How you can limit the spread of invasive weeds in Summit County

Scotch thistle is one of plants considered a noxious weed, which is invasive and harmful to local plant life.
Colorado Weed Management Association/Courtesy photo

As the weather warms and Summit County’s greenery comes back to life, so do harmful plants, and local officials are asking the public to remain informed of invasive species of weeds that can harm local wildlife.

Currently, the state of Colorado has identified dozens of plants as “noxious weeds,” or plants that out-compete natives for light, space and nutrients. Ryan Cook, an invasive plant species manager for Summit County, said that in Summit County many of them are commonly found along trails or on private property. Some of the most common ones include Canadian thistles, musk thistles, mullein an ox eye daisies.

“They all come from different places. For instance, we have Chinese clematis. Obviously it originally came from China,” Cook said. “There’s also Canadian sisal from Canada and Russian knapweed. These plants came from other nations and were introduced into Colorado, and they basically don’t have any natural enemy in the Rocky Mountains. They choke out our native wildlife for water and can sometimes make some of our native flowers go extinct because they’re so aggressive, and they don’t have anything to keep them in check.”

Noxious weeds are categorized into three lists based on how aggressive they are: List A, List B and List C. Cook said List A weeds are the most aggressive, and the state of Colorado has put 25 types of weeds into List A. In Summit County, there are two types listed in this category, myrtle spurge and orange hawkweed.

“If someone sees, on a hiking trail or private property or something, an A-list weed, the weed department wants to know about it immediately,” Cook said. “We need to get that out of the county, like pronto.”

For B- and C- List plants, Cook said that those should also be eradicated, but they’re lower on the priority list. Those include plants such as thistles, false chamomile and ox-eye daisies, which are also pretty common on local trails. Because the county brings in so many tourists from across the nation and abroad, the county is considerably more vulnerable to various types of noxious weeds being brought in. Cook said it’s common for seeds to come in from cars, animals or even attached to clothing. When those tourists recreate on trails and other public lands, the seeds go with them.

Despite this, Cook said hikers can still actively help Summit County’s weed issues.

“If you’re going to pick it, that’s great,” he said. “We love the help of picking it, but if you just lay it down on the side of the trail, and it’s already flowered, then those seeds are just going to blow around anyway. Really, all you’ve done is killed that one plant. But some of these plants have — I’m not kidding —20,000 seeds per plant. Those blow around or get in the waterways and travel around that way or on animals or whatever, and just spread all over the place.”

Unfortunately, he said, a lot of noxious weeds in the county look like harmless wildflowers. In the past, local municipalities would even plant certain noxious weeds in landscaping projects, which Cook said didn’t help, and he added that many personal gardens in Summit County could unknowingly have noxious weeds that are mistaken as just flowers.

“It’s funny — many of them are pretty,” Cook said. “I wouldn’t say that thistles are. Those get kind of mean, but a lot of these that are on the list produce a pretty little violet flower, orange flower or whatever, but once you know what they are, it’s like, ‘Oh my gosh, this is not good.'”

This year, the department will not offer its Backpack Donor Program, which traditionally allowed the general public to borrow herbicide to use on private property, due to a liability issue that suspended the program. However, the department’s cost-sharing program will continue.

Members of the Forest Health Task Force also discussed noxious weeds at its monthly meeting on April 20. John Taylor, who has served on the Colorado Department of Agriculture’s Noxious Weed Advisory Committee and won an award last October for his work to eradicate noxious weeds locally, said at that meeting that he has spent a significant amount of time removing musk thistles.

“I spent over 90 hours on the (Dillon Reservoir) peninsula last year, and most of that was cutting musk thistles at the base, clipping off the heads, and picking up the seeds before they germinate,” Taylor said. “The only thing that I would like to bring up is that they did not start in the wilderness. It did not start in the areas that people haven’t been to. They start down here with us, and they work their way up. The thing that’s important is they’re within the town area, and those weeds are thick. When we start a trail, we’ve got to make sure we get the weeds out of there first.”

Residents are encouraged to visit the county’s weed department website, at, to help familiarize themselves with noxious weeds that are found in the area.

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