“Good plants gone bad” is how an LSU AgCenter forester describes plants that have invaded the Louisiana landscape and created problems for farmers, forest owners and homeowners.
Hallie Dozier, an assistant professor in the School of Renewable Natural Resources, says the introduction of foreign plants often causes problems because they don't have natural enemies to control their spread.
The top two invasive plants in Louisiana are Chinese privet, also known as ligustrum, and Chinese tallow tree, often called popcorn tree. Chinese privet is particularly bad in forest service corridors for power lines, pipelines and access roads. Dozier calls it “the worst forest invader.”
“Things that make a good ornamental plant make a good invader,” Dozier said. “It flowers and fruits a lot and produces lots of seeds.”
Flowering plants are attractive to pollinators — primarily honeybees — and to “dispersers” — primarily birds, she says. “So people who like bees and birds in their yards like these plants.”
In addition to flowering profusely and producing seeds, other characteristics of invasive plants include the ability to thrive under a wide variety of growing conditions and tolerate significant above-ground damage.
Invasive plants can cost agricultural producers and landowners tens of millions of dollars for weeding, herbicides and research to control them, Dozier said.
In natural, uncultivated areas, these plants can be equally challenging, she said. Left unchecked, the Chinese tallow tree can transform woodland ecology, change the hydrology and even affect the microclimate where they grow.
For example, after Chinese tallow trees began growing in wet prairie soils in southeast Louisiana, the environment was transformed to woods, changing the ecosystem. “That changed all the rules for all the animals,” Dozier said.
Dozier said insect and disease resistance are common among invasive species. These traits not only make them hardy in the wild but also allow them to thrive in cultivated locations.
Kudzu was introduced for erosion control in Southern landscapes that had been denuded as a result of railroad expansion in the late 19th century. Now, kudzu engulfs almost anything in its vicinity, growing as much as a foot a day under the right conditions.
Other common attributes of invasive plants include being evergreen, so they grow through winter; not being picky about light or soil conditions, so they grow almost anywhere; and fast-growing, so they're difficult to manage. In addition, many also reproduce asexually — through rhizomes and underground roots — so they can be spread by cuttings as well as by seed.
Invasive species provide competition for tree growth in Louisiana forests, and some species, such as Chinese tallow, get in the way because loggers won't harvest it, said Bill Gallagher with GR Forestry Inc. in Amite, La.
Gallagher said he has seen privet as tall as 30 feet in streamside management zones. “You could ride a horse under it,” he said. “The canopy is so dense nothing can grow under it.”
Gallagher estimates that without good site preparation, a tree stand could be as much as a third less in value after 30 years because of invasive plants.
Cogongrass is a recently introduced species becoming a problem in Louisiana, primarily in the Florida parishes, where conditions are conducive for its growth, Dozier said. It's moving west from Florida and Alabama, where it was introduced as forage for grazing cattle.
“Unfortunately, it's only good for grazing when it's small,” Dozier said. “When it grows taller than a few inches, it becomes rough and unpalatable because of silica in its leaves.”
Experts believe cogongrass has been moved mostly by the tires of heavy equipment working in pipeline and power line rights-of-way. The plant reproduces by rhizomes as well as seeds, so small clumps can be moved unintentionally on vehicles and grow where they're dropped.
“Cogongrass is in the top 10 worst weeds in the world,” Dozier said. “Worldwide it costs farmers a lot and causes ecological damage in woodlands — pine forests in particular.”
Cogongrass is starting to appear as a forest problem in Louisiana, mostly in power line and pipeline rights of way, said Cal Baker of the U.S. Forest Service in Pineville. The grass takes over a site and chokes off the forest floor, eliminating habitat for wildlife.
“It's now taking off in Louisiana,” said Baker, who's on the ecosystem conservation staff in the Kisatchie National Forest.
He said he's seen patches of cogongrass south of Leesville near Colfax and is concerned about its effects on forestlands. The grass forms a dense mat about a foot high and has a high silica content.
“It crowds out every other species and competes with establishing trees,” Baker said. “It's a struggle to get other species back in the ground.”
Baker said the Forest Service is trying to keep the invasive grass out of the forest by monitoring and using herbicides before it gets out of hand.
Baker said kudzu and trifoliate orange are other invasive species that trouble forests in central Louisiana. And privets are “just everywhere,” he said. “Controlling them is like swatting mosquitoes.”
Baker also said Japanese climbing fern, Chinese tallow trees and mimosa trees are problems.
“With kudzu, we still have the ability to control it with early identification,” he said.
Baker said controlling invasive plants requires a collaborative approach among all concerned.