Get Wild: Thistles — the good, bad and ugly |

Get Wild: Thistles — the good, bad and ugly

Jim Alexander
Get Wild



A thistle flower seeds at Dickeys Landing in July.
Photo by John Taylor

This time of year, some bad thistles are in full bloom around Summit County. They’re ready to spew thousands of seeds to continue their spread. These thistles, musk and plumeless thistles, are invasive, noxious weeds that take over entire fields and render the land useless to native wildlife and plants.

Musk thistle is the lead of these invaders: It grows tall and can have dozens of bright, purple seed heads on each plant. The pretty seed heads can produce hundreds of seeds, so a large plant might produce as many as 20,000 seeds. This time of year, you’ll begin to find the flowers producing white fluff, and the winds will begin to carry these seeds to new locations where they will settle and grow.

These seeds are not just a problem for next year. Some will settle into disturbed ground and grow new plants in the spring, but others will lie dormant and may sprout anytime over the next 10 years. As a result, musk thistle infestations develop a seed bank, or an area where the thistles will grow and spread for year upon year if they are not stopped. So it is important to limit the spread of these seeds. The seed heads can be pulled and bagged, and the rosettes can be pulled out of the ground to prevent growth next year.

Plumeless thistle is a cousin to musk. It has a similar purple head and a huge prickly stalk. Like musk thistle, plumeless thistles can take over a field and ruin it for our native wildlife and plants.

But there are other thistles in Summit County that should not be disturbed. Canada thistle is another invasive, noxious weed. This pest is smaller than the plumeless and musk thistles. It has little purple flowers that distribute seeds. It is important to limit their distributions, but it’s not good to pull those plants. Canada thistle is a perennial, and pulling it makes its roots grow faster. So Canada thistle needs professional treatment.

The other thistles to leave alone are native thistles. We have several native thistles that we should keep and encourage in Summit County. These are generally smaller and less attractive than the invasive thistles. Native thistles generally have smaller flowers, which tend to be white or gray, and they have brown stems. It is imperative to leave those alone.

A patch of thistles is pictured last week near Cherry Creek Reservoir.
Photo by Jim Alexander

The Summit County Weed Program, U.S. Forest Service and groups like the Eagle Summit Wilderness Alliance’s WeedSpotters work year after year to limit the spread of noxious thistles by treating the existing infestations. Small to moderate infestations can be controlled by pulling off the seed heads, putting them in a sealed plastic bag and sending them to the landfill. Larger infestations are treated by professional exterminators who use chemical and/or biological agents.

You can help limit the spread of these, too. A simple way is to stay away from the weeds. If you walk through the weeds or let your dog romp in the weeds, the seeds can be spread. They might stay on your shoes or your dog’s fur, and then you’ll later deposit the seeds in new locations to grow next year.

You can also join a group like the WeedSpotters and learn to locate, record and manage weed infestations. Let us know if you’re ready to join the fight by emailing

Jim Alexander

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