For April. So we’re a little behind, but because we go to press the month before our issue date … well, let’s get to it. Thistle’s a perennial problem, after all, and not just in Minnesota.
A significant part of thistle’s preferred territory is rangeland or pasture, but both Cirsium arvense (Canada thistle) and Carduus acanthoides (plumeless thistle) are highly invasive and can threaten streambanks, roadsides—and established landscapes, if vigilance and maintenance aren’t practiced.
Both bear pink to purple flowers atop their stems, and the flowers mature into tufted seeds that are easily dispersed by wind. While Canada thistle is a perennial, plumeless thistle is a biennial. Both are thugs. Canada thistle forms immense colonies, with roots that can grow 10 to 12 feet per year. It often bears multiple stems that appear smoother than those of plumeless thistle. Plumeless thistle, on the other hand, grows spiny bracts directly beneath its flower.
Management can be frustrating, but preventing seed from maturing is key. For plumeless thistle, mowing at the flower bud stage can prevent seed production, but this step needs to be repeated several times during the growing season. Canada thistle also can be mowed, but this, too, must be performed continually throughout the season. Fall and spring foliar herbicide applications can be effective.
Note that not all thistles are non-native, nor invasive. Many native thistles provide sustenance for wildlife, as well as act as pollinators. The lesson here? Know your thistles.