Canada thistle is a non-native perennial forb, which originated in Eurasia and North Africa. This very aggressive weed spreads by rhizomes and seeds. The extensive root system produces as much growth under-ground as there is growth above ground, making this noxious weed a tough one to control.
Canada thistle grows from 1-4 feet tall, and pale purple flowers bloom during June, July or August, depending on our weather and moisture. The flower head of Canada thistle is the size of the end of your pinky finger, making it easy to distinguish from other thistles.
Canada thistle infests rangeland, pastures, cropland, ditch banks, roadsides, stream and lake banks and disturbed areas. It is common to find Canada thistle in and around irrigated cropland.
Control of Canada thistle can be quite difficult, due to the extensive root system. Tillage is not recommended, as it only spreads the root segments, which will start new patches of Canada thistle. There are several herbicides recommended for use on Canada thistle. The EC-130 Guide for Weed Management produced annually by the University of Nebraska Extension recommends the proper timing and amounts of herbicides used for Canada thistle control. Always read and follow the label instructions, as the herbicide label is the law.
This is a link to the 2013 edition: http://www.ianrpubs.unl.edu/live/ec130/build/ec130.pdf
Persistence is the key to Canada thistle control. Herbicide will need to be applied according to the label. Follow up treatments will be necessary, with monitoring for several seasons.