, virulent species may try to blossom in the autumn, making it an ideal time for herbicide. Use a broad-range herbicide that will kill the thistles. Cultivate the soil where the thistles have bee growing with a rototiller, then spray the area thoroughly with the herbicide.
Assess the thistle area the following spring. Reapply more hay to continue to smother the thistles if you detect any growth. If you see none, prepare the planting area as you desire.