Hydrilla won't do it with the lights on.

That's what specialists working to curb the plant's sex life said yesterday, as the federal government turned on its newest weapon to combat the so-called monster weed: a 500-watt lamp.

"This will fool them the hydrilla plants into thinking it's a short night," said Lars W.J. Anderson, as he hooked up a quartz lamp at the edge of Alexandria's Belle Haven Marina yesterday.

Anderson, a U.S. Department of Agriculture weed researcher, said because hydrilla sprouts its reproductive buds as nights lengthen in late summer, the lamp should disrupt its reproductive patterns and halt further growth.

Hydrilla, which covers much of Alexandria's shoreline waters like a rich, green carpet, has drastically diminished boating activities at the Belle Haven Marina and elsewhere, said marina manager Steven M. Jennings. "The motor boats can't even come in during low tide because the hydrilla is so thick," said Jennings.

If hydrilla continues sprouting at its current feverish rate, it will cover 34,000 acres of the Potomac by 1995, the Army Corps of Engineers predicts. Currently, the Corps said the weed spreads over 600 Potomac acres.

"Everybody's glad to see a lot of plants in the Potomac," said Anderson, acknowledging that some biologists believe hydrilla is not quite the Godzilla many government experts believe. But, he said, it must be stopped because it is "an exotic species that grows out of control."

Those who favor eradicating the weed cite the millions of dollars clogged river access and thwarted fishing, swimming and boating activities have cost the government and private owners in Florida and California. Those who favor leaving the plant alone say it is a healthy sign of a cleaner Potomac, and a welcome addition to the river's ecosystem.

"It's very aggressive in the water," said Vernon V. Vandiver, a University of Florida aquatic weed specialist. Vandiver said he eagerly awaits the Alexandria lamp experiment. "It would be a lot safer and cheaper" than other methods of hydrilla control if it worked, he said.

So far, the Army Corps of Engineers has used an expensive mechanical harvester and chemicals that have raised safety concerns to wipe out hydrilla.

"It all sounds like folly to me," said Douglas Land, the proprietor of the Alexandria Marine Services, about the hydrilla control methods. "I think hydrilla is like rats. Once they're entrenched, you can't get rid of them."

On July 17, Virginia, Maryland and District officials met and tentatively endorsed an Army Corps of Engineers cost-sharing plan that allegedly could rid the Potomac of 340 acres of hydrilla and cost about $3 million. The officials are scheduled to meet again this fall to iron out what method will be used to carry out the plan.

Two overhead quartz lamps, and two underwater lights encased in plastic will be turned on at midnight at Belle Haven, Anderson said. In 45 days, he will analyze the results.