Ranchers and land managers are used to managing invasive plants on land.
Have you taken stock of what might be

Invading Your Water Space?

By Lorie Woodward Cantu

Mesquite, ashe juniper, salt cedar, prickly pear and a host of non-native grasses have changed the landscape of Texas. Managing these aggressive plants is a never-ending challenge on millions of
acres of rangelands.

Water managers are fighting similar battles with invasive aquatic species on lakes, rivers and
ponds across Texas. While giant salvinia, water hyacinth and hydrilla are not household names,
they pose a significant threat to the state’s water supply, its fisheries and recreational economy. Eurasian watermilfoil and water lettuce, while a nuisance, are less troublesome because they grow
more slowly.

“In general terms, these aquatic invaders reproduce and spread rapidly and persist,” Dr. Earl
Chilton II, program director of aquatic enhancement for the Texas Parks and Wildlife Department
(TPWD), says. “They succeed because the environment is favorable to them and there is a lack of
natural predators, herbivores, competitors, and diseases that normally regulate their populations.”

For instance, giant salvinia, which has been found in 20 locations in Texas, can double its area every
7 days, he says. Because of its aggressiveness, it is one of the biggest threats.

In the fall of 2009, giant salvinia flushed out of tributaries and backwater areas into Toledo Bend Reservoir on the Texas-Louisiana border. A mat estimated at 2,000 acres in size formed before the plant could be treated and brought under control.

Chilton recounted another experience with a private lake in East Texas. The 6-acre lake was separated from Swinney Marsh by a levy. The marsh had been infested by giant salvinia, but the lake had not, until a cow crossed the levy and got into the marsh. She picked up a few small giant salvinia plants and transferred them back to the lake. Within a month, the 6-acre lake was completely covered by giant salvinia.

In a severe infestation, the giant salvinia, which is a floating fern, can pile up on itself, creating mats that are 3 feet thick, he says. These vegetative mats remove oxygen for fish and can lead to fish kills. They impede boat traffic and recreational fishing. They shade out or crowd out native plants, drastically altering the ecosystem. Hydrilla and water hyacinth have similar impacts.

Additionally, these aquatic invaders are water hogs, Chilton says. Water hyacinth, with its huge floating leaves, provides an excellent example. Studies have shown that water bodies covered with water hyacinth lose 1.5 times to 13 times as much water to these plants as open water does to evaporation, he says.

Tenacious invaders
Hydrilla, which can be found in at least 100 Texas water bodies, illustrates the difficulties of containing and eradicating these invading plants, he says. Hydrilla can grow from plant fragments, picked up and transferred by boats, fishing equipment, birds and animals. The plants, which grow from the bottom, form tubers. The tubers, even under stressful conditions such as a water-table-lowering drought, can remain viable for a decade or more and resprout when conditions are right.

“Vigilance is important, especially if a landowner has people or animals coming and going,” Chilton says.

While the state has outlawed the possession of any of these plants (many were introduced through the commercial aquarium or nursery trade), they are now established in the wild and can be transferred by equipment that comes into contact with them.

It is vital that you monitor any boats that you allow into your lakes or ponds, including canoes and kayaks, he says. While TPWD has posted warning signs at infested lakes to encourage recreationalists to be aware and thoroughly clean their boats and equipment, not everyone complies.

“Once one of these invaders is introduced, the only hope for eradication is to catch the infestation early,” Chilton says. “If the plant establishes itself, then it becomes a control and management issue.”

Strategies to control aquatic invaders
There are 3 strategies for controlling and managing invasive aquatic plants. The regimen, which can be a combination of all 3 treatments, depends on the plant species and the severity of the infestation. The primary treatment options are water level manipulation, biological controls and herbicides. In some cases mechanical removal is also used.

Because aquatic invaders depend on having a certain water level to survive, water level draw downs are effective control measures. For instance, water managers control the Eurasian watermilfoil infestation in Lake Austin by drawing down the lake level 12 feet every other year. The draw down lasts for 1 to 1.5 months depending on conditions. During this time, the sun and wind kill the plants.

Herbicides can be effective, albeit expensive, control measures. It is important to match the herbicide to the invader and to carefully calculate the area and/or volume of the treatment area. Professionals with Texas AgriLife Extension can provide guidance. One danger with any chemical control method is the chance of oxygen depletion after the treatment caused by the decomposition of the dead plant material. Oxygen depletions can kill fish.

Biological controls also exist to help control invasive aquatic species. Giant salvinia, hydrilla and water hyacinth have natural predators in the form of giant salvinia weevils, water hyacinth weevils and hydrilla flies. The giant salvinia weevil has been an effective control in other parts of the world, and TPWD recently completed 2 rearing facilities to ensure that it could produce the insects in adequate numbers, Chilton says.

Hydrilla, on the other hand, is more effectively controlled by grass carp, an Asian fish that has been introduced to the U.S. to eat unwanted aquatic plants. A grass carp can eat up to 3 times its body weight in plant material per day. It’s important that the number of grass carp be matched to the size of the infestation. Too few grass carp will not have an effect and too many grass carp can clean up the infestation so effectively that they then begin to feed on native plants and disrupt the ecological balance further, Chilton says.

Because grass carp are non-natives and such voracious eaters, only sterile fish can be released under a permit from the TPWD. On average the department issues about 1,000 permits annually, Chilton says, giving an indication of the scope of the problem.

Protecting private waters from invaders
While TPWD is the lead agency for control on public water bodies, the department does not have funding for control on privately owned lakes and ponds. Landowners are responsible for containing and controlling these species on their own land.

“The best way to stop the spread of invaders is through education,” Chilton says. “People need to be aware of what plants cause problems and avoid spreading them. If they see invasive plants in areas where they’ve not been before, they need to contact their local Texas Parks and Wildlife Department office, so we can begin taking appropriate measures on a broad scale and advise the landowners how they can proceed locally.”

For more information about invasive aquatic plants, visit texasinvasives.org and AQUAPLANT: A Pond Manager’s Diagnostic Tool at aquaplant.tamu.edu.