What is a good method for getting rid of Canada thistle?
Question:
I have wildflower gardens that wraps around my house. In one section, I am battling what I believe to be Canada thistle. Initially, I was cutting off the stems at the base and spraying the remains with 30% vinegar. However, because I have such a large patch of it I found that to be too time consuming. Lately, I have been cutting off the stems and removing the plants. I have noticed that even the areas where I sprayed vinegar, several new shoots have developed around the dead stalks. Is there a better method? Or something else I can use to destroy the plants? This is the most invasive plant that I’ve ever dealt with. Any suggestions would be greatly appreciated.
Answer:
Canada thistle (Cirsium arvense) can indeed be quite a headache in the home garden/landscape–it’s an aggressive perennial that spreads by seed and underground rhizome, and stores extensive resources in its root system in order to survive stressful conditions. Horticultural vinegar will not control it (in fact, the vinegar only kills plant tissue it comes into immediate contact with, so it works primarily on small, young, annual weeds), so I recommend discontinuing that approach.
The Univ of MN Extension has a nice page on the plant here, including suggestions for chemical control. It’s a persistent, resilient plant, so you’ll need to be dedicated to a longer term strategy. Keeping the plant stressed is one key component to exhausting its stored resources, so cutting it back and/or mowing is very helpful, and definitely do not let it flower and go to seed (if it does flower/seed, remove that specimen by hand as mowing will simply spread the seed around). This strategy can be effective over time, but you’ll have greater success combining it with one of the herbicide treatments mentioned in the link above. If the amount of thistle is too daunting for you to manage alone, consider contacting a ME-licensed pesticide applicator for help. As always when using chemical interventions (including horticultural vinegar), read carefully and follow conscientiously all instructions on the label.
Best of luck, and happy gardening.