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Highly invasive aquatic weed discovered in the Animas Valley

Concerns raised about Eurasian watermilfoil spilling into Animas River, Lake Nighthorse
Rod Cook, owner of Cook’s Land Services, said it may take multiple treatments to completely eradicate an invasive aquatic plant found in the Animas Valley.

A highly invasive aquatic weed has been discovered in a number of ponds in the Animas Valley, putting the heat on to eradicate it before it enters the Animas River and, potentially, Lake Nighthorse.

Ben Bain, La Plata County’s weed control manager, said he was alerted last fall that several ponds north of Durango possibly had what’s known as Eurasian watermilfoil, instantly becoming a high priority for his department.

Because there are certain species of native watermilfoil, Bain sent samples to a lab to confirm it was the Eurasian strain, which came back positive. Not long after, he made plans to start eradicating it.

“We’re in a situation where we want to get rid of it,” he said. “Because once it’s in a large body of water, it’s pretty much impossible to get rid of.”

While common on the Front Range, this was the first confirmed presence of Eurasian watermilfoil in La Plata County.

Eurasian watermilfoil can outcompete and outgrow native plants, and wreak havoc on water infrastructure. Weed managers are working quickly to remove it from ponds north of Durango.

Eurasian watermilfoil is native to northern Europe and Asia, but made its way to the U.S., likely though watercraft, in the early 1900s. Now, it is a constant concern for getting into waterways, because once it does, it quickly takes over.

The invasive weed aggressively outgrows and outcompetes native aquatic plants. It can then clog irrigation systems and ponds, and even push out native species of fish and other aquatic life.

Russ Howard, manager of the Animas-La Plata Operations and Maintenance Association, which manages Lake Nighthorse, said a potential Eurasian watermilfoil infestation poses a significant risk to the reservoir.

Eurasian watermilfoil was first brought to the U.S. in the early 1900s, and has since spread all over the country. It was found in La Plata County for the first time last fall.

“It grows so uncontrollably, it has the ability to clog our (infrastructure),” Howard said. “The best thing to do is get rid of it, but as you try to eradicate it, you just break it up into fragments, which can start a whole new plant. That’s what makes it so invasive.”

A grant for $6,000 was secured to help fight the Eurasian watermilfoil, Bain said. So far, four ponds have been confirmed to have the invasive weed, and another two are awaiting confirmation results from a lab.

It is nearly impossible to say how the Eurasian watermilfoil arrived in Southwest Colorado, Bain said, but typically, it spreads by people dumping aquariums into natural waterways.

“It’s a common aquarium plant, and it grows easily,” he said.

Rod Cook, who worked for the county’s weed management department for nearly 25 years before retiring in 2017 and starting a private business, Cook’s Land Services, took the lead in the fight against the aquatic pest, conducting the first treatments this summer.

Cook said he has treated four ponds so far with an aquatic herbicide to remove the Eurasian watermilfoil, but the success of the effort won’t truly be known until next growing season to see if the plant greens up.

“This is our first time ever dealing with the plant, so we don’t know yet,” he said. “Usually, though, you don’t ever get rid of something with just one treatment. It most likely will take multiple treatments.”

Cook said water in the four ponds that were treated is used to irrigate hayfields, so there is less of a risk of draining into the Animas River. But the two other ponds awaiting test results are closer to the river.

“You just don’t want that weed in the Animas or in Lake Nighthorse,” he said.

Weed managers are working fast to eradicate the invasive Eurasian watermilfoil before it gets into the Animas River, and possibly into Lake Nighthorse.

Bain said some of the ponds are within a couple hundred feet of the Animas River, and even if the water doesn’t directly drain into the river, the proximity alone is a concern.

“But we are optimistic,” he said. “We’re hoping to get a successful treatment this year, and expect the plants to be taken care of.”

Howard said that luckily, there’s a construction project on Lake Nighthorse’s intake structure, so there’s no active pumping into the reservoir, which could bring in the Eurasian watermilfoil.

“Ideally, they’ll have it resolved (by next spring), so when we pump in May and June, we won’t pump fragments of (the weed) into Lake Nighthorse,” Howard said.

Bain said it is likely there could be more ponds with Eurasian watermilfoil present, and he encouraged landowners who suspect it to reach out to his office.


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