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A thistle is a type of flower with rough, spiny, leathery leaves and an erect, many-branched stem bearing flowers in umbels at the top.There are about a dozen purple-flowered spiny thistle species that occur in Oklahoma. Oklahoma’s Noxious Weed Law can raise concern among landowners if they do not know which thistles on their land they are required to control. The purpose of this publication is to describe the introduced thistles, selected common native thistles and provide information about them. There are other plants that are referred to as thistles, such as the Russian thistle (Salsola tragus), bull thistle or bullnettle (Cnidoscolus texanus) and spiny sowthistle (Sonchus asper), which are not included in this identification guide.
Native thistles seldom form troublesome populations to rangeland and cultivated crops because insects and diseases keep them in check. They also play an important role as a nectar source for hummingbirds, butterflies and other native pollinators as well as providing seeds for birds. Commonly encountered native species include wavyleaf (Cirsium undulatum), distributed statewide; yellowspine (Cirsium ochrocentrum), in western Oklahoma; tall thistle (Cirsium altissimum), in central and eastern Oklahoma; and yellow thistle (Cirsium horridulum), in southeastern Oklahoma. Four other native Cirsium species occur in Oklahoma, but are rarely encountered. Two other native plants commonly mistaken for thistles because of their purple flowers are Leavenworth’s eryngo (Eryngium leavenworthii) and American basketflower (Centaurea americana) although they are not closely related to thistles.
Musk thistle (Carduus nutans) invaded Oklahoma around 1960. Populations are fairly common in northeastern and central counties with some isolated problems statewide. It has large (2 inches to 3 inches), single, nodding flowers at the end of each branch with long triangular bracts beneath the petals. The stem has spiny wings. In 1991, several Extension educators began collecting and releasing two thistle-feeding weevils collected from established populations in Missouri. These weevils were originally introduced into the U.S. as biological control agents for musk thistle in the 1970’s. The musk thistle flower weevil (Rhinocyllus conicus) and the musk thistle rosette weevil (Trichosirocalus horridus) successfully decreased the severity of infestations in the northeastern Oklahoma counties. These two weevils have since been collected and released in other Oklahoma counties. Since their introduction, OSU Extension educators have redistributed more than 900,000 weevils throughout Oklahoma. (Source: extension.okstate.edu)