Adapted from Jane Mangold and Hilary Parkinson, August Weed Post, Montana State University
There are five common exotic thistles (excluding Centaurea spp., both the starthistles and knapweeds) in the western U.S. that are problematic to some degree across a variety of habitats. In addition to exotic thistles, there about 160 native thistle species in North America, some of which can be difficult to distinguish from the troublesome exotics! Why is it important to distinguish exotic from native thistles? Exotic thistles can spread quickly, especially with disturbance, they have poor forage value, and their sharp spines can injure livestock and limit recreational activities. In contrast to exotics, native thistles are rarely or ever reported as invasive and play an important role in the ecosystem. For example, birds feed on thistle seed, and some birds time their nesting around thistle flowering because they use the downy seeds to line their nests. Bees, wasps, flies and beetles feed on thistle pollen and become food sources for other wildlife, and some native thistles are forage for deer and elk. Answer the following questions to distinguish five common exotic thistles from many native thistles.
1) Does the thistle have rhizomes?
Yes? It’s the exotic Canada thistle (Cirsium arvense). Heads are small and clustered, and there are no spiny wings on stem. Note: native Flodman’s thistle, Cirsium flodmanii, is a taprooted perennial, spreading by horizontal roots, which may appear rhizomatous. Heads are not densely clustered, but grow as one to two heads at stem tips.
No? Continue to question 2.
2) Does the thistle have spiny wings the entire length of the stem?
Yes? It’s one of four common exotic thistles. Review photo's above and key diagnostic features below.
- Bull Thistle (Cirsium vulgare). Narrow, needle-like bracts, leaves green on upper and lower surface.
- Musk thistle (Carduus nutans). Broad triangular bracts point outward or down, heads are often nodding. There may be an expanse immediately below the flower head without spiny wings, but you will see them lower on the stem.
- Plumeless thistle (Carduus acanthoides). Flower heads in clusters so each flower stalk is ≤0.75 inches, each head measures ≤1 inch in diameter
- Scotch Thistle (Onopordum acanthium). Foliage silver gray in appearance, plants may grow up to 12 feet tall, spiny wings especially prominent.