Thistle Control with Cultivation, Cropping and Chemicals

Abstract

Canada thistle used in this study is generally called Cirsium arvense (L.) Scop. (2, 5, 12) and the perennial sowthistle was a mixture of Sonchus arvensis L. and S. uliginosus Bieb. (2, 5). The two species of sowthistle are very similar. However, S. arvensis has long glandular-hispid peduncles and involucres covered with lead colored glandular phyllaries (5) and a chromosome complement of 54, while S. uliginosus has slenderly turbinate to cylindric pale involucres and glaborous white-margined phyllaries (5) and a chromosome complement of 36. All 3 species are perennials that spread by means of horizontal roots and seeds. They have umbellate heads. The flowers of Canada thistle are purple and those of sowthistle are yellow. All produce copious amounts of pappus. Seeds attached to the pappus are disseminated over large areas by air currents. One or more varieties of Canada thistles can be found in most of the northern states of the United States and the southern parts of most of the Canadian provinces (5). Heavy infestations are found in Ohio, Illinois, Indiana, Michigan, Wisconsin, Minnesota, Iowa, North Dakota and South Dakota, while lighter infestations are found in Nebraska and Missouri (9), and it is more common than any other weed in Montana, Idaho, Oregon and Washington (7). Perennial sowthistle is, in general, common to the Dakotas, Minnesota, Wisconsin and northern Iowa and Illinois (9). Although S. arvensis is generally thought to be the more common species, S. uhliginosus is more common in North Dakota (11) and western Canada (6). Canada thistle is dioecious and sowthistle is monoecious, but both are cross pollinated (2). Single plants propagate by means of roots and seeds and develop into patches. Seeds are also scattered, which gives a sprinkling of plants throughout the landscape. The density of the infestation is related to the distance from the parent patch, which creates a different problem than that associated with leafy spurge, Russian knapweed, hoary cress or field bindweed. Although the parent patches (weed nurseries) are similar to infestations with the other weeds, many acres are infested with sprinklings of thistles.