Thistles. They’re thorny and they’re tough, but they’re not all bad. In fact, of the nine species of Cirsium found in Missouri, there are several native species that occur on prairies and other habitats. They are rich food sources for birds, butterflies, and moths in late summer to fall and provide nesting materials for birds and small mammals.
Native species include field thistle (Cirsium discolor), which can be abundant in open woodlands or savannas undergoing restoration, and in prairie plantings, but is not likely to persist in high numbers. Another common native species is tall thistle (Cirsium altissimum). A native inhabitant of prairies, it seldom grows in great abundance. In summer, it provides a nectar source for butterflies and later, seeds for visiting goldfinches and other birds. Thistledown, the fine hairs (or pappus) attached to the mature seeds of this and other thistles, is used by a variety of birds and small mammals for lining nests.
Non-native, invasive thistles include spotted knapweed (Centaurea stoebe
ssp. micranthus), musk thistle (Carduus nutans), Canada thistle (Cirsium arvense), bull thistle (Cirsium vulgare), and Scotch thistle (Onopordum acanthium ssp. acanthium), with the first two species being the most widespread in the state. See these and other noxious weeds on the Missouri Noxious Weed list. Treatment of non-native thistles is recommended so the plant doesn’t set seed and spread. If spot treatment is not possible, you can cut down the flower stalks to prevent seed development.
This article on thistles from the Missouri Prairie Journal provides detailed drawings and a key to help you identify thistles. To identify native vs. non-native thistles, an easy rule of thumb to follow is to check the underside of the leaves. If the underside is whitish, the thistle is native; if the underside is green, it is non-native.*
See a video of butterflies visiting tall thistle at MPF’s Carver Prairie and read more in our thistle blog post.
*an exception to the “green underside = non-native thistle” rule is the native swamp thistle (Cirsium muticum), which grows in uncommon, small wetland areas in the Ozarks called fens. It is native, the underside of its leaves are green, and it is known from only seven counties in Missouri. It is a host plant for the swamp metalmark butterfly, as is Cirsium altissimum.
Photos above of bumble bee on native thistle and whitish underside of native thistle leaf by Hayley Howard. Photo of cloudless sulphur butterfly on a native thistle by Bruce Schuette. Photo of goldfinch nest with thistledown by Frank Oberle.