Invasive Weeds Are Spoiling Lake Tahoe

Published December 1, 2005

The world-famous clarity of Lake Tahoe in Nevada is under assault from invasive Eurasian milfoil weeds. The aggressive weed is choking out native plants and fish and is threatening to create an algae epidemic in previously pristine waters.

The answer, say researchers and local residents, is modern aquatic herbicides that have proven environmentally safe and effective at removing milfoil in other regions of the country.

Tahoe Keys Overrun

The Tahoe Keys, a system of inlets at the south end of Lake Tahoe, are nearly 100 percent infested with milfoil, according to the September 15 Tahoe Daily Tribune, and the weed is spreading.

“Phalanxes of a tall, dark green weed called Eurasian water milfoil are choking off oxygen to fish, encouraging algae growth, and smothering native aquatic plants in at least 16 locations in Lake Tahoe, including Emerald Bay and the Truckee River,” the Daily Tribune reported.

The weed takes root on the lake bottom at depths of up to 20 feet, sending long, entangling tendrils to the surface. Unchecked, the weed completely takes over the infested area and squeezes out virtually all other aquatic life. In addition to being unsightly and deadly to aquatic life, it has also proven deadly to human swimmers who have unwittingly been entangled in the weed.

Herbicides Safe, Effective

The answer, according to scientific analyses, is treatment with advanced aquatic herbicides.

Charles Goldman, a Lake Tahoe researcher who has studied the lake for 46 years, explained to the Daily Tribune that modern herbicides eradicate milfoil while leaving native plant and animal species unaffected. Moreover, the chemical breaks down quickly after eliminating milfoil, leaving lake waters pristine. The benefits of such herbicides far outweigh any asserted risks, Goldman said.

Local residents believe herbicides are worth a try.

Dick Horton, who owns a marina on Lake Tahoe, told the Daily Tribune, “I’m happy to look at anything that might work. It’s about as bad as it can get.”

Other means of attacking the milfoil, such as mechanical harvesting, are not only more expensive but also counterproductive, observed Holly Crosson, a University of California at Davis scientist who managed Vermont’s milfoil control program for nearly 20 years.

“It’s a known fact that mechanical harvesting spreads the plant,” Crosson told the Daily Tribune.

Nature Conservancy Supports Herbicides

In Virginia, environmentalist groups are joining forces with state officials to employ aquatic herbicides to fight back against destructive invasive plant species.

The Nature Conservancy and the Virginia Department of Conservation and Recreation are spraying Habitat, a newly developed herbicide, in ponds, lakes, and marshes throughout the state to counter an aggressive invasive reed that is choking out native plants and aquatic wildlife.

“The reed is a nuisance,” explained the September 16 Richmond Times-Dispatch, “because it crowds out other marsh grasses and native plants that migratory birds favor as rest spots and food sources. It is classified as an invasive species by the state, blamed for hurting biodiversity and bird abundance.”

Early results of herbicide use on the problem plants are promising. “Generally, it’s retreating where we treat,” reported Curtis Hutto, natural areas steward for the Virginia Department of Conservation and Recreation, to the Times-Dispatch.

The Nature Conservancy’s Joe Scalf, who is coordinating the environmentalist group’s application of aquatic herbicides in Virginia, told the Times-Dispatch that on Virginia’s Eastern Shore alone, the Conservancy plans to treat about 350 acres of wetlands to eliminate the invasive reed.

Minnesota Considers Treatment

After years of failing to control milfoil through alternative means, the Minnesota Department of Natural Resources is considering the use of aquatic herbicides in Lake Minnetonka.

Spreading rapidly since first being spotted in Lake Minnetonka in 1987, the milfoil persistently “frustrates boaters and swimmers, crowds out native plants and damages the lake’s ecosystem,” reported the September 7 Minneapolis Star Tribune.

The weed “catches fish that die, and trash, and it gets kind of nasty,” said Marcia Holmberg, natural resources coordinator for the Minneapolis Parks and Recreation Board.

Efforts to control the weed through human and mechanical removal have largely failed. As a result, local residents are looking into aquatic herbicides.

“Given the fact that milfoil continues to spread, I understand the idea they want us to be more aggressive,” Dave Wright, manager of the Minnesota Department of Natural Resources, told the Associated Press.

Congressman Jim Ramstad (R) helped organize a community information meeting on August 25. The consensus of the participants was that current efforts to contain milfoil are not working and that producing better results must be a top priority. Lake Minnetonka Conservation District board members would like to begin treating portions of the lake with aquatic herbicides.

Washington Experience Promising

Herbicide treatment to eradicate aquatic weeds will be highly successful in Minnesota, Virginia, and Tahoe if a recent treatment program in Olympia, Washington is indicative of likely success elsewhere.

Olympia’s Capitol Lake was overrun by milfoil before government officials decided to treat it with herbicide. In the beginning, “It was a highly controversial exercise here, and it stirred up a significant level of citizen interest,” Nathaniel Jones, a planner with the state Department of General Administration, said.

“Technically speaking, it was tremendously successful in eradicating milfoil,” Jones reported. “We were hoping for and expecting a 90-95 percent kill rate. To our pleasure, we had a kill rate of 98-99 percent. We are not aware of any negative side effects. Plus, we went to some fairly extensive means to ensure the herbicide did not escape into nearby marine environments.

“While we do not know if the original strident opponents have changed their way of thinking, we do know that the community as a whole is very pleased that the noxious weeds are gone,” Jones said.

James M. Taylor ([email protected]) is managing editor of Environment & Climate News.

For more information …

Several recent issues of Environment & Climate News have reported on the success of treatments with aquatic herbicides. See, for example,

“EPA Approves Chemical Control of Aquatic Weeds,” July 2005,

“Herbicides Winning Against Invasive Weeds,” July 2005,

“Aquatic Weeds Overwhelming Nation’s Lakes,” August 2005,

“Aquatic Herbicides Ring Up More Success Stories,” October 2005,