‘We aggressively go after it’: Missouri biologists work to control invasive hydrilla plant | St. Louis Public Radio

‘We aggressively go after it’: Missouri biologists work to control invasive hydrilla plant

Jun 26, 2018

Shane Creasy stands on the edge of a lake and casts a plastic beaker full of thick white herbicide into the water.

The herbicide slowly fans out across the surface of the lake, as Creasy, a fisheries technician with the Missouri Department of Conservation, peels off his protective gloves. The target, an invasive aquatic plant known as hydrilla, is a tenacious adversary that takes years to eradicate.


Like other invasive plants, hydrilla is a vigorous competitor. By some estimates, its slender stems can grow about an inch a day, up to 30 feet in length.

“It has a very can-do attitude,” said Denise Otto, a fisheries management biologist with the MDC. “It only needs one percent sunlight, which means it photosynthesizes earlier in the day than all other plants, and it can grow in up to seven percent salinity.”

As it grows, hydrilla blankets the surface of ponds and lakes with a dense mat of greenery. Not only does it crowd out native plants and profoundly change ecosystems, it can also clog water intake systems and restrict water flow, causing flooding.

The plant, which is originally native to southeast Asia, first appeared in Missouri in 2012 in a private nine-acre lake in Greene County. Since then, it has popped up in four other counties across the state, including St. Louis County.

“Every call that I get from somebody who thinks they might have hydrilla, I personally go out and take a look at,” Otto said. “Not only is it important for us to find it, but once we find it, we will check streams and lakes in a five-mile radius surrounding where we know that we have it. We aggressively go after it whenever we find it.”

The MDC has been treating this particular lake in northeast Warren County with aquatic herbicide for the past five years. The state covers the cost of the herbicide, which will total roughly $10,800 for this five-acre lake.

When it comes to hydrilla, researchers say an ounce of prevention is worth a pound of cure.

“Preserving ecosystems from the devastating effects of invasive species such as hydrilla will always be more desirable and successful than efforts to restore ecosystems once they have been severely altered or destroyed,” Otto said. “Restoring damaged ecosystems is very expensive, labor intensive and complex.”

Hydrilla can grow up to an inch a day, rapidly blanketing the surface of lakes and ponds with a thick mat of greenery.
Credit Missouri Department of Conservation

Before the treatment started, landowner Brian Cohoon said almost half of the lake was covered in hydrilla.

“At least 20 to 25 feet out from the shore, all the way around, was just green,” Cohoon said. “We would just have this circle in the middle of the lake that was clear. It would cause issues with clogging up the overflow, so then the lake would actually try to run over the dam in a few areas.”

One of Cohoon’s relatives who works for the U.S. Army Corps of Engineers was the first to discover the hydrilla invasion. As an aquatic plant enthusiast, the relative had collected some of the plant from Cohoon’s lake and brought it back to grow in his aquarium.

“In his office, they had a meeting about this invasive species,” Cohoon said. “They’re showing some slides and pictures [of hydrilla], and he goes, ‘I have some of that in my aquarium!’”

The original source of the hydrilla remains a mystery, Cohoon said. Because hydrilla is listed as a federal noxious weed, it’s illegal to sell or plant it in the U.S.

Humans often move hydrilla from place to place accidentally, either as a hitchhiker on the undersides of boats or mixed in with native species purchased through the aquatic plant trade.

Because hydrilla can rapidly regenerate from small, potato-like root tubers in the sediment, the MDC will continue monitoring this lake even after the herbicide treatment is finished.

“Those [tubers] can remain viable for longer than ten years,” Otto said. “Just because we don’t necessarily see it doesn’t mean there’s not something lurking dormant in the sediment.”

Missouri residents can report possible hydrilla sightings to the Missouri Department of Conservation at hydrillareporting@mdc.mo.gov

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