Leafy Invaders Creep Into US Soil and Sink Tough Roots

American forests, roadsides, prairies, vacant lots, wetlands - even backyards - are under siege from a seemingly unstoppable army of foreign invaders.

These attackers are plants that have been transplanted from overseas and are reproducing so quickly and are so robust that they are crowding out native American plants and literally taking over the countryside.

If the trend continues, an ever widening sphere of indigenous plants and animals across North America could be pushed toward extinction, experts warn.

"This is not just a question of aesthetics, it is the balance of the natural world," says Richard Moyroud, conservation chairman of the Florida Native Plant Society. "There is a disequilibrium. The balance has been upset."

The invaders have names like leafy spurge, purple loosestrife, and Chinese tallow. Some are beautiful trees that can grow 50 feet tall. Others are grasses or herbs with attractive flowers. But they all have one thing in common: the ability to rapidly reproduce at the expense of native plants.

Estimates are that invasive foreign plants are spreading at a rate of 4,600 acres a day. The problem is widespread: 61 percent of national-park supervisors reported problems with foreign plants last year, and 12 percent of Nature Conservancy land stewards - overseeing more than 1,500 private preserves - said invasive foreign plants now pose the most dangerous threat to the ecological health of their preserves.

The explosion of pest plants in the US is the result of several causes - from exotic species escaping from gardens to misguided efforts to stop erosion along new highways by employing foreign vines.

In some cases, plant seeds and seed-laced grasses have been used as packing materials by foreign businesses. In other cases the seeds may have been carried by migrating birds, or hitchhiked in dried mud on the soles of hiking boots of travelers returning from overseas. Many of the invaders were simply transplanted by globetrotting horticulturists, who didn't realize what they were doing.

Some foreign species are so invasive, experts say, because they lack natural predators that held plant populations in check in their home ecosystem. Once in the US, there are no bugs, birds, or other animals to eat the plants because North American animals have never seen them.

"This is a green version of pavement," says Maria Minno, an environmental consultant in Gainesville, Fla. When foreign plants invade they crowd out native plants and drive animal life away. "The biodiversity is just gone," she says.

A study released last year by the Arlington, Va.-based Nature Conservancy found that invasive foreign plants and nonnative animals contributed to the decline of 42 percent of 958 threatened or endangered species. For 18 percent of species on the list, invaders were "the major cause of endangerment."

Of the roughly 4,000 nonindigenous plants in North America, several hundred are seen as potential pests, and 50 are considered particularly dangerous.

But getting rid of these dangerous plants can be difficult and expensive. Cadres of self-described weed warriors are sprouting up across the country.

Faced with thousands of acres of these plants, weed warriors have had to abjure the traditional pull-them-out-by-the-roots technique in favor of more sophisticated, expensive methods.

One of the most promising campaigns began recently in Florida, where scientists are encouraging hundreds of imported insects called Australian weevils to attack thick stands of melaleuca trees. The trees, brought to Florida's wetlands from Australia to help drain the water from tracts of swampland, worked better than anticipated. Soon melaleuca began to take over large sections of the Everglades, leaving dry land. The stands were so thick that it took a chain saw to walk through them.

The hope is that by bringing the tree's natural predator to Florida, the melaleuca population can be brought under control.

Plant specialists say much progress has been made in realizing and combating the problem. But the US still hasn't discovered a way to prevent invasive pest plants from entering the US.

Greg Jubinsky, chairman of the Florida Exotic Pest Plant Council, says the US needs a screening process to weed out potentially harmful plants before they arrive. He says the Chinese tallow tree was identified as a pest in the 1970s, but it took 18 years until it was widely recognized as a destructive plant. "The natural areas are being bombarded with the seed from these plants," he adds. "We can't spare 18 years to determine whether a plant is going to be a pest or not."