How do you kill Canada thistle?

Answer 1

Canada thistle [Cirsium arvense (L.) Scop.] has attractive purple flowers and sharp, dark green leaves. It easily is confused with native thistles. But it's distinguished by its aggressively fast spread horizontally and vertically through an area, at the expense of almost everything else. For it can reach heights of five feet. And it can alienate all other growth by its chemical releases that are toxic to plants other than its own kind. The plant needs female and male flowers, to produce seed that germinates. But the flowers aren't found on one plant. So viable seed production can be controlled if an area ends up with only female, or only male, plants. It may be difficult to make all the necessary identifications. And so a specialist tends to be needed to tell the difference between the female and male plants, and between the exotic form and the natives. But Canada thistle also grows by its strong root system. For the roots may grow up to 18 inches in one growing season. And through adventitious roots, fresh, new, young shoots burst through the soil surface every 3-6 inches. Culturally, Canada thistle can't handle waterlogged soils. So overwatering may help kill it. Conserve on water by watering with collected rain water, or recycled household grey water. Neither can Canada thistle handle shade. For it prefers an open site with full sun. So crowding and shading may help kill it. Generally, any two out of the three-pronged strategy of repeated cutting, burning, or brushing with Glysophate will wear down invasives and weeds. But the burning and the mowing of Canada thistle must be timed to starve the root starch reserves, and to keep the plant from setting seed. And Glysophate tends to be most effective when brushed onto the stems instead of the leaves. Heavy-handed applications of salt will kill Canada thistle. But they also will kill everything else in the area too.