Chiller of an idea becomes a killer for lake weeds

Two years after Eden Prairie's Anderson Lakes were drained in an experiment with natural weed control, rain is finally filling them up again and early results are encouraging:

The weeds, after back-to-back cold treatments, seem to be in retreat.

Northwest and Southwest Anderson Lakes were drained in the fall of 2008 to expose the lake beds to a winter freeze in an attempt to kill unwanted curly-leaf pondweed. The freeze targeted burrlike buds embedded in the lake bed that allow the weed to reproduce.

A recent inspection found curly-leaf pondweed in only six of 130 sample locations, said Randy Lehr, senior manager of water resources for Three Rivers Park District that owns land along the lakes. Also encouraging is growth of more plants in the lake and on the shoreline, Lehr said.

Cattails along the lake perimeter have been joined by bullrushes, smartweed and broadleaf arrowhead, Lehr said. More plant diversity is good for birds. A yellow-headed blackbird that had not been seen in the area in recent years has returned.

Ultimately, for the drawdown to be considered a success, the lake water must meet state standards for phosphorous, native plants would be growing in the lakes, and curly-leaf pondweed would lose its dominance, Lehr said.

The same 130 sample locations will be visited again this fall. A final conclusion on the benefit of draining the lakes will take several years of monitoring, Lehr said. "I am encouraged by what we see so far."

Before the drawdown, weeds covered 45 to 50 percent of the area of the lakes -- blocking light to native plants early in the growing season. Those plants died off in midsummer, releasing nutrients that fed algae that turned the lake pea green.

The lakes were supposed to refill after the winter of 2008-09, but last summer's drought kept water levels low and prevented officials from getting out on a boat to measure results. The low water level exposed the lake bottom to another freeze last winter, providing the method a second chance to work.

"We sort of got back-to-back drawdowns," said Kevin Bigalke, administrator of the Nine Mile Creek Watershed District, which drained the lakes.

Now the lakes are coming back. At the end of May, Southwest Anderson Lake was about a foot shy of its original depth and Northwest Anderson Lake was about half full, Bigalke said. June rains also have raised levels.

Eden Prairie urged the attack on curly-leaf pondweed because of concerns about the degrading lake-water quality. Three Rivers Park District suggested the nonchemical approach because the lakes were shallow -- 8 to 10 feet deep -- and easily drained.

That will offer a point of comparison between the two weed-control methods. Bigalke said he thinks that the undrained area is where the pondweed is showing up now. It will be subject to follow-up herbicide treatments for the next several years.