It works! Shining bright lights on a bunch of Potomac River hydrilla weeds in the dark of night inhibits the plants' libidos and limits the number of their offspring.

Federal researchers installed lights in August at the Belle Haven Marina just south of Alexandria. The results are in: The reproduction of the green aquatic weed, widely viewed as a nuisance, was cut in half when 500-watt lamps were trained on it for an hour each night, starting at midnight.

But the main researcher, Lars W.J. Anderson, is going to have to persuade marina operators, the Army Corps of Engineers and assorted local hydrilla haters that stringing lamps above affected waterways is practical.

"We think there is practical application, particularly in localized areas," said Anderson, a plant physiologist with the U.S. Agriculture Department. Because hydrilla sprouts its reproductive buds in late summer's longer nights, the lights trick the plant into acting as though it is not time to reproduce.

Two types of lamps were tested -- 500-watt floodlights and small bulbs encased in plastic tubing somewhat resembling Christmas tree lights. The floodlights did best.

"There was a 50 percent drop in the number of new tubers [roots] produced when the lamps were turned on for one hour at night," Anderson said. "The next time, if we use stronger lamps and start earlier, we may get even better results."

Army engineers and other hydrilla experts are somewhat skeptical.

Because lamps require nearby electrical outlets and must be attached to a steady object, they are not part of the Corps' new plan to eliminate 340 acres of hydrilla from the Washington area's most traveled waterways, which are often far from shore.

"I don't think anyone's taking it seriously, because how are you going to hang the lights out in the middle of the river?" said one Washington area hydrilla expert.

The Corps plans to use a mechanical harvester beginning in June to literally mow down the weed at a cost of $3 million over the next decade. All that to counter the advance of the American strain of an underwater Asian plant that was virtually unheard of here until it appeared in the Potomac two years ago. It has since spread over 1,900 acres of the river.

Anderson says he will travel to Alexandria this spring hoping to persuade other government agencies that shining 1,000-watt lights into the water at midnight is the answer to ridding at least the river's shores of hydrilla.