Soundlessly the invaders came, to wage their battles across the American landscape. They arrived from all over the globe: from Russia, Europe, Asia, even Africa. Clothed in luxurious garb of petal and leaf, some foreign plants were carried by the colonists and carefully nurtured; others hitchhiked as seeds hidden in the food of livestock or in the soil used as ballast in old ships and then dumped on our shores.
Today, foreign plants have so insinuated themselves into the American landscape that in some cases, they define it: most people consider bright blue chicory, oxeye daisies, Queen Anne’s lace, dandelions, and crabgrass as “typical American” plants; Los Angeles is well known for its palm trees. But none of these plant species existed in pre-Colonial America. American fields, forests, and marshes are filled with invaders and vagahonds.
While many of these plants have been around for more than two centuries, biologists say their numbers have skyrocketed in the past twenty years. With an explosion in bulldozing, road building, and other construction, the opportunistic aliens quickly colonize the altered landscape, taking over drained and flooded wetlands, roadsides and cleared lots. Trucks and railways hasten their spread, carrying seeds of the foreigners in hayloads. As a result, foreign plants that gained a toehold a century or two ago have run rampant; new introductions take off almost immediately.
Some of these aliens, such as oxeye daisies, have become among our most beloved wildflowers. But other foreign plants have become public enemies: clogging waterways, disrupting natural ecosystems, and strangling agricultural crops. According to the USDA, the ten most serious crop-threatening weeds in America today are all foreign plants; American farmers lose more than $10 billion worth of crops each year to introduced weeds.
The invaders are so successful because in coming here, “they left their own biological control agents behind,” says Gerald Henke, a U.S. Forest Service range conservationist responsible for noxious-weed control. Without predators like the insects that kept their numbers in check at home, the aliens spread unimpeded.
“They keep expanding exponentially,” says Norm Reese, a USDA research scientist in Montana. Take, for example, leafy spurge, a toxic Eurasian plant that sickens cattle and irritates skin. “In the ’40s, a few places started finding it. In the ’50s, it was classified as a noxious weed. And in the 1980s, all of a sudden you realized you were in serious trouble.” Now the weed is found from California to New York.
In Alabama, kudzu, a high-climbing Japanese vine, covers trees, railroad beds, cars, buildings—“anything that sits still long enough,” according to Dr. Jim Miller of the U.S. Forest Service at Auburn. Growing at the horror-movie rate of a foot a day, it fetters stationary railroad cars, overtakes homes, and brings down transformers and telephone wires with its weight. The vine’s spread, Miller estimates, is costing Alabama, Georgia, and Mississippi between $50 million and $175 million a year in lost timber revenues alone.
In Massachusetts, the attractive purple loosestrife, a Mediterranean native with a tall crown of pink-purple flowers, has rendered some waterways in wildlife refuges virtually useless. Taking over marshes and ponds once inhabited by American cattails, bulrushes and sedge, purple loosestrife has usurped the native plants that provided food and cover for rare native birds like the American bittern, the common moorhen, and the king and sora rails. Where loosestrife now reigns, these birds have vanished.
In North Carolina, multiflora rose, a bush imported from eastern Asia, has overrun more than 2 million acres of meadow. The spine-covered beauty was once encouraged as a natural fence; today, it envelopes the fields it was supposed to enclose, rendering them impassable and useless to man and beast.
Perhaps the greatest irony of the alien invasion is that many of the noxious plants arrived as pampered, invited guests, intentionally imported and carefully nurtured by Colonists and their followers. Even folk hero Johnny Appleseed was guilty of this mistake. Besides leaving behind orchards, he also introduced a nasty weed—dog fennel—that plagues Midwestern farmers to this day.
Today, farmers, foresters, and agricultural agents are discovering that many of the plants they encouraged are now nearly impossible to get rid of. Kudzu, for instance, was introduced to the South in the 1920s as a forage crop for pigs, goats, and cattle—a situation Jim Miller likens to “taking a lion cub home for a pet.” By the 1970s, its graceful purple blossoms had overrun seven million acres in Alabama, Georgia, and Mississippi, covering fields, forests, and homes. “I get calls from little old ladies who tell me their husbands have died and the kudzu’s taking over their houses. They get nightmares about it,” Miller says. By the time kudzu was recognized as a problem, many patches had developed root systems weighing up to three hundred pounds, stretching twenty feet underground.
Managers of the Parker River National Wildlife Refuge on Plum Island in Massachusetts are still trying to figure out what to do about the purple loosestrife choking their three freshwater impoundments. Saving the pools from total takeover, explains Bob Secatore, assistant manager of the 4,650-acre bird sanctuary, “isn’t just a matter of dollars, it’s how to go about it.” Burning or running over it with tractors won’t kill it, and little data are available on pesticides’ effectiveness on loosestrife.
Getting rid of some of the unwanted aliens has called for some unusual approaches. Hydrilla, an aquatic weed imported from Central Africa and Southeast Asia as an aquarium plant, spread through two hundred thousand miles of California and Florida canal and river systems until many waterways were completely clogged. To the rescue came a hydrilla-eating fish, the grass carp. (The carp are sterile hybrids, so they cannot become pests themselves.)
USDA researchers have also tried importing foreign bugs to eat foreign plants, but scientists have to be extremely careful they don’t select solutions that become worse than the problem. For instance, to try to check the growth of the musk thistle, a thorny weed from Europe with bright pink flowers that grows up to eight feet tall, researchers looked at eighty-nine species of foreign insects known to feed on the plant. Of those, only twenty-six were detrimental to the weed, but twenty-one of those species could also damage the U.S. artichoke crop, a plant distantly related to the musk thistle.
Some insect controls have proved very useful; one European beetle, Chrysolina quadrigemina, has cut the spread of the Klamath weed (a.k.a. goatweed) by 90 percent. The Eurasian plant’s oil glands sicken cattle, and it was such a serious threat to western rangeland that during World War II, banks would not loan on Californian properties infested with it. A statue of the beetle now stands in tribute to its success.
Many scientists agree that most of the foreign plants which have already become well established are here to stay. But at least there is this consolation: we gave Europe poison ivy.