The baby blue, paddle-wheeled, weed-cutting vessel pulled away from shore and headed for shallow waters where Piscataway Creek flows to the Potomac River a few miles south of the Woodrow Wilson Bridge. The contest between plant-eating machine and an underwater herb called hydrilla was under way.

The 37-foot mowing contraption, known as a harvester, edged into a dense patch of submerged grasses, lowered its scissorslike blades a few inches off bottom and began hauling tons of brilliant green hydrilla from the creek. "It's very, very thick -- thick as I've seen it," said harvester operator John Tippett.

Summer has become hydrilla season along a 23-mile stretch of the Potomac, including Alexandria's waterfront, the Mount Vernon shoreline, the Bolling Air Force Base marina and parts of the Prince George's County riverside from the Wilson Bridge to Marshall Hall.

Since it was first sighted here in the early 1980s, the fast-growing, exotic plant has overwhelmed most native species along key sections of the Potomac, led to scientific studies and set off a wide-ranging controversy over the hazards and ecological benefits of submerged vegetation.

Sailors, power boat owners, marina operators and homeowners living along the waterfront have denounced hydrilla as a noxious weed that clogs access to the Potomac. Fishers, bird watchers, biologists and water quality specialists have praised the plant as a sign of the river's restored health.

"It depends on who you are -- how you feel about the hydrilla," said Errigh LaBoo, an Army Corps of Engineers inspector overseeing the hydrilla mowing project. "For the environmentalists, it's great. For the boaters, it's a nightmare. It's called a Godzilla."

As a compromise, federal, state and local officials have adopted a strategy of mowing pathways through hydrilla beds near public piers, marinas, boat ramps and other recreational areas. The work is designed to provide access to the Potomac for boaters without causing environmental damage, officials say.

"You can see how thick it gets and why it could be a real nuisance," explained Michael P. Sullivan, senior water resources engineer for the Metropolitan Washington Council of Governments, which has helped coordinate hydrilla control measures.

Then he pointed to catfish, crayfish, birds, a watersnake, turtles and other wildlife drawn to the thriving beds of hydrilla. "It's just really home to a lot of creatures," Sullivan said. "This is a good example of how rich the aquatic life is."

The origins and habits of this bottom-rooted, branching, perennial herb are only partly understood, according to biologists and other officials.

Hydrilla verticillata -- named because its slender, tooth-edged leaves radiate from stems in verticils, or whorls -- is thought to have reached the United States from Asia. It was reported in Florida in the late 1950s and later spread across the Sun Belt and up the East Coast.

The hydrilla strain discovered in the Potomac five to six years ago differs from the Florida variety in reproductive mechanisms and other characteristics. The type found along the Potomac has male and female flowers on the same plant rather than on separate plants.

Hydrilla is believed to have been carried to the Washington area by migrating birds and on the bottoms of trailer-borne boats. Some hydrilla was inadvertently planted at Dyke Marsh on the Potomac's Virginia side by National Park Service scientists experimenting with aquatic plants.

Hydrilla spread rapidly in shallow areas. According to studies by the U.S. Geological Survey, submerged plants, which occupied only 100 acres in 1983, were found covering 3,600 acres in the past two years. Hydrilla nearly doubled its reach along the Potomac from 12 miles in 1984 to 23 miles last year.

The weed, which takes root close to shore and along sand bars, began crowding out less aggressive native species, like wild celery and coontail. Hydrilla accounted for 56 percent of the aquatic vegetation in 1984 and 94 percent last year, according to the Geological Survey.

Scientists have attributed hydrilla's prolific growth and dominance over other plants to several factors, including its ability to reproduce in various ways, tolerance for low light levels, high rate of photosynthesis, capacity for absorbing bicarbonate particles along with carbon dioxide, and habit of forming dense beds that shade out other species.

Hydrilla can reproduce from plant fragments, which float downstream and take root in new spots. It can grow from bulblike tubers extending into the river bottom and turions lodging above the sediment. Tubers and turions survive in winter, when other parts of the plant die. Hydrilla can spread by rhizomes, or runners. And, scientists say, it apparently reproduces by flowers and seeds.

"All these things come together to give a plant like that a tremendous reproductive advantage," said Virginia Carter, a biologist for the Geological Survey.

Nonetheless, hydrilla does not appear to be invincible, scientists say. It probably cannot survive in the Potomac at depths greater than 10 feet because it will not get enough light. It is unlikely to flourish upstream where fast currents would wash it away.

Downstream, scientists say, hydrilla probably will stop short of the Gov. Harry W. Nice Memorial Bridge at Rte. 301 because the river is quite salty. Hydrilla's growth may vary from year to year. When rainfall is heavy enough to dilute the river's saltiness, hydrilla may spread downstream. In times of drought, it may retreat upstream, they say.

Because hydrilla is a relatively late bloomer, other underwater plants that come up earlier seem to have retained a foothold in the Potomac, officials say. Eurasian watermilfoil, another aquatic grass viewed by boaters as a nuisance, tends to get a head start in spring.

Hydrilla's ills have been widely cited. Boating advocates say the plant can clog the cooling systems of power vessels and can form floating mats thick enough to block a sailboat. Logs, dead fish and unsightly trash often collect in the hydrilla mats.

Hydrilla also causes unwanted shoals to form.

"It gets so dense that you can't get any boats through. Ducks walk on it," complained George H. Stevens, who manages the Belle Haven Marina, a private venture set up on federal parkland south of Alexandria.

Hydrilla poses a "drastic threat" to use of the river, contended Marsha D. Crossley, who heads the Potomac River Yacht Clubs Association, adding, "It's on the move this season."

Fishermen view hydrilla differently. "It's the best thing that could have happened to the river," said Ken Wilson, vice president of Outdoor Life Unlimited, a fishing and hunting guide enterprise. Largemouth bass and other fish thrive near hydrilla, he said. "The grass beds are veritable fish factories."

Federal and University of Maryland researchers recently reported finding between three and 11 times more fish in parts of the Potomac with underwater vegetation than in areas without plants. Fish spawn in hydrilla beds, small fish seek shelter there and large fish go there to find food, officials say.

Ducks and other wildfowl feed in hydrilla patches, officials say. The plants provide habitat for snails, crayfish and other shellfish. Hydrilla helps provide oxygen for aquatic life, stabilizes the river bottom and tends to improve water clarity.

Hydrilla has been labeled a sign of renewed health for the Potomac, "For the environmentalists, it's great. For the boaters, it's a nightmare."

-- Errigh LaBoo

which became nearly devoid of submerged plants in the 1970s, apparently because of pollution.

When government officials began devising tactics to contend with hydrilla three years ago, they considered dredging, dousing the plants with herbicides and such novel techniques as using nighttime lights to impede its reproduction. In the end, mowing was found most acceptable to officials here.

This year marks the first full summer of hydrilla mowing in the Potomac, officials say. Last year, they said, the project had a late start and never caught up with the weed. The job has been turned over to a newly launched, local company called Friends of the Waterfront.

"We got into this because we're sailors," said Virginia state Del. Jim Dillard (R-Fairfax), the corporation's president.

The firm was formed by 18 sailors and waterfront homeowners from the two sides of the Potomac who were worried about hydrilla. "We knew it was going to destroy sailing if somebody didn't do something about it."

The group has cleared paths through hydrilla along Piscataway Creek, the Mount Vernon shoreline, Little Hunting Creek and Swan Creek. Work is scheduled at nearly 30 marinas, piers and other areas. The hydrilla mowing and research are expected to cost federal and local governments about $250,000 a year.

The mowing vessel attacks about two acres of hydrilla a day, carting off nearly 20 tons per acre, officials say.

The weed has been dumped at a Prince George's sand and gravel pit for use in reclamation.

"Everybody's happy to see us coming," said Mike Arnold, a retired federal official who is vice president of Friends of the Waterfront. "We're getting the weeds cut out."