A fourth south metro lake will be drained in an attempt to revive its water quality and fight an invasive pond weed, but results on other lakes suggest it won’t be a permanent solution.

In late September, Apple Valley will drain 37-acre Long Lake under Pilot Knob Road into adjoining Farquar Lake. The goal is to expose Long Lake’s bottom to a hard winter freeze that will kill the curly-leaf pondweed that is fouling the water.

“If this works, there are going to be a lot of happy people,” said Long Lake Watershed Association President Paul Habegger. He said he and his wife, Kate, thought twice about having their son’s graduation party outside because they knew guests would be looking at the jungle of weeds in the lake.

Draining lakes for the fall and winter and allowing them to fill up again in the spring is the most dramatic tool used in Minnesota’s campaign to restore water clarity and bring back native aquatic plants. The Department of Natural Resources drew down 40 lakes in outstate Minnesota last year to keep them hospitable for waterfowl.

But in the metro area, drawdowns are rare: The DNR has given permits for just 10. Among the most recent were two of the Anderson Lakes chain on the border of Eden Prairie and Bloomington and Cleary Lake in the Three Rivers Park District.

A $130,000 drain pipe was installed on Long Lake in 2009 with a $60,000 grant from Vermillion River Watershed Joint Powers Organization. The city paid its share with a storm drainage utility fund collected from residents. There is no direct cost to lakeshore homeowners.

Water was drained from the lake to put in the pipe, but the weather did not cooperate that year, and residents did not see much improvement in the spring.

Best results come from a dry, cold winter in which the lake bottom is not insulated by snow and freezes solid.

It may be necessary to drain the lake repeatedly, every four years or so, to get results, said the city’s natural resources coordinator, Jeff Kehrer.

“There isn’t one project out there that is going to cure all our problems. It’s going to take a few more drawdowns. There is no chemical out there, either, that has long-term control over curly leaf,” Kehrer said.

“It took many, many years to degrade these lakes, and it will take many, many years to get them back to where they belong.’’

Pondweed has taken hold

Herbicides and mechanical harvesting are often used to curb curly-leaf pondweed, which is now so common on metro lakes that the DNR has said that eradicating it is not a realistic goal.

In the Three Rivers Park District, a successful drawdown of Cleary Lake in 2003 and 2004 reduced curly-leaf pondweed and significantly improved water clarity for about six years, said John Barten, Three Rivers’ natural resources director. “You can get some really good results if everything works right,” he said. The park district is now considering repeating it in the next couple of years, he said.

The 2008 drawdown in the Anderson Lakes chain had mixed results. Northwest Anderson, where the bottom froze down 10 to 18 inches, got more benefit, showing improved water quality and more native plant diversity than southwest Anderson, which could not be drained as completely and froze down just 6 to 10 inches, said Nine Mile Creek Watershed District Administrator Kevin Bigalke.

Kehrer expects to drain Long Lake down to about 1.5 feet in the deepest part.

Although lakeshore property owners hate to see the water go and worry about disturbing the eagles, blue herons and other wildlife that frequent the lake, 19 of 22 of them signed a petition asking for the treatment.

“If we had a magic wand, we would prefer not to do this, but the water quality is terrible,’’ said Paul Habegger. “You wouldn’t dare swim in it.”

The Minnesota Pollution Control Agency added Long Lake to the state’s list of impaired waters in 2002. Farquar Lake is also on the impaired list, also has curly-leaf pondweed and is not expected to be harmed by receiving Long Lake’s water, Kehrer said.

When the Habeggers moved to Long Lake in 1990, the 4.5-foot-deep lake was clear all year. Now pondweed forms thick mats on the surface through June and July, making it difficult to canoe and kayak. Because it is the first plant to come up in the spring, curly-leaf blocks light to more slowly growing native plants, and when it dies back at the end of July it releases phosphorous, which promotes green algae growth.

Long Lake is burdened by too many pollutants, including phosphorous from lawn fertilizer. “The lake needs to go on a diet. It’s overnourished,” said John Erdmann, research scientist for the Minnesota Pollution Control Agency.

The first part of the diet is the drawdown, which will aim to kill the pondweed and the rough fish — including bullheads — that dominate the lake and contribute to its poor quality. Then in the spring, when the lake fills back up, the city will activate two in-ground filter cells, built at a cost of $140,000, with a $40,000 grant from the watershed and $20,000 from Dakota County, to remove phosphorous from stormwater before it goes into the lake.

The DNR wants the drawdown completed before mid-October to allow frogs, toads, salamanders and turtles to move to surrounding ponds for the winter. Kehrer said it may take three weeks for the water to fully drain.