Canadian thistle in Pennsylvania

Question:
I have a patch of Canadian Thistle (Cirsium arvense) that I want to eradicate. I was told that changing the soil pH might help. Is that true? What is my best option to control this weed?

Answer:
Canadian thistle (Cirsium arvense) is a cold-hardy perennial weed with a range that covers the entire northern US. It spreads by seed as well as aggressive underground rhizomes that can spread outward at least 15 to 20 feet. One reference even stated that thistle has allelopathic chemicals in its root tissue that helps to fend off other vegetation from establishing in its place. From book and internet descriptions Canadian thistle is tolerant of a wider range of soil types so I do not see how changing soil pH would be a strong enough deterrent to prevent the weed from continuing to spread. Not to mention if you make the soil too acidic (below 5.5) or too alkaline (above 7) then you run the risk of reducing all nutrient availability in the soil to the point where nothing will grow except the most noxious and determined of weeds.

For best control of Canadian thistle a combination of mechanical and chemical controls is the best option. Horticulturists and agriculturists alike are looking into natural biological controls that go after Canadian thistle, but most studies still appear to be in trial stages. Mechanically what you want to try and achieve is destruction of the carbohydrate storage tissue in the plants. Again, this type of thistle is most aggressive because of its long, carbohydrate-storing rhizomes. If the thistle is in an area that can be mowed, mow every three weeks to repeatedly cut back and weaken the plant’s tissues. Farmers usually keep livestock grazing in fields of thistle to achieve the same outcome. Canadian thistle typically flowers from late June through August, so do your best to prevent it from flowering and setting seed. Carbohydrate stores in the plant are at their lowest as the plant begins to flower, so June is a good time to enforce strong mechanical control. Even though not my favorite suggestion by any means, chemicals can be used in addition to mechanical control to really combat the aggressive nature of this plant. Foliar herbicides can be applied during the spring or fall, but most suggest that fall application works better. A systemic herbicide works to kill both the photosynthetic parts of the plant, leaves and stems, as well as the tough rhizome tissue underground. Slowly weakening the production of spreading rhizomes as well as deterring the plant from spreading seed, I believe you can begin to combat the thistles aggressive nature and hopefully get it under control after a few years.