Thistles are usually one or several types of noxious weeds that can quickly invade cultivated soil. Some types, like sow thistle, grow closely to the ground until they flower, while other varieties, like Canada thistle, can grow up to 4 feet tall. Thistles are covered with fine spines that can really sting if touched or stepped on, and some people might have allergic reactions to thistles. Getting rid of a thistle infestation with a rototiller might take several seasons to be successful.
Dig up any small thistle plants as soon as you can see them. Don't just pull them up because the roots can survive and grow into more plants. Instead, dig down as deeply as you can under the plant and pull it out of the soil. Smaller plants are easier to deal with because the spines are underdeveloped.
Clip the flower heads off older thistle plants. Ideally, you should clip them off before they have a chance to bloom and start spreading. Collect the flower heads in plastic bags and throw them away. Wear gloves to protect your hands from the spines.
Use a rototiller on the thistles only if the infestation is very bad. For tilling to be effective, you will have to do it about once a month until the plants flower because many varieties of thistles spread with rhizomes. This means that any cut-up bits of the plant that remain on or in the soil can grow into new plants.
Till the soil 4 to 6 inches deep to get under the root systems of most types of thistles. Cutting up the roots of varieties like Canada thistle help propagate the plant, so it's best to pull them out as completely as you can.
Rake up all the thistle parts you can see and dispose of them immediately after tilling.
Avoid tilling after the thistles have started to flower. You can make the infestation worse by spreading and burying the seeds.