Most thistle plants that are native to the United States are non-invasive plants that do not give gardeners trouble. But species of thistle that were introduced from Europe are invasive plants that can move in and take over a garden like a set of bad neighbors. According to North Dakota State University, the best way to eliminate thistle is to prevent its spread. One of the best ways of controlling thistle is to cultivate the ground in which it grows.
Mow down thistles using a lawn mower that catches all refuse. Thistles spread through seeds and at the roots. By keeping thistles mowed, you don't allow plants to spread by seed.
Start the rototiller and cultivate thistle roots by slowly pushing the rototiller over the area in sections.
Work any remaining thistle roots free by combing the cultivated ground with a garden rake. Since thistles can return from the taproots, it is important to remove all roots from the soil.
Repeat steps 1 through 3 monthly throughout the growing season to prevent thistles from returning.