MSU Extension Guide Helps Distinguish Native Thistles From Exotics

BOZEMAN – A new publication from Montana State University Extension is designed to help identify invasive exotic thistles and verify those that are native to Montana. People often think that the only good thistle is a dead thistle. While this can be true for well-known invasive exotic thistles like Canada or musk thistle, some thistles are Montana natives and better off alive than dead due to their beautiful flowers and foliage and the habitat they provide for wildlife. Five exotic thistles and ten native thistles grow in Montana. Why is it important to identify invasive exotic thistles and differentiate them from natives? Invasive exotic thistles can spread quickly, especially with disturbance, have poor forage value, and their sharp spines can limit recreational activities and injure livestock. From a weed management perspective, even differentiating among exotic thistles is important because a perennial rhizomatous thistle will require different control measures than taprooted annuals or biennials. Compared to exotic thistles, native thistles do not spread quickly with disturbance, are rarely or ever reported as invasive and are important for wildlife. For example, birds feed on thistle seed, and some may time their nesting around thistle flowering, using plumes on seeds to line nests. Bees, wasps, flies and beetles feed on thistle pollen and become food sources for other wildlife. For some large ungulates like elk, native thistles are a source of forage. The new publication can be used to verify whether a thistle is native or exotic before implementing thistle control strategies. The publication includes a tutorial of how to use a simplified key and a brief description of the important anatomical features and terms that are needed to successfully identify the thistle. Printed copies of the thistle publication are free and can be ordered from Montana State University Extension Publications at (406) 994-3273, or at http://store.msuextension.org/. Additionally, an electronic version can be downloaded from extension publications at http://store.msuextension.org/ and search for EB0221.