Killing Canadian Thistle

Asked June 25, 2019, 12:20 PM EDT

I have a lot of what seems to be Canadian Thistle. I am unsure because some of the stems are spiny, rather than smooth. See photos. Also, it does not necessarily grow in clumps. The partly-opened lavender-colored flowers are 1/2 inch across now, near the end of June. The infestation took hold long ago, on sunny ground below a pine and a spruce tree. It has extended well into my yard which is grass and other plants. I followed my city's recommendation to pull the plants and put thick newspaper down below surface level. I find now, through experience and reading, that this is far from the correct approach. I hesitate greatly to buy the array of chemicals recommended, and to apply the ones that require (to my knowledge) expert handling. Your advice would be much appreciated.

Hennepin County Minnesota

4 Responses

Canada thistle usually does not have spines along the stem. However, all the other non-native thistles do, and the native ones do not. So you should try to get rid of it. Check this link:
https://www.friendsofthewildflowergarden.org/pages/plants/thistlecompsheet.html

Repeated pulling will eventually weaken the plants. Don't let it go to seed. Spot treatment with glyphosate (Roundup), but you may have to repeat. Glyphosate is non-selective, so it will kill anything green that it touches. Read here:
https://www.dnr.state.mn.us/invasives/terrestrialplants/herbaceous/canadathistle.htmlhttps://www.minnesotawildflowers.info/flower/canada-thistle
https://www.dnr.state.mn.us/invasives/terrestrialplants/herbaceous/bullthistle.html
https://www.mda.state.mn.us/plants/pestmanagement/weedcontrol/noxiouslist/canadathistle

I had attached photos to show you that some of the stems were smooth and others had spikes. Also, I asked about how to apply the array of chemicals that other websites (USDA and Colorado State U) advise, They recommend Roundup BEFORE blooming time. Is there a company that can be hired to deal with this?
Thank you,
Ellen

Lawn care companies are licensed to use herbicide products homeowners can't use. If you are looking for a company to do this job, that would be where I would start.

Herbicides work best when temperatures are moderate (60-80). Also, herbicides work best when the plant is actively moving energy from the plant to the roots. But Canada thistle is a tough plant, so herbicides both in the spring and fall may be necessary. There are various protocols and herbicides. Read here for another take on this from Purdue:
https://www.extension.purdue.edu/extmedia/FNR/FNR-436-W.pdf

Thank you very much!