Chesapeake Quarterly
Travels with Hydrilla
The Unnatural History of an Accidental Invader


DC Reflecting Pool - photo by Michael W. Fincham
The famous Reflecting Pool on the National Mall once held an underwater grass called hydrilla, as did the nearby Constitution Gardens Lake. An invasive species now found in 30 states, hydrilla is native to countries in the Indian Ocean region. Credit: Michael W. Fincham.

EARLY ONE AUGUST MORNING IN 1980, four young scientists from Florida tried to get some sleep on benches near the Washington Monument. Soon enough the Park Police rousted them and they began wandering along the north side of the National Mall. When they reached the lake at Constitution Gardens, the scientists paused, took off their shoes — and waded into the water. When the police caught them this time, they were pulling plants out of the lake and studying their roots.

"This is hydrilla," they told a perplexed policeman with little interest in aquatic plants. It was an accidental discovery but an important one, made by sleep-deprived scientists who had driven all night to make a meeting.

Hydrilla, they explained, was a non-native plant found mostly in India, Sri Lanka, Pakistan, and other countries bordering the Indian Ocean. What was it doing on the National Mall?

"We didn't know what hydrilla was," said Richard Hammerschlag, the Park Service scientist who had supervised the plantings. "It wasn't on our radar. It wasn't up here."

Hydrilla was suddenly large on his radar — and not just because it was here on the National Mall. Hammerschlag knew hydrilla had spread through much of Florida, clogging canals and creeks and lakes with thick green mats of vegetation. And he knew Park Service scientists had recently put this same invasive grass out on the Potomac River.

Hydrilla has been confusing scientists ever since it arrived in America. In 1960 two young scientists found hydrilla clogging the Snapper Creek Canal in south Miami, but they had no idea what this mystery plant was or where it came from. Bob Blackburn had been working in Florida for one year. His partner, Lyle Weldon had just arrived. They were the first scientists to study this plant in American waters, but it would take them five years to figure out what it was.

Blackburn and Weldon were part of a group newly organized by the Agricultural Research Service of the U.S. Department of Agriculture. Eastern Florida with its subtropical weather and numerous waterways was ground zero for invasions of aquatic plants from around the world. The new hires were supposed to discover and disable the invaders.

A little detective work turned up the culprit behind the canal invasion. A homeowner told Blackburn she put the plant in the water. Something that looked so pretty in her aquarium would also look nice in the canal. Where did she get the plant? From an aquatic plant dealer who bought plants from overseas and sold them here.

Worried by its rapid spread, Blackburn and Weldon sent plant samples to the University of Florida and the Smithsonian Institution in Washington, D.C. and got the same answer back from scientists at both institutions: this fast-growing plant was probably American elodea.

The second outbreak was 300 miles away in Crystal River, and no detective work was needed to pinpoint the source. An aquatic plant dealer bragged to Blackburn that he had been growing these plants in the river so he could sell them to people with aquariums and backyard water gardens. He knew the plant wasn't American elodea — it was being sold as "Indian Starvine."

When Blackburn and Weldon kept finding tubers and turions, they went looking for a third opinion and got the same answer: this was elodea "with strange growth characteristics." Finally a fourth opinion paid off when they sent samples to Harold St. John, a world authority on aquatic plants. Six months later they got the telegram: this was not elodea. It was hydrilla, a plant native to the Indian Ocean region. At the launch of their careers these young scientists could now take credit for documenting the arrival of a new aquatic plant in North American waters.

They were already driving around Florida observing and photographing new outbreaks. It was work that brought Blackburn and Weldon into a partnership unusual in science. As Blackburn describes it, Weldon was a research partner, then a friend, and finally almost a brother. They worked together, traveled together, and published together.

And they dove together. Hydrilla, they quickly saw, was "a canopy former," and they wanted to see what lay beneath. To see the underside Blackburn and Weldon began making scuba dives wearing air tanks, weight belts, and regulators. Sinking below the surface, they discovered mats that could grow four to six feet thick, blocking out the sun and sending long vine-like tentacles twisting down to the bottom. It was, Blackburn remembers, like diving into a dark cave.

Hydrilla, they discovered, could snake along the bottom quickly, almost secretly, using vinelike runners. Its long stems then reach upwards toward the surface where they suddenly branch out in all directions, interlacing to form a canopy. It replicates through seeds, buds, and roots, but also through broken shoots that float away, sink, and quickly latch onto the bottom with fine, threadlike roots. By spreading through fragments, the "perfect weed" became a fast traveler. During the 1960s it showed up throughout the state. During the 1970s it would range throughout most of the south.

On February 1, 1970, the partnership ended. Blackburn and Weldon dove into a hydrilla-jammed lake next to a Naval base near Orlando and quickly lost sight of each other under the dark canopy. When Blackburn surfaced he saw no sign of Weldon and dove again. He finally found his partner floating dead under the canopy, tangled in long, twisting hydrilla vines.

Blackburn's guess: "He got under the canopy and got confused." Separation, entanglement, confusion, panic. His mouthpiece lost, his lungs filled with water.

Blackburn never dove again. "I lost my feel for it that day."

As Kerry Steward motored down the Potomac River in the summer of 1982, he was amazed at how far hydrilla had spread along this huge river. A colleague of Blackburn and Weldon, he was a veteran of the hydrilla campaigns in Florida, now come north to consult with the National Park Service. Traveling the Potomac with a team of local scientists, he was finding an invasive species that was first discovered 1,000 miles to the south.

A local scientist riding with him found the scene breathtaking — even frightening. Small bays and coves were covered wall-to-wall with green mats — like golf fairways laid over parts of the river. It was, he thought, an ecological nightmare, an opinion shared by a lot of angry boaters, marina operators, and waterfront homeowners along more than 20 miles of shoreline. The "hydrilla wars," as the locals called them, were well launched by now in the nation's capital. Newspapers were running headlines about a new "monster" seagrass that was invading the river.

Where did the "monster" come from? And how to get rid of it? Those were the questions Steward was hired to solve, according to Richard Hammerschlag, the Park Service scientist who brought him to Washington. Like any good detective, Steward and his team collected samples, sent them off to a lab, and interviewed local witnesses.

Was the National Park Service a culprit? At Dyke Marsh, a cove of the Potomac just south of Alexandria, one of their scientists had been testing a pondweed that looked like American elodea. Looks can be deceiving, as Steward knew. The plants he was pulling out of the Potomac looked to his eye like hydrilla, the same plant that caused so many problems in Florida.

Was there, Steward asked, another, less obvious culprit? Who had sold hydrilla to the National Park Service?

Don Schmitz knows the name of the man who first brought hydrilla to Florida, but he's not telling. The co-author of a history of aquatic invasions, Schmitz did enough detective work to track down the first culprit, agent zero for the hydrilla epidemic. Schmitz not only found his man, he got his confession — but only by promising anonymity.

What Schmitz heard was a tale of mistaken identities and accidents. The mistakes began with a dealer for tropical fish and aquatic plants who imported hydrilla from Sri Lanka (then called Ceylon) to St. Louis, Missouri, thinking he was getting a species of anacharis, a green plant commonly sold for aquariums. Impressed with the plants, he airmailed six bundles to Tampa, Florida, where another dealer, unimpressed, ordered them thrown away.

The accidents began when his son failed to follow orders, first storing the exotics in a Tampa canal near the airport — and then promptly forgetting about them. When the Tampa dealer, let's call him agent zero, later discovered the plant spreading throughout the canal, he promptly changed his mind. Finally impressed, he began marketing his mystery plant to other dealers, calling it "Indian Starvine." And some of those dealers began storing it in canals before selling it for aquariums and ponds.

This roundabout route is fairly typical for invasions of many exotic plants, according to Schmitz. With the expansion of air freight after World War II, the rate of invasions into America picked up dramatically. Plants that used to die on long ship passages now arrived ready for transplanting to new waters.

Much of the transplanting was done by dealers working in the aquarium plant industry. "Dealers deliberately seeded the waterways," says Schmitz. They would import exotics, often storing them in creeks and canals, a tactic that saved them the cost of maintaining so many ponds and tanks. Later they would harvest plants out of the canals and sell them to homeowners for their aquariums and water gardens. Months or years later many of their customers would empty their aquariums into creeks and canals near their home, seeding new waterways.

Once in the water, hydrilla traveled easily to new waters, carried across land as a hitchhiker on boats and boat trailers. Arriving in a new location, any fragment could quickly take root and start snaking along the bottom.

When Schmitz finally tracked down the Tampa dealer, his agent zero, they met in a restaurant for sandwiches. The dealer, now well into his nineties, made his confession but got no absolution from Schmitz, who outlined all the damage done by hydrilla, all the millions spent trying to eradicate it or control it in Florida's waterways.

"I had to ask him, how do you feel about all that," says Schmitz. "He leaned back and said, 'Whoops!'"

As he investigated the hydrilla invasion of the Potomac River, Kerry Steward suspected he was seeing another big whoops: another chain of accidents and mistaken identities that could be a repeat of the Florida invasion pattern. And one possible link in the chain was a local commercial dealer in aquatic plants.

Where, he asked, did Park Service scientists get the mislabeled hydrilla they put in the Potomac River? They told Steward they kept their elodea look-alikes at the Kenilworth Aquatic Gardens, a small riverside park along the Anacostia River — but they originally brought those plants in from Lilypons Water Gardens, a commercial dealer in Adamstown, Maryland. Lilypons in turn often used a supplier in Texas.

Steward sent plants from both Kenilworth Gardens and Lilypons to his Florida lab for culture and analysis. The results showed that plants from both sources were hydrilla, not elodea, and both had identical enzyme patterns, more evidence that Lilypons was the probable source of hydrilla. The chain became clear, says Hammerschlag: "Lillypons to Kenilworth Gardens to Dyke Marsh."

And the last link in the chain was the scientist who put hydrilla in the Potomac at Dyke Marsh.


In the spring of 1980 Horace Wester had a plan. He would put elodea in floating cages, place the cages in the river at Dyke Marsh, and see how the plants fared under different conditions. These cages, five feet long and made of wood, would float, holding the plants above the dark bottom and closer to the light. His hope was to begin restoring underwater grasses to the Potomac. His mistake was using elodea plants that were actually hydrilla.

It was a mistake made by experts around the country, but it carried sad irony for a scientist as experienced and observant as Horace Wester. A native of the Washington, D.C. area, Wester had spent a 40-year career trying to preserve or restore the capital city's endangered natural resources. During the onslaught of Dutch elm disease he helped save Washington's famous shade trees by identifying a single tree that seemed resistant. He discovered it standing in front of the Freer Gallery of Art, and his work led to the cloning of that tree and the cultivation of an American elm species. The "Jefferson" elm became one of the cultivars that the Park Service uses to keep the National Mall lined with large shade trees.

Two rivers run through the nation's capital, and Wester as boy and man spent time on both. He remembered an Anacostia River with marshes full of wild rice and a Potomac River with hundreds of acres of underwater grasses. According to Stephen Syphax who worked with him on both rivers, Wester spent years growing test plots of wild rice in hopes of restoring the Anacostia marshes of his youth. Then he went to work on the underwater grasses of the Potomac.

He carried his passions for these rivers into retirement. By the spring of 1980 he'd been officially retired for three years when he started floating cages of hydrilla in the Potomac.


The monster weed that invaded the Potomac River never went away, but over time hydrilla became hated less, perhaps even loved a little.

Fishermen came to like it first, since they were often catching fish near hydrilla beds. Bird lovers were seeing wading birds walk out on the hydrilla mats where they'd start pecking at the grass and at the fish that emerged along the edges. And scientists were getting high-visibility readings when they dropped their secchi disks, good evidence the beds were trapping floating sediments and helping clear the water.

All of which brought some relief to scientists with the National Park Service who'd taken a lot of heat for releasing this non-native marauder into the Potomac. "What actually happened, essentially to my consternation, was that people like hydrilla," says Richard Hammerschlag. "Hydrilla rapidly filled an ecological void."

Several native species have also reappeared in that void, perhaps helped along by hydrilla. More evidence of that came two years ago when Nancy Rybicki reported her findings from an annual survey she's been running since 1985 for the U.S. Geological Service. While two non-natives, hydrilla and Eurasian water-milfoil, account for 60 to 90 percent of the grass acreage in the upper tidal Potomac, their dominance has decreased over time. As water quality improved in the river, the coverage of several native species, especially wild celery and coontail, increased — slowly, but steadily.

Hydrilla in the Potomac never became the scourge it was in Florida where the campaign to kill it continues. The wide, flowing Potomac is not a Florida canal or creek or lake that can be easily covered over. "We have a different perspective," Rybicki says. "We lost our grasses, so when something came back we were excited about it."

The hydrilla wars abated, but they never ended. Scientists may be pleased, but boaters and sailors and waterside homeowners are still unhappy when hydrilla blocks them off the water. "People are trying to spin it and make a positive out of it," says Jamie Hamilton who owns a dock-building company, "but I don't see it." Watching kids swimming through hydrilla, he even wonders whether someone could drown in the stuff.

Living with hydrilla means mowing it, so Hamilton also owns a grass-mowing boat now, one of several along the river. In quiet shoreline coves from Dyke Marsh down past Mount Vernon and Mason Neck, marine contractors will fire up heavy, clunky mower boats this summer and begin chomping channels through the thick hydrilla mats. The dead, smelly grasses will probably end up in a landfill.

Fishermen will motor out through these cleared-out channels. In waters where Horace Wester once used the wrong plants to restore a barren river, they will drop anchor along the edge of some large hydrilla beds. And there they will start catching fish.

June 2009
vol. 8, no. 2
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