Exotic plant control requires a variety of tools

WEST PALM BEACH — Exotic plant control was the topic for the Water Resources Accountability and Collaboration (WRAC) meeting May 28 online meeting.

SFWMD STA and Invasive Species Section Leader LeRoy Rodgers said invasive plant species have a big impact on the SFWMD’s mission.

“We want to protect the progress of Everglades restoration efforts from the encroachment of invasive species,” he explained.

“We want to protect our flood protection and water supply system from nuisance vegetation,” he said. In addition, “stormwater treatment areas rely on vegetation to reduce phosphorus from those constructed wetland system.”

Invasive exotic species are plants that are not native to the ecosystem. Introduction of these plants causes or is likely to cause economic or environmental harm or harm to human health, he explained.

Nuisance vegetation is the term used for native plant species that become problematic in disturbed areas or interfere with infrastructure. For example, the native cattail, due to nutrient loading, has become a very dominant and aggressive plant in parts of the Everglades and interferes with wildlife habitat.

South Florida is rather famous for a wide variety of invasive species issues, he said.

The mild climate, international commerce, large ornamental plant industry and dense ares of human population are all factors that contribute to the problem of invasive species.

It’s a “perfect storm for biological invasions,” he said.

Not all exotic plants are bad for the environment, he continued. SFWMD has documented 1,005 non-native exotic plants within the district. Of those, only 76 are considered category 1 invasive species and only 18 are widely established or locally abundant on SFWMD managed lands and waterways.

Proliferation of these species leads to alteration of structures and processes in the environment, he said. For example, Everglades marshes have been completely taken over with Australasian melaleuca trees. Invasive vegetation can also clog waterways. Vegetation on the levees must be managed, he added, so the levees can be inspected.

Examples of invasive plants include:
• Floating aquatic weeds are predominantly water hyacinth and water lettuce. These plants rapidly reproduce and quickly dominate open water habitat, he said.
“In our STA program, these are considered very undesirable because they strongly impact the health of the vegetation that we need to remove phosphorus from the system,” he added.
High nutrient concentrations in Florida waterways accelerates the growth of these weeds, said Mr. Rodgers.
• Another aquatic weed that is very well known is hydrilla, he said. “Left unmanaged, hydrilla can aggressively fill an entire water body.” It can slow the flow of water and clog water control structures.
• Old World climbing fern is native to Southeastern Asia. This plant rapidly dominates all of the plant communities, he said. The plant is very flammable and will allow wildfires into areas that would otherwise not be at risk of fires.
• Napier Grass dominates disturbed upland areas, forming dense thickets. “This is a major issue on our levees,” he said.
Vegetation control tools used by SFWMD include prevention, biological, mechanical, chemical and cultural controls, he said.
• “Prevention is a very important part of controlling invasive plants,” he said. Decontamination protocols are used to prevent spreading of “hitchhiker” plants by boats and equipment.
“Prevention is only successful as the involvement of the public,” he added.
• Biological control is the use of other organizations to reduce the plant population. For example, SFWMD uses sterile grass carp to consume aquatic weeds in canals. Insects that evolved with an exotic species can also be used to control that species. “Biocontrol agents do not always eradicate the invasive species,” he explained, but they do control the growth of the exotic species.
• Mechanical control is the physical removal of vegetation. SFWMD uses harvesters and shredders. “You have to take care to minimize sediment disturbances and you have to deal with large quantities of plant matter to dispose,” he explained.
• Chemical control is the use of EPA-approved herbicides.
• Cultural controls include prescribed fire, management of nutrient inputs, water depth changes and planting desirable plants. Fire is part of the natural system for many Florida ecosystems.

He said they also used a combination of tools to manage invasive plants. This is called integrated pest management. This has allowed SFWMD to reduce use of chemical herbicides.

“Florida Fish and Wildlife Conservation Commission (FWC) has been highly criticized for their vegetation management program in Lake Okeechobee,” said Gary Ritter of Florida Farm Bureau. He said it appears SFWMD has made more progress with mechanical harvesting. He asked what SFWMD does with the vegetation biomass they remove from the water.

Mr. Rodgers said SFWMD coordinates with other agencies such as FWC. SFWMD Vegetation Management System Section Administrator Francois Laroche said FWC has the lead role for plant management on Lake Okeechobee.

He said the disposal of vegetative biomass is an issues with mechanical harvesting. “Currently we are depositing the excess biomass on spoil islands within the lake,” he said.

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