Hi!

Please send photos of the area with the thistle problem so we can see how to best advise you.  You can attach photos directly to your reply. Are you specifically prevented from using glyphosate or all systemic herbicides? 

Thanks!

Jamie

Thank you for the photos. This will be a challenging, labor-intensive and/or lengthy removal process, especially given that there are desirable plants amid the thistle. Any systemic herbicide (which kills the roots) will injure any plant it contacts, so even if you do not use glyphosate, the risks to desirable plants remains. We don't have a particular active ingredient recommendation beyond glyphosate, which we understand you can't use, since it's typically the go-to option given its efficacy and relative non-toxicity and non-leachability once it enters the soil. A Penn State publication does list some alternatives to glyphosate, but note that all are hazardous to use near desirable trees (and likely shrubs as well). Pesticide manufacturers periodically alter their formulations or product labels, so the list of suitable options can change over time. Check any systemic herbicide product you consider to verify its label includes Canada Thistle and the type of area you're treating.

In comparison with systemics, organic herbicides are contact-only and will be no different in impact than clipping the stems off by hand at ground level; neither will affect the thistle's root system, which is extensive and tenacious. Resprouting will occur repeatedly from either method, and these new shoots must be removed promptly each time they appear in order to starve-out the root's energy stores. Smothering the planting won't work because of the desirable plants present, and even if you were to sacrifice them for the sake of thistle control, using a cover to deprive thistle of light tends not to work because it's too vigorous of a weed. (Not only can it easily keep creeping to the edge of the cover and resprout, but efforts to manage thistle this way haven't worked even after a year of smothering.)

You can dig the thistle out to reduce the population, but be aware that all this will do is temporarily remove top growth so any herbicide used can be applied in lesser quantity. Root fragments of Canada Thistle easily resprout, so in general, we don't suggest digging as a method of control unless you can dig up the entire colony and treat re-sprouts with systemic herbicide.

At a bare minimum, do not let these plants mature enough to bloom and go to seed. As it is, any seed already present in the soil can remain viable for well over a decade.

Since Canada Thistle is a noxious weed in Maryland, the MD Dept. of Ag. may be able to assist with its control. You can contact them regarding what exactly this entails, which might be working with a county-level noxious weed control program.

Miri