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American goldfinches relish the seeds of native and non-native thistles, sorting them one by one with their beaks and tongues. (Stephen Jones / Courtesy photo)
American goldfinches relish the seeds of native and non-native thistles, sorting them one by one with their beaks and tongues. (Stephen Jones / Courtesy photo)
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Thistles — those prickly plants goldfinches, bees, and butterflies love and most folks hate — are not all noxious weeds.

Lynn Reidel, grassland plant ecologist with the Open Space and Mountain Parks Department, says that well-meaning members of the public often dig up native thistles — usually along trails — thinking they are invasive, non-native thistles.

Butterflies and a bee on a wavy leaf thistle. (Bill May/Courtesy photo)

Thistle protection signs saying “Please don’t judge a plant by its spines” have been posted along several trails such as Lions Lair, Joder, Skunk Canyon, Homestead, Flatirons Vista/Doudy Draw, and South Mesa where damage to native thistles has been noted.

Of special concern are native wavy leaf thistle (Cirsium undulatum) and yellowspine thistle (C. ochrocentrum) that are being mistakenly destroyed when confused with non-native musk and Scotch thistles. Plant ecologist Pat Butler has written a guide available online from the Colorado Native Plant Society titled “Colorado Thistles: the good, the bad and the ugly” with photos to help distinguish the good from the bad. She writes that Colorado has 24 native thistles and half a dozen non-native invasives. Sharon Bokan wrote an article on managing thistles in the June 9 issue of The Camera and also advised resisting the urge to pull every prickly plant you see.

Even folks who know what they are doing, can run afoul of the law against destroying plants on Open Space land. A good friend of ours who is a plant expert was arrested and fined last year while pulling myrtle spurge, a scourge on the land and one of the most noxious of noxious weeds. She just got off probation and — with a permit now — has recruited a cadre of like-minded friends to help pull myrtle spurge.

Naturalist Steve Jones praises thistles, members of the sunflower family that provide a bounty of protein for seed-eating birds and nectar for pollinating insects. As for problem thistles he writes, “The real culprit, as usual, is us. In areas where humans have restored native vegetation, thistles cause fewer problems while sustaining some of our most colorful birds and insects.”

Ruth Carol and Glenn Cushman are the authors of “Boulder Hiking Trails,” published by West Margin Press.