These have been released by private parties to attack the thistles, and this demonstrates how biological controls can help and then move far beyond their original control areas. Understanding Canada thistle is a major step in learning how to fight it, Knight said. While the seeds have low viability, it has a root system best visualized as a thick hemp doormat. If you try to dig it out or plow it under, hundreds more root systems will begin to thicken and mature, Knight said. Other weeds, Knight said, are annuals or biennials and can be eliminated by pulling.
Examples are bull thistle, mullein and tumble mustard.
These plants are dependent on widespread seed dispersal and high viability of the seed. Any seed heads cut before maturity are just that many less to worry about later. The waxy leaves and stem of dalmation toadflax also make it almost totally immune to herbicides. Leafy spurge also has a very extensive root system sustaining it as a perennial, with the added problem that the catapult type seed heads can move the infestation along at the rate of about 15 feet a year in any direction. It's becoming a real problem, especially along roads and watercourses.