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Researchers Test Fungus To Treat Weeds, Protect Orchids At Filoha Meadows

Sep 18, 2018

Denise Wilson and Rea Orthner paint herbicide on thistles in Filoha Meadows Nature Preserve.
Credit Elizabeth Stewart-Severy / Aspen Public Radio

Filoha Meadows Nature Preserve is tucked between steep mountain cliffs and the Crystal River; the open space is a flat expanse of wetlands. On a recent brisk fall morning, plant ecologists Rea Orthner and Denise Wilson lead the way across the swampy meadow. We’re here because of the stream orchid, which blooms in penny-sized flowers in mid-summer.


"It's very colorful. It has magenta and some white and some pink and some yellow,” Wilson said.

Wilson is an expert on stream orchids, which are pretty secure on a global level, but very rare in Colorado, in part because they depend on wetlands.


“It has to have wet feet," Wilson said. "It can't survive in a place where it's not always wet.”

Many of Colorado’s wetlands freeze in the winter, but Filoha Meadows sits right next to Penny Hot Springs. The water comes out of the ground at 130 degrees, keeping soil warm enough for the stream orchid to survive the winter.

Last year, Rea Orthner noticed that orchids are not the only plants thriving in the wetland. The Canada thistle, a notorious noxious weed, grows in tall, thick patches and covers a lot of ground.   

“Noxious weeds can outcompete plants for moisture, sunlight, have root competition, nutrients," Orthner said. "The question popped into my head, 'Should we be doing something to control noxious weeds in Filoha Meadows to protect these rare plants?'”

It’s a real challenge to treat weeds in wetlands: You have to be careful that herbicides affect the native plants or don’t contaminate the water. Many herbicides are not permitted near water at all.

"It's a very sensitive area with the orchids, and we just want to be super duper careful," said Gary Tennenbaum, director of Pitkin County Open Space and Trails, which owns and manages Filoha Meadows.


The county hired Orthner to find the most environmentally sustainable way to remove the Canada thistle. Orthner designed an experiment to test three ways to get the thistle under control: Pulling them by hand, applying herbicides and using a biological control — that is, introducing a natural enemy of the thistle.

Rea Orthner measures an experimental plot in Filoha Meadows.
Credit Elizabeth Stewart-Severy / Aspen Public Radio

Orthner has divided the wetlands into segments that are each seeing one of these treatments. They all have their challenges, she explained as she put on gloves and pulled a thistle out of the earth.


“This is part of the underground stem," Orthner flipped the weed upside down and pointed to the roots. "See how it’s just broken off there?”

Pulling thistles by hand is not highly effective; it’s impossible to get the entire underground root complex. Each pull depletes the roots, but it takes years to fully destroy a thistle. Plus it would take an army of volunteers to pull out all thistles in this meadow, and that’s not ideal, either.

“The last thing you want to do with a rare plant is endanger it further by trampling and compaction of soils, especially orchids,” Wilson said.

Orthner and Wilson spent an entire day pulling out thistles from two experimental plots, and another applying herbicide to two more. It's slow work; they use kid-sized paint brushes and small buckets to carefully dab the chemical on each thistle. They aren’t spraying it, like you would in most places.

“This herbicide could kill this orchid,” Orthner explained.

So it’s tedious — and potentially hazardous.

But there’s a third variable in this study.


Karen Rosen is a biological control specialist for the Colorado Department of Agriculture. She works with puccinia punctiformis, a rust fungus that’s a natural enemy of the Canada thistle, but doesn’t impact any other plants.


“It cannot live without Canada thistle; they're co-evolved," Rosen said.

The rust fungus gets into the root system of the thistle and kills the stem. It’s been used in Colorado since 2013, and its effectiveness depends on the site. It works best in high elevation riparian areas, just like Filoha Meadows.


"It just seems like an ideal spot, and with those endangered orchids, you know, there really is not a lot of options,” Rosen said.

Next spring, researchers will begin measuring the effectiveness of each treatment. Orthner plans to count all the stems of both the stream orchid and Canada thistle, and compare that to this year’s numbers. But it could take several years to see what gives the rare pink flowers the best chance to beat out the thistle.