Canada thistle has been on the rise in recent years across Illinois. This is a noxious weed in Illinois, which means you are required by law to control it on land you own or work. Canada thistle is a prolific seed producer; it propagates from underground rootstocks that sprout from any root pieces left after weeding. Theoretically, it can be eradicated by hand-pulling or mowing at least three times a season. Timing is everything; however, a proper mowing schedule is difficult to achieve.
Herbicides can save a lot of time and effort. A 2% solution of glyphosate (Roundup, for example) is effective, but usually more than one application is necessary. In landscape beds, glyphosate must be applied selectively with a wick applicator or some other method of wiping it on, using at least a 30% solution. Bentazon (Basagran) does a fair job on Canada thistle, while dichlobenil (Casoron) provides control in some woody ornamentals. One of the best chemicals for control of Canada thistle, clopyralid (Stinger), unfortunately is not labeled for a landscape or nursery use. However, it is very useful in noncrop areas and abandoned fields surrounding the nursery where Canada thistle often forms large stands.
(Adapted from Buckeye Yard & Garden, April 23, 1998)