Thistle weeds have invaded 2 of our perennial beds. The past 2 years I have...

Asked April 27, 2013, 11:30 PM EDT

Thistle weeds have invaded 2 of our perennial beds. The past 2 years I have dug them up, trying to get at the roots, but they keep coming back, and they are very aggressive and fast growing. What can I do to get rid of them?

Montgomery County Maryland

1 Response

We assume you have Canadian thistle, which is designated a non-native invasive plant in Maryland--and for good reason. As you see, it is extremely hard to dig up this pest because it can regenerate from but a small piece of root.

Because your thistle is in an establish bed, you have fewer options (you can't smother it for years, for instance). You need to either remove all or some of your plants to save and then kill the entire bed, or use a systemic herbicide to kill each thistle while being extremely careful not to get the the herbicide on your desirable plants.

The systemic herbicide recommended for Canadian thistle is one containing glyphosate. You'll see glyphosate listed as an ingredient on the label. Systemic herbicides are transported down to the roots to kill the roots. This happens most efficiently in fall. Plan 2-3 applications 3-4 weeks apart when the weeds are actively growing. Apply with a very controlled spray or paint it on. If you spray, remember that glyphosate is a total vegetation killer and will kill any of your plants it contacts. You can fashion a shield of plastic or cardboard to protect your desirable plants.

Go to the Weeds Gone Wild website's factsheets more extensive information about controlling Canadian thistle. Never let it go to seed!