NEW MARKET, Va. -- Some of the most colorful exotics arrived in the colonies destined for such planter notables as Thomas Jefferson and James Monroe.
Hundreds of other plant imports came later, brought to the New World from Asia and Europe because they excelled at doing certain jobs, such as covering unsightly spots in yards or woodlots.
Others worked their way into the ground through the good intentions of neighborhood nurserymen who were trying to help solve one gardening dilemma or another. Still others were brought across borders by birds, dumped into harbors along with ship ballast water or were scattered from the tattered pant cuffs of overseas visitors.
But that was the innocent then; this is the invasive now.
Many of the exotic plants have grown wild over the intervening decades and are shouldering aside or literally smothering native species.
The prolific nonresidents have been declared planta non grata: noxious weeds accused of reducing wildlife habitat, destroying pastures and rangelands, screening utility line rights-of-way, choking lakes, streams and canals.
They are changing entire eco-systems, hybridizing, creating wildfire hazards where none previously existed or further drawing down water tables in semi-arid areas.
Losses to harmful non-indigenous plant species in the United States run in the hundreds of billions of dollars, the Congressional Office of Technology Assessment says.
An estimated 400 of the 958 species listed as threatened or endangered by the federal Endangered Species Act are at risk, for the most part, because of competition with non-natives, according to a Cornell University study.
The reference book "Flora of North America" says one-fifth to one-third of all plant species growing north of the Rio Grande arrived from other continents.
Granted, not all non-native plants are bad. Wheat, tulips, peonies, rice and corn, among others, have become outstanding citizens.
But other exotic plants can be deadly, particularly to resident species that can't compete.
The purple loosestrife is Problem Plant No. 1 on many preservationist hit lists.
Pioneers used it as a medicinal herb to help treat dysentery or ulcers and sores. But the colorful perennial can produce up to 2.7 million seeds per plant per year and spreads across an estimated 1 million acres of wetlands annually, federal officials say.
Blanketed by kudzu
And then there's kudzu, the consummate clinging vine. The leafy, high-climbing plant was introduced into the United States from Japan at the Philadelphia Centennial Exposition in 1876. There, it was promoted as a forage crop and ornamental. Farmers in the South were paid as much as $8 an acre in the mid-1930s to plant kudzu for reducing soil erosion.
But it was declared a pest weed by the U.S. Department of Agriculture in the 1950s after growing as much as a foot per day, blanketing everything in its path and uprooting or snapping trees beneath its weight.
A coalition ranging from environmentalists to government agencies, garden clubs to commercial growers, is working toward halting the spread of the insidious invaders.
Among other things, they're urging gardeners to:
Avoid disturbing natural areas, which increases their vulnerability to non-native species.
Leave behind any plants or fruits you may find desirable when traveling abroad.
Become a volunteer in the hand-to-hand campaign to remove invasive plants.