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In the News

October 19, 2006

Fishing for a solution for a destructive aquatic weed

Weed specialists are never off duty — even when they’re fishing.

   Eurasian watermilfoil

Eurasian watermilfoil
Photo by Jack Kelly Clark

While trout fishing in the Fall River in Shasta County, weed scientist Joe DiTomaso noticed Eurasian watermilfoil in the water. “In the areas that had this exotic plant, there were few fish, and the guide indicated that fishing was very poor in those areas,” says DiTomaso, a University of California Cooperative Extension specialist in Plant Sciences at UC Davis.

Fall River is a world-renowned trout fishery nestled in the mountains between Mounts Shasta and Lassen. Almost entirely spring fed, the Fall River winds for 16 miles, usually through private agricultural land. In August 2003, Eurasian watermilfoil infestations slowed the river’s water flow and flooded 300 acres of grazing land when a levee collapsed.

Since its introduction into the United States, this submerged aquatic plant has spread throughout many rivers and reservoirs. The plant is native to Europe, Asia, northern Africa, and also occurs in Greenland. In North America, Eurasian watermilfoil is found from Florida to Quebec in the East, and California to British Columbia in the West.

Milfoil can form thick underwater stands of tangled stems and vast mats of vegetation on the water's surface, especially in shallow, nutrient-rich water. These mats can limit boating, swimming, and fishing. The plant can disrupt the ecology of a water body by crowding out important native aquatic plants needed to provide food for a healthy fishery. It can also potentially reduce property values. Milfoil generally does not produce mats on the surface in water more than 15 feet deep, and doesn't usually grow in water more than 20 feet deep.

This attractive plant has feathery underwater foliage and was once commonly sold as an aquarium plant. Milfoil reproduces rapidly and can infest an entire lake within two years of introduction.

Since Eurasian watermilfoil appears to rely heavily on sediment-based nutrients for growth, DiTomaso, Dave Spencer, a USDA-Agricultural Research Service scientist, and Thaddeus Hunt, a UC Davis agronomy and horticulture graduate student, compared the differences in sediment nutrient levels in infested and uninfested sites in the river to predict the areas that are susceptible to invasion.

By identifying environmental factors contributing to invasiveness of Eurasian watermilfoil, this study will point to ways of predicting what stream conditions are susceptible to invasion, leading to enhanced capabilities for preventing new invasions in wildland aquatic habitats.

Hunt measured the seasonal change carbohydrates in root crowns and shoots of Eurasian watermilfoil populations in Fall River to optimize the success of plant harvest techniques and reduce the need for herbicide treatments. “The low point in root crown carbohydrates can be used to determine the best timing for mechanical control efforts since plants will have less energy reserves to recover from cutting,” says Hunt.

In late summer and fall, the plants become brittle and naturally break apart. These fragments will float to other areas, sink, and start new plants. Milfoil will also grow from fragments created by boaters or other disturbances during any time of year. It appears to be spread primarily through boating activity. Once established in its new home, water currents may carry milfoil fragments and start new colonies within the same body of water.

“Understanding the seasonal pattern in the Fall River milfoil populations is essential to the success of any control strategy to limit its further spread,” says DiTomaso. “Once the carbohydrate reserves reach their low point, mechanical cutting and harvesting can provide some control of the invasive species by limiting successful recovery of the plant. Even if plants do recover, a subsequent cutting can further deplete the energy reserves. Preliminary results indicate that the best time to cut the milfoil is mid-July to mid-August, but more testing is required.”

Spencer and Greg Ksander, biological science laboratory technician, are using a remotely operated, radio-controlled boat to collect digital pictures of the riverbed and plant species, as well as Global Positioning System coordinates that correspond with the digital pictures, to create distribution maps. These maps are used to determine where to collect soil samples for analysis.

“Our long-term goal is to better understand what riverbed conditions are likely to lead to the invasion of Eurasian watermilfoil and to determine the most effective timing for control of the invasive species using mechanical methods,” says DiTomaso.

The UC Exotic/Invasive Pests and Diseases Research Program, a collaboration between the UC Statewide Integrated Pest Management Program and the UC Riverside Center for Invasive Species Research, funded this project. U.S. Department of Agriculture, Cooperative State Research Education and Extension Service, funds the program.


High-resolution image (132KB) "Eurasian watermilfoi." Photo credit: Courtesy of UC Statewide IPM Program, by Jack Kelly Clark. Photos are for use with this release only. All other uses see Legal Notices.


Stephanie Klunk, Communications Specialist
UC Statewide IPM Program
(530) 754-6724

Joe DiTomaso, Weed Specialist
UC Cooperative Extension
Plant Sciences
UC Davis
(530) 754-8715

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