In the News

Boaters Can Help Prevent the Spread of Eurasian Watermilfoil
date posted: December 22 2005

Lake Tahoe Report #78
Aug. 2, 2004
By Susan Donaldson,Water Quality Education Specialist,University of Nevada Cooperative Extension

By now, I¡¦m guessing that most boaters and recreationists have heard of Eurasian watermilfoil (Myriophyllum spicatum), an invasive weed spreading around Lake Tahoe¡¦s rim. This water weed grows rapidly and forms a dense canopy at the water¡¦s surface, interfering with recreation, inhibiting water flow, and tangling in boat propellers. It has become a major headache in local marinas such as the Tahoe Keys, costing tens of thousands of dollars each year in control costs. A 2003 survey of the lake found that it is expanding rapidly along the western end of Emerald Bay, and has colonized most marinas and some protected coves and beaches.

As the name suggests, Eurasian watermilfoil is not native to the United States. Widespread in its native continents (Europe, Asia, and North Africa), some reports suggest it was first imported to the United States in the late 1800s. It has spread rapidly across the country with help from boaters, who accidentally transport plant fragments. Today it is established in 45 states. After the flood of 1997, the weed escaped from Lake Tahoe into the Truckee River, and has been documented as far downstream as Verdi.

Eurasian watermilfoil roots in shallow lake bottoms or slow-moving areas on streams and rivers. It grows into a 10-foot to 15-foot long vine. Feather-like leaves grow in whorls of four around the stems, each leaf having 12 to 21 leaflets. When removed from the water, the leaves collapse limply around the stem. New weeds are capable of growing from tiny fragments as little as 1 inch long, allowing it to spread rapidly. Eurasian watermilfoil also produces reddish flowers in midsummer, which produce seed and contribute to its spread.

What¡¦s the big deal about this weed? It grows prolifically and competes aggressively with native aquatic plants, forming dense mats that shade the water and cool the water temperature. Eurasian watermilfoil¡¦s rapid growth and eventual decay degrades water quality and uses up oxygen needed to support fish populations. There is concern that it also accelerates nutrient cycling by moving nutrients such as phosphorus out of lake sediments and leaking them into the water, or returning them during the decay process, contributing to algae growth and impaired lake clarity. The weed also creates problems for boaters, recreationists, managers, and landowners, as the dense, matted growth restricts boating, water-skiing, swimming and fishing.

To compound the problem, the weed is difficult to control. Marinas use mechanical harvesters to mow the weed below the water¡¦s surface to maintain boat access. The biomass is removed and landfilled, but the process releases plant fragments that can add to the spread of the weed. At other locations, a variety of methods has been attempted, including mechanical harvesting, diver-assisted dredging, insect biocontrols, drying and freezing the water body, and chemicals. Unfortunately, we don¡¦t yet have an ideal control method for Lake Tahoe. The Tahoe Keys Marina is experimenting with the SolarBee, a floating solar-powered water circulator that pumps water from deep in the water column and circulates it to the surface, affecting water biology and chemistry. The SolarBee seems to help suppress the growth of the Eurasian watermilfoil, but does not eliminate it.

Our best option at this point is to try to prevent further spread within Lake Tahoe and to other local waterways. Boaters, this means you! Below are some steps boaters can take to help stop the spread of Eurasian watermilfoil:
„X When leaving infested areas in Lake Tahoe, stop for a moment and run your boat engine in reverse. This helps knock off plant parts and leaves them in the infested area, preventing spread.
„X Remove all aquatic vegetation from your boat and trailer before launching at or leaving any boat ramp. Check the propeller, air intakes, tires, etc. and remove plant parts.
„X Drain the boat, engine, and any other reservoirs.
„X If possible, high-pressure wash your boat and trailer. Alternatively, leave the boat and trailer in the sun to dry for at least five days before re-launching.
„X Dispose of plant parts away from the lake or other waterways.
„X Pass the word among friends and neighbors so they¡¦ll help control this weed.