Canada thistle brings a thorny problem to any landscape where it appears. This prickly beast grows from seed that can blow into your yard, or it can sprout from root pieces, which sneak in with bulk topsoil or mulch loads. Size varies, with many mature plants reaching 5 to 8 feet tall. In a single season, one plant can produce a 20-foot-long root system, and it only takes one piece of root to produce a plant. Control through weeding, but dig carefully and deeply to get the horizontal root. After digging, if another sprout appears, pull it, too. Or use an herbicide. The best time to spray is as soon as leaves break ground. Spray repeatedly through the growing season, and you will eventually kill it.