With most of the growth spreading unchecked on the Louisiana side, where Texas residents say the authorities have been preoccupied with Hurricane Katrina recovery, local advocates raised $35,000 for a two-mile net, put up in June, to seal off Caddo Lake’s more contaminated eastern half.

“We just stuck our necks out,” said Paul Fortune, a contractor who has lived his whole life on the lake. “We just did it.” But propagating leaves still float through gaps left open for boats, and are spread by the boats themselves.

Image Residents in East Texas are working to stop a noxious weed from smothering it. Credit... Michael Stravato for The New York Times

In one area of Louisiana, along a thicket of cypresses called the Big Green Brake, the Salvinia has already grown out into the lake as a luminescent green crust over the water. “It’s at the stage where it starts to lose its eerie beauty and starts to look like a real monster,” said Mr. Canson, the prow of his motorboat poking cracks in the matted covering like an icebreaker. Even flamethrowers have failed to kill it, he said. And beetles that devour the plant elsewhere die in the Texas cold.

Now chemical weapons have been thrown into the battle.

Mike Turner, a burly boat mechanic who calls himself part of the “Caddo Navy,” has set aside his business to go out daily in his small boat for $25 an hour to spray Salvinia infestations with a government-approved herbicide mixture of diquat and glyphosate and surfactants to make it stick to the leaves.

“It gets in the water hyacinth and it hides, like it’s a thinking animal,” said Mr. Turner, removing the surgical mask that protects him from the chemicals.

“I’m finding stuff that was not there two days ago,” he said, mopping his brow in the rising morning heat. He said he felt the task was hopeless at first and considered moving but changed his mind. When he was born 40 years ago, he said, his parents dipped his feet in the lake, and he did the same 12 years ago with his newborn daughter, Patte.