LSU reversing gains of invasive weed

We sprayed it, we dried it, we burned it and we salt-watered it,” Ben Welch said as he stood next to his air boat in the Cameron Creole marsh near Cameron, La.

The “it” Welsh referred to is Salvinia molesta or giant salvinia — an invasive weed that forms dense mats, which sometimes are several feet thick. The weed was first identified in Cameron Parish in 2001.

The fast-growing aquatic plant can choke a waterway and kill plants and animals underneath its thick covering. It was first discovered in Louisiana at Toledo Bend in 1998 by a biologist with the Sabine River Authority.

Dearl Sanders, a weed scientist with the LSU AgCenter and research coordinator of the LSU AgCenter's Idlewild Research Station at Clinton, La., said the weed can clog irrigation systems and even compete with rice crops if it's introduced into rice fields. Native to South America, the plant has been a major problem for Brazil's rice industry and poses a threat to Louisiana farmers.

Welsh, owner of Welch's Airboat Service, has a contract with the Cameron Parish Police Jury and the Cameron Parish Drainage Board to control giant salvinia in 4,000 acres of the marsh known as the Henry-Davis property. The infested region essentially is blocked off from other areas by high levees, roads and ridges.

“We need to prevent salvinia from moving out of the area,” Sanders said.

When it was first identified in Cameron Parish, giant salvinia was 4 to 5 feet thick against bridges, and observers identified an area 5 to 6 miles long and as wide as 2 miles on the Cameron Drainage Canal.

Since then, Sanders and Kevin Savoie, an LSU AgCenter area fisheries agent in Southwest Louisiana, have been working with the local agencies and Welch to control the weed.

In the hope of finding a less-expensive alternative to chemical control, Sanders and Savoie tried salt water this past spring. The Henry-Davis property was pumped nearly dry and flushed with salt water from the Intracoastal Waterway.

“Salt water did more good than anything else,” Savoie said.

The giant salvinia hasn't reappeared in areas of the marsh where salt water was introduced, officials said.

On the other hand, in areas where levees held back the salt water, giant salvinia presents a never-ending battle being fought with chemicals and other management tools.

Sanders, who said the Cameron Parish infestation is the only one he knows that's close to salt water, is recommending the area be pumped dry and flushed with salt water yearly for about three years.

“Where salt water can get, we just annihilated it,” Welch said.

In other spots, Welch still fights the weed with frequent herbicide applications.

One area was completely covered in May, but after spraying, it was mostly clear in mid-July, he said. All that remain are “little pockets.”

“It's like hide and seek,” Welch said. “It's not there one day, but it's there later.”

Since May 1999, Sanders has been looking for the best method to control giant salvinia. He found that a herbicide called Reward — which is available for use in the state — is the best means of controlling the weed.

One drawback, however, is expense. Reward costs about $80 per acre for the chemical, and the application cost adds even more to the expense.

In Cameron Parish, the Louisiana Department of Wildlife and Fisheries has supplied the Reward, and the local agencies have paid for the application.

One aspect of marsh management is controlled burning, Savoie said. Landowners periodically will burn off the dead, dry grasses on dry ground that emerges during periods of low water.

It was during a burn that Welch discovered fire is not a friend of giant salvinia.

When his crew burned some dry cover, the heat “baked” the salvinia, Welch said. That salvinia didn't come back.

“We have the benefit of a contractor who knows marsh management,” Savoie said about Welch.

The Henry-Davis property is used for duck hunting and to graze cattle, Savoie said. But when open water is covered with giant salvinia, the ducks can't land. And even if they did, they'd have nothing to eat.

Since the salvinia control program started, wigeongrass and other grasses that provide food for waterfowl and wildlife are coming back. “We are putting the area back into productivity,” Savoie said.

In addition to its threat to recreation and fisheries by clogging lakes, bayous and other waterways, giant salvinia threatens agriculture throughout southwest Louisiana because it can get into the waterways used in rice production. Sanders said in one area of Brazil, the weed was responsible for a 30-percent loss in rice production.

“There's a direct water link between here (Cameron Parish) and rice-growing areas of the state,” Sanders said. “That's where the threat comes in. We're looking at a 30 percent to 40 percent rice yield reduction potential and as much as 75 percent reduced water flow in canals.”

Rick Bogren writes for the LSU AgCenter.

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