Throughout the winter, we kept the bird feeder full of seed. We had a seed mix for the larger birds and another for the finch feeder. The finch mix was said to be niger thistle seed. So when thistly plants promising yellow flowers popped up at the base of the bird feeder, our first guess was thistles. Wrong. It turned out to be safflower, and although they have spines on them, safflowers are not thistles. Neither is niger for that matter. Niger (Guizotia abyssinica) is an African oil seed that’s imported and sterilized by subjecting it and the weed seeds that come with it to 250-degree steam. This makes it safe to feed without introducing foreign weed seeds—theoretically anyhow. Apparently some of our birdseed was unsterilized safflower.
But now I’m thinking about thistles. In our prairie garden we have two six-foot-plus-tall thistles growing that look a lot like the bull thistle (AKA common thistle or spear thistle) used as the symbol of Scotland. Yikes, you say. Kill them! Kill them! They’re classified as noxious weeds! But wait—what kind of thistle are they really? Because although the bull thistle is an invasive from Scotland and looks a lot like this one, Iowa has six native thistles of its own. Ours turned out to be field thistles (Cirsium discolor). As native plants, they may have come in our prairie garden seed mix.
Thistles are in the same family as sunflowers. Field thistles are biennial plants that spend their first year as a rosette of leaves near the ground, and the second with a tall stalk that branches near the top and produces spikes of pink to purple flowers in late summer. The flower heads are very prickly, with spines that stick straight out. Native bees and butterflies love thistles because of their deep wells of nectar inside the flowers. The name “discolor” refers to the leaves. They are long and deeply lobed, and look white on the underside due to a profusion of tiny hairs. They’re dark green on the upper surface, and they are spiny on the edges. When the seeds mature in late summer, the goldfinches will be along to hang on the thistle and eat the seeds. What the goldfinches don’t get will ride the wind on thistledown to new locations.
There’s another, less benign thistle patch on Owl Acres. It’s kept down by mowing it off, but it doesn’t go away. It’s the dreaded Canada thistle (Cirsium arvense).
This noxious weed, also known as creeping thistle, was accidently brought to North America in the 1600s probably in grain shipments. Everyone quickly realized that it would spread inexorably, crowd out desirable plants, and be nearly impossible to get rid of. The New Englanders labeled it Canada thistle, thinking that French traders from Canada were responsible for introducing it. The Canadians point out that it appeared in Canada and New England at about the same time, so it wasn’t their fault. No matter its origin, everybody agrees that it came from Europe, and that it’s here to stay.
Low-growing, wavy, spiny leaves of a creeping thistle, growing in the yard in front of the barn. It gets mowed and isn’t allowed to go to seed. Photo by Author.
Since those early days, creeping thistle has spread throughout all of Canada and the United States. It likes open areas and disturbed soils such as pastures, cultivated fields, logged forests, riverbanks and roadsides. It spreads by flinging up to 1500 seeds per plant into the wind, each one afloat on its thistledown. If that’s not enough, and apparently it isn’t, once a creeping thistle gets established, it creeps. It sends roots out in all directions, with shoots that emerge in spring and grow through the summer into seed-bearing extensions of the mother plant. Left to prosper, the original plant can turn into a whole colony of prickly invaders.
Farmers hate them because they degrade pastures and can colonize fields. They’re very hard to control. Our little patch over by the barn is kept down by mowing, but it persists, waiting, I’m sure, for a moment of negligence on our part when it will spring up to its full height of six feet and produce more glossy, spiny leaves and prickly purple blooms.
Goldfinches love all kinds of thistle seeds, and coordinate their nesting cycles so that the babies will hatch at the peak of thistle seed production. Bees, butterflies and other pollinators love thistles for their generous nectar and pollen, and some gardeners appreciate the thistle blooms in late summer. The state of Iowa, however, has labeled them all as noxious weeds and requires landowners to control them. The six native species are not aggressive invaders and do not harm local landscapes. As of 2019, Iowa has taken a somewhat more lenient view of native thistles although they’re still listed as noxious weeds.
We’ll keep mowing off our creeping thistles, but I think we’ll leave the native ones in our prairie garden alone.
Photo by Author. Alt text: Karen is dwarfed by a tall field thistle, which was allowed to grow in the prairie garden this year because the flowers are gorgeous and it’s a heavy nectar producer.