Theresa Gunter, in knee boots, can almost walk on the thick weeds that blanket one of her former fishing holes across the Mississippi River from Vicksburg. Giant salvinia, the fast-growing aquatic fern is having a very good year — so much so that the water weed has grown so thick that some ponds resemble a field.

“This pit had some humongous white perch. Beautiful perch,” said Gunter, squishing through the marshy tarp of plants. The “pit” is a borrow pit, one of hundreds created when soil was “borrowed” to create the river levees. They’ve been excellent fisheries for decades, but are severely threatened.

Giant salvinia, which is native to Brazil, has crept northward in Louisiana as far as Madison Parish and the pits where Gunter and her family used to fish. It is tenacious and fast-growing, experts say, and left unchecked can take over a body of water the way kudzu — another invasive plant species — has overtaken forests and undergrowth in the South.

Giant Salvinia covers most of the bar pits on land the Gunter family leases as pasture for cattle.

“The cows love it,” she said. “You can see where they wander out to eat it.”

Sometimes, however, the weed is so thick the grazing animals get tangled and must be extracted using a tractor. “You put a chain around their bellies and just pull them out,” Gunter said.

Giant salvinia has spread across much of Louisiana in recent years and is known to grow in Mississippi ponds, too. Once introduced, the plant rapidly swallows the surface of a pond or a lake, crowding out native plants, fish and animals.

State wildlife agencies have declared war against the weed, whose tenacity and growth rate liken it to a green creature from a horror movie.

The fern leeches nutrients from ponds, chokes off sunlight from fish and turns open water into marshland, ruining it for boaters and swimmers, too.

“A boat just doesn’t stand a chance in it because it’s so thick,” Gunter said.

Sad at the loss of her favorite fishing spots, Gunter was alarmed the speed at which the plants spread. She is concerned at what she says is lack of awareness about the plants, which can take over a body of water in a matter of weeks. Actually, there is a widespread awareness of Giant salvinia among state and federal agencies — but there’s not a lot that can be done about it.

“The plant has an amazing ability to grow under ideal conditions,” said Michael Pursley, a biologist with the Mississippi Department of Marine Resources. “It can double the size of the area it occupies in four or five days.”

Pursley patrols waterways in Mississippi’s three coastal counties, and keeps an eye out for giant salvinia, hydrilla, water hyacinth and other invasive plant that threaten aquatic ecosystems.

Giant salvinia, which threatened several lakes on Mississippi’s coast before the storm surge from Hurricane Katrina poisoned most of it with salt water, threatens to return, he said.

“Fisherman can spread it pretty easily with their boats and their boat trailers,” he said. Posters warning people to clean plants from boats and propellers have been posted at public launches, he said. A cleanup effort is under way on the Pascagoula River, he said.

“It’s a very aggressive plant and it has the potential to rapidly cover the entire surface of the water,” said Dennis Reicke, a biologist at the Mississippi Department of Wildlife and Parks. He said information on the plant has been distributed to scientists, but not to private boaters.

Private lakes in Mississippi have not been surveyed for giant salvinia, Pursley said.

“As long as its not impacting waterways, it’s really the responsibility of the landowner,” Pursley said.

Giant salvinia was probably was introduced to North America via the exotic plant trade, shipped here from the Amazon Basin.

“You can actually purchase this plant on the Internet,” said Linda Nelson, a researcher at the U.S. Army’s Waterways Experiment Station in Vicksburg. “People don’t realize when they put it in their water garden – they think it’s just an innocent little plant.”

Nelson, who is part of a team that studies invasive marine plant species, researched ways the plant can be killed using herbicides in 2006.

Though she still has giant salvinia plants at the experiment station, research has stalled. “We didn’t get funded this year,” she said.

Attacks on the plant must be complete because it will return if not completely removed. Chemicals and machines have been used to remove giant salvinia, and scientists have even tried introducing other nonnative species to eat the fern.

In 2003, The Hawaii Department of Land and Natural Resources, using herbicides and industrial equipment, cleared 285 acres of the plant from the state’s largest freshwater body, Lake Wilson. Giant Salvinia had overtaken 95 percent of the lake’s surface area.

“They used front-end loaders and tried to pull it out of the lake,” Nelson said. Boats would round up the weeds and haul them to dredging machines on shore. “It was very costly.”

In 2005, the Mississippi Department of Wildlife, Fisheries & Parks introduced weevils to lakes affected by giant salvinia, in the hopes that they would eat the plants. Several days after the bugs were released, Katrina hit, and salinity killed off most of the plants, said Pursley.

Nelson said the plant has the potential to wreak havoc if left unchecked.

“If there are any remaining plants they’re going to come back again and take over like they did in Louisiana.”

“It’s spreading and its taken over,” Gunter said. “I know people that have spread different chemicals on it; it can’t be killed.”