Big, Bad Thistles


When I spy a thistle in my yard—or in the open space next door—my first inclination is to annihilate it. Pull out the weed killer. Put on gloves and yank. Dig out the roots. Sure, some have imposing purple flowers, but I’ve learned that if you delay your war, the thistles will conquer. Then I started reading up on thistles.

It turns out that, yes, some thistles are malicious characters, with spreading roots, glove-piercing thorns, and prolific offspring. They’re nasty enough to earn themselves noxious weed status. Some are “merely” weeds—annoying but not striving for world domination. And some thistles are native wildflowers, just like columbines, lupine, and paintbrush. They provide food for wildlife (especially finches and various pollinators), enhance the landscape, and are not the least invasive. The question is, which is which?

Thistles are in the sunflower family, and are closely related to cornflowers (aka bachelor buttons). Thistles have sharp prickles on their leaves, stems, and sometimes even the flowers, making them hard to pull even when small. They typically have an involucre—a structure resembling a miniature pineapple that is found at the base of each blossom, as shown below. Artichokes are delicious thistles—we eat the flowers before they open. (There are a few plants with “thistle” in their common names that aren’t related; we won’t worry about those here.)

While Colorado has 20 native thistles, there are also a number of invasive species to watch for. Today I’ll focus on the two species that share the common name Scotch Thistle (Onopordum acanthium and O. tauricum, shown at top). As you can guess, Scotch thistles originated in Europe and western Asia. They migrated here as stowaways in the early 1900s, and have since spread throughout much of the U.S. They’re now on Colorado’s noxious weed “B” List, meaning they’re likely here to stay, but eradication is mandated to prevent them from spreading even further.

These can be huge plants. O. tauricum reaches 6 feet in height while O. acanthium can soar to twice that. (Those in our local field are much shorter, probably due to the current drought.)

Both species have a distinct mid-rib on their spiny leaves, which in the case of O. acanthium are covered with dense, gray, woolly hairs. Both species have stems edged by broad “wings” that end in spiny tips. From June to September, 1- to 3-inch, violet-red flowers appear along a central stem, attached by short stalks .

Scotch Thistles are biennials—they germinate and overwinter as a low-growing rosette (that can be 2 feet in diameter), then enlarge, bloom, and die the following summer. They’re prolific bloomers, with each plant producing 100 or more flowers and leaving a legacy of 14,000 seeds.

These and other invasive thistles often grow into dense thickets that livestock (and humans) can’t penetrate, destroying acres of rangeland. In our landscapes, they can defeat even the most determined gardener. As the seeds remain viable in the soil for up to 30 years, it’s best to attack them in their more vulnerable first season. If any plants survive to bloom, be sure to remove them before the seeds mature. At that point, be sure to bag them and toss them in the trash. If left lying around, even decapitated flowers can still bring their seeds to maturity.

Scotch Thistles have been the national emblem of Scotland for 700 years. I’m glad someone appreciates them, but we in North American wish they’d stayed there!

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