Invasive Plant Lays Claim To Wild Areas Of South Florida

November 12, 1999

LOXAHATCHEE NATIONAL WILDLIFE REFUGE—An exotic vine that already covers 40,000 acres of South Florida wilderness is on the brink of an expansion that could make it the worst weed threat yet to Florida’s native plants, University of Florida researchers say.

Old World climbing fern, or Lygodium microphyllum, could quickly become the dominant plant in Florida wildlands because of its fast rate of growth and lack of natural enemies, said Ken Langeland, a researcher at UF’s Center for Aquatic and Invasive Plants, a part of the Institute of Food and Agricultural Sciences.

“Old World climbing fern is the worst of the worst,” Langeland said. “People who have worked to eradicate melaleuca and Brazilian pepper, two of our worst plant pests, say Old World climbing fern is much worse.

“It’s the worst weed we’ve seen in Florida’s natural areas to date.”

Old World climbing fern scares researchers and land managers so much because of how quickly it has gotten out of control. It reproduces via almost microscopic spores, which can be carried anywhere by wind or by wildlife.

“In the last six years, we’ve seen a hundred-fold increase in Lygodium in South Florida,” said Mark Musaus, manager of the Loxahatchee National Wildlife Refuge near West Palm Beach. “Left unchecked, it will be astronomical in the next few years.”

In Loxahatchee, the vine has climbed into the crowns of cypress and pines on tree islands, a unique feature of the refuge habitat. With a single leaf capable of reaching lengths of more than 100 feet, the vine knits quickly into a dense canopy. Sunlight can’t penetrate, and shrubs and ground covers die.

The fern canopy becomes so heavy that trees collapse. Some of the islands have become so infested that, from the air, they look like they have been draped with a giant green cloth. In short, where Old World climbing fern becomes established, nothing else can survive.

Dan Thayer, director of vegetation management with the South Florida Water Management District, says Lygodium’s devastation is cloaked in green, making it look innocent to the public.

“It’s hard to convince people that something green is bad,” Thayer said. “This is green, it’s pretty, it’s not a smokestack so it looks like it should be there. So people don’t realize it’s one of the worst forms of pollution around.

“This weed is one of our biggest fears,” Thayer said. “The district has purchased land because we wanted to preserve it, and this weed is destroying those lands.”

Flood won’t kill the fern, nor will drought. And the presence of the vine poses an elevated fire risk. Native plants that normally would survive naturally occurring fires, die when the vine is present because fires burn hotter. Massive clumps of the vine also break off easily, carrying burning vegetation on the wind and putting more acreage at risk of fire.

The only natural enemy of the plant is frost, making Central Florida its northern limit.

Old World climbing fern was first documented in South Florida near Jupiter in the late 1950s. It was brought from its home in Southeast Asia and Australia as an ornamental plant.

Because its spores travel on the wind, the fern is showing up in some of Florida’s most pristine and remote locations. Langeland said the fern recently was discovered in Everglades National Park.

“Our experience with other invasive plants has shown that plant populations tend to hit a critical mass and then begin an exponential rate of expansion,” Langeland said. “Lygodium has reached that critical mass.”

Eradicating it, said Thayer, would “cost more than people want to hear.” The focus, he said, should be on preventing its spread.

At Loxahatchee, Musaus is ready to wage war. Lygodium already is smothering more than 20,000 acres of the refuge’s tree islands, a habitat crucial to songbirds.

“If we don’t make this our No. 1 battle now, it won’t make a difference what else we’re working on, with wetlands or wildlife or other needs,” Musaus said. “If we’re overcome with Lygodium, nothing else will matter.”

Langeland said some herbicides destroy Lygodium, but they must be used carefully to avoid killing native plants. And even when Lygodium is killed, its thick curtains of vines are left behind, still blocking sunlight and taking about five years to disintegrate.

Researchers hope to discover natural enemies to help control Lygodium, but that process is time-consuming and expensive, Langeland said.

“There’s a cost to not doing anything, though,” Langeland said. “We need to examine our mind-set when we think of invasive plants. There is a cost to the natural environment that we can’t even put dollars and cents on.

“There’s a value of the natural habitat,” Langeland said, “just for the sake of having the natural habitat.”