Getting to know the different thistles
Thistles are stout plants that, in addition to their telltale spiny leaves, produce distinctive round, pinkish-purple, pincushion-like flowers. These emerge in summer from pineapple-like buds at the tips of the stems.
Unfortunately, there’s no single trait that separates a native thistle from an invasive, non-native one. The only way to tell what thistle you have is to consult a weed-identification field guide or search online for the differences in leaf shape, plant size, flower makeup, and other traits that set each thistle apart from one another.
One difference among the four worst weedy thistles is that three of them are biennials (plants that grow from seed the first year, then flower and die in the second) and one is a winter-hardy perennial.
Bull, musk, and Scotch thistles are all biennials, which form low rosettes of prickly leaves during their first summer and fall. Those rosettes overwinter, then send up flower stalks that typically reach three to four feet tall – even up to 10 feet for the particularly spiny Scotch thistle.
The flowers produce seeds that are spread by wind to create new plants the following year. Some thistle plants can produce more than 100,000 seeds per plant.
Canada thistle is a true perennial that dies back in winter but comes back with a vengeance the following spring. This one is especially aggressive because, in addition to producing seeds, its roots colonize the surrounding soil. Creeping horizontal roots that can survive winter underground send up shoots that quickly form dense patches.