Answer Man: Return of the Green Menace

Whatever happened to the hydrilla peril? It seems that a decade or so ago we were being told that if we didn't take drastic measures immediately, this weed would overtake the Potomac and make it possible to drive across the river and obviate the need for a replacement for the Wilson Bridge. Clearly, this has not happened. Is hydrilla still considered a threat to the health of the Potomac?

David Stonner, Washington

When and if a safe and reliable time machine is ever invented, the second thing Answer Man is going to do is hop in, go back to the mid-1980s and hand today's column to all the hydrilla-phobic scientists and reporters who were pulling their hair out over the green menace. (The first thing? There are a few memories from Answer Man's high school prom that still nag at him.)

Early stories about Hydrilla verticillata -- an exotic, invasive species from Asia -- called it a "monster," the "Godzilla of aquatic plants." Unless it was stopped, these shrill stories suggested, hydrilla was going to colonize every last drop of H ₂ O in the Washington area, right down to the gloop in your basement sump pump and the water in your ice cube trays. It already had done something close to that in Florida, where waterways were so choked by the thick, green mats formed by the fast-growing plant that boats couldn't move.

How did it get in the Potomac River in the first place? There are several creation myths. One is that someone dumped an aquarium into the river, hydrilla being used by tropical fish hobbyists. Another is that a tiny piece of hydrilla caught a ride in on a duck's foot. Another is that a well-meaning Park Service scientist planted a bit of hydrilla near Dyke Marsh, little knowing that he was setting off a tuberous time bomb.

There appears to be some truth to this last version. Back then -- the early 1980s -- the Potomac needed subaquatic vegetation, or SAVs in biologist parlance. Water plants are an important part of a healthy river. For decades, though, the Potomac had been too polluted for any to grow.

But right about the time that hydrilla came on the scene, the Potomac was starting to rebound and so were SAVs, said Army Corps of Engineers biologist Robert Blama. "The scare was that what vegetation was making a comeback would be out-competed by hydrilla," he said.

How to fight the green menace? Some experts suggested chemical warfare, giving hydrilla a nice taste of such herbicides as potassium endothal, which was described by one Department of Agriculture scientist as "pretty safe." (Pretty safe? Hey, that's good enough for me!)

Another idea was to lay down mats atop the hydrilla to choke it of sunlight. But the mats needed to have slits in them to keep gases from building up underneath, and the hardy hydrilla just pushed its way through the openings.

They looked at sending divers down to yank out the hydrilla by hand. They considered dumping sterile hydrilla bud-eating Chinese grass carp into weed-choked bodies of water. But who could guarantee that all the carp were 100 percent "fixed"?

A researcher discovered that shining bright lights on the hydrilla for an hour each night messed with the plant's libido and cut back the production of roots by 50 percent. Then someone realized that most of the problem hydrilla was out in the middle of the Potomac and that the bright-light method would require impractically long extension cords.

Soon a sort of vigilante justice sprang up. Owners of socked-in marinas started cutting the hydrilla themselves. This was pretty much the course of action that the Army Corps of Engineers recommended. A special hydrilla mower was deployed in 1986, and it's been operating every summer since, its blades slicing the plants just four inches above the bottom of the Potomac. The corps' mower clears a path to public use areas.

But something interesting happened. As it became accustomed to its new home, the hydrilla stabilized. It not only didn't take over the Potomac, it proved to be beneficial. The hydrilla set up "microenvironments" that allowed native SAVs -- wild celery, coontail -- to get a toehold. It helped clear the Potomac of sediment, which settled to the bottom of the water column as it flowed through the stands of hydrilla. Microorganisms and small fish started to move in.

And then bigger animals came. Anglers noticed that the bass fishing improved enormously. The number of swans, ducks and geese counted in an annual waterfowl census nearly tripled between 1983 and 1985.

"It doesn't mean we should go out and plant [hydrilla] to create fish habitat," said Peter Bergstrom, a fishery biologist at the National Oceanic and Atmospheric Administration's Chesapeake Bay office. And such invasive aquatic plants as Eurasian water milfoil and water chestnut are still a problem.

There's no denying, though, that hydrilla didn't turn out to be the catastrophe many expected.

"All our concerns were for naught, I think," said Claire Buchanan, associate director of aquatic habitats for the Interstate Commission on the Potomac River Basin. "It's sort of typical of the human condition. Things you don't know, you fear."

So let me offer this prediction for a headline from 2024: "Snakehead Fish Cures Cancer, Grows Hair, Melts Away Cellulite and Coaches Redskins to Super Bowl Win."

Answer Man cannot answer every question about the Washington area, but he might answer yours. E-mail answerman@washpost.com, or write John Kelly, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071.