Thistles have been introduced in the 48 contingent states since the early 19th century. Originally from Eurasia, as far reaching as Mongolia and southward to North Africa, these plants have made their way to the United States most likely as stow-a-ways in ballast water of ships. There are two types of thistles, biennial and perennial. Biennial thistles have a life cycle that only lasts for two years and a perennial thistle’s life cycle lasting greater than two. Commonly occurring biennial thistles in Pope County are Plumeless, Bull and Musk thistle. Plumeless thistle, Asteraceae Carduus acanthoides, is the major noxious weed that has management priority. It is one of the shorter growing thistles with upright stalks typically between 1 to 4 feet in height. The dark green leaves are deeply lobed. The spines on the stem and on the edges of the leaves give it a winged appearance. Bull thistle grows to a height of 6 feet displaying alternate, deeply lobed and greatly toothed leaves with a large stiff spine at the leaf tip. The characteristic look is defined by downward pointing spines of the bull thistle. Musk thistle contains large reddish brown bracts under the flower head that resemble pine cones. The flower head droops at an angle up to 120° giving a nickname of Nodding thistle. Biennial thistles solely reproduce through seed production. They can produce 8,000-120,000 achene seeds depending on thistle type and favorable environment. Perennial thistles differ in reproduction methods because they reproduce through seeds and through the roots sprouts. Commonly occurring perennial thistles are Canada, Flodman, and Wavyleaf thistles. Canada thistle can flower from the bolting stage after 16 hours of a light period. Flodman and Wavyleaf thisltles are native to North America. One characteristic of Wavyleaf thistle is the gray appearance gained from its immensely hairy stems. The flower head of the Wavyleaf thistle also contains yellow spines whereas the Flodman thistle has defining leaves that are 90° lobed and are shiny green in color.
Control for both biennial and perennial thistles is essential. Prevention is key. Keeping pastures and road right-of-ways healthy with dense native stands is a good way to prevent invasion. If and when thistles begin to invade and displace native species cultural controls such as mowing, tillage, crop rotation and controlled burns can be implemented. Controlled burns will establish a more uniform cover to prepare the area for herbicide treatment. Biological controls, for example various varieties of weevils and gall flies, take many years to establish a population and have been found to be ineffective. Chemical control is most valuable at rosette stage or before fall frost when mature plants are vulnerable. Due to perennial thistle plants having an established reproductive root system, it is imperative to also treat or remove the roots. Therefore, control of biennial and perennial thistles cannot be carried out using the same method. It is always best to use an integrated management program when controlling invasive weeds. If you have any questions or concerns about thistle species and ways to manage them please contact either Jessica Hoheisel with Pope Soil and Water Conservation District at 320-634-5327, Barry Bouwmann with Pope Land and Resource Management at 320-634-7791, or Andy Albertson with Swift Soil and Water Conservation District at 320-843-2458.