Recent Flooding, Cold Weather Curbing Invasive Giant Salvinia Growth in East Texas

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AUSTIN – In 2017, Caddo Lake was covered by more than 6,000 acres of invasive giant salvinia that was reproducing rapidly due to a warm winter and unusually long growing season. Access for anglers, boaters and waterfowl hunters was blocked on much of the lake, and hired contractors struggled to keep up with the demand for herbicide treatments due to the sheer size of the growing giant salvinia mats.

Two years later, Texas Parks and Wildlife Department (TPWD) biologists report giant salvinia at Caddo Lake has been reduced to just over 1,500 acres.

“We can thank Mother Nature for the cold snap and subsequent flooding that caused a massive decrease in giant salvinia on Caddo Lake and at our other infested East Texas lakes,” said John Findeisen, Brookeland Aquatic Habitat Enhancement Team Lead. “A hard freeze in January 2018 made a huge dent in the giant salvinia, freezing much of the plant material and loosening the dense mats.  This was followed by high water inflows a month later, pushing the remaining giant salvinia to the open lake where it was destroyed by wind and waves and washed up on shore.”

Findeisen said the outlook for the 2019 treatment season looks good with much of the giant salvinia flushed out from the recent flooding. For the remaining giant salvinia present in East Texas reservoirs, Findeisen said herbicide applications will do the majority of the heavy lifting to keep it under control. Since September 2017, TPWD and its contractors have treated 18,390 acres of giant salvinia statewide.

“Natural events that help reduce the amount of giant salvinia in lakes and flush it out are great, but there is still a need for herbicide treatments to maintain control of the remaining giant salvinia,” Findeisen said. “Additionally, we are utilizing giant salvinia weevils in areas where flooded timber is too thick to navigate a spray boat or areas where the giant salvinia is mixed with beneficial vegetation that we want to preserve.”

Since September 2017, TPWD and the Caddo Biocontrol Alliance have released a total of 394,616 giant salvinia weevils to help control the invasive plant in East Texas lakes. Although they are a piece of the overall control effort, Findeisen said one of the problems with relying heavily on weevils is they cannot tolerate as cold of temperatures as the giant salvinia, and weevil numbers decreased substantially after the January 2018 cold snap.

“We did document some over-winter survival last year but for the most part we are having to completely rebuild most of the giant salvinia weevil populations,” Findeisen said.

Boaters have a critical role to play to prevent giant salvinia from spreading to any new lakes. Before traveling from lake to lake, boaters need to clean, drain and dry their boats, trailers and gear to ensure they aren’t transporting any invasive species. A short video tutorial on how to do this properly can be viewed online at

Boaters recreating on one of the 22 lakes infested with giant salvinia should be particularly vigilant about taking these actions. Texas lakes currently infested with giant salvinia include Caddo Lake, Toledo Bend Reservoir, Sam Rayburn Reservoir, Lake Texana, B.A. Steinhagen Lake, Lake Murvaul, Lake Palestine, Timpson Reservoir, Lake Naconiche, Lake Fork, Lake Nacogdoches, Lake Athens and Martin Creek Reservoir.

“We need boaters to take the necessary steps to keep it from moving into new lakes,” Findeisen said. “I am hopeful we are going to see less of it spreading this year because we are seeing an increase in piles of giant salvinia around boat ramps, which suggests people are doing their part to help protect our lakes by cleaning their boats and equipment.”

In Texas, transporting prohibited invasive species is punishable by a fine of up to $500 per violation. Boaters are required to drain all water from their boat and onboard receptacles before leaving or approaching a body of fresh water in order to prevent the transfer of aquatic invasive species like giant salvinia. Other species of concern in Texas include zebra mussels, crested floating heart, water hyacinth, yellow floating heart, hydrilla and American lotus.

Because early detection is an important part of reducing or eliminating the presence of giant salvinia, TPWD encourages boaters to report new sightings to (409) 384-9965,, or via the online report form.

Visit for more information or text TPWD GS for updates on giant Salvinia.

For pictures of giant salvinia, visit the TPWD Flickr.