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1. Redefining the Weed
by Janet Marinelli
Used to be, gardens were tiny enclaves in a vast wilderness. Beleaguered gardeners struggled daily against the forces of nature, not the least of which were weeds—hence the traditional definition of a weed as any plant from outside the garden that ends up inside the garden where it isn't wanted. But, boy, have the tables turned. Today, around the globe, shrunken fragments of once-awesome wilderness are hemmed in by human-dominated land. Now it is our activities, including our gardens, that threaten natural areas and the creatures they harbor. Hundreds of species that we've carried from their native ranges to new areas, including prized horticultural plants, have overrun native vegetation. These have become the true weeds of the modern world.
People have been rearranging the planet's flora for centuries. Many of the exotic plants we've introduced by intention or accident have been beneficial to us and ecologically benign. But a small percentage have run rampant. Gaining a foothold first in areas disturbed by human activities, they moved into natural areas where they've not only driven out indigenous species but in the worst cases radically altered the ecosystems they've invaded.
In 1993, after an extensive review of exotic species, the Congressional Office of Technology Assessment (OTA) concluded that pest plants and animals have an effect not only on natural areas but also on agriculture, industry and human health. In its report, Harmful Non-Indigenous Species in the United States, the agency noted that from 1906 to 1991, just 79 problem plants and animals caused documented losses of $97 billion, and that a worst-case scenario for a mere 15 potentially high-impact species could cause another $134 billion in future economic losses.
Thousands of non-indigenous plant species are known to persist outside of cultivation in the United States. How many of these are plant invaders? In the course of researching this book, we tallied the species on the two most comprehensive national natural areas weed lists ever compiled: one of plants being reported as problems on Nature Conservancy preserves nationwide, and another compiled by the National Association of Exotic Pest Plant Councils, an umbrella organization of state groups concerned about the ecological impact of invasive non-native species. After submitting the combined list to various state weed authorities for their additions and subtractions, we arrived at a total of just over 300 plants invading wildlands in the 49 continental states and Canadian provinces. (We didn't have room for the plant invaders of Hawaii, which could easily fill an entire volume.) According to our calculations, about half of the 300 continental plant invaders were brought here to beautify our gardens. A much tinier fraction of native plants are showing signs of invasiveness, but there is considerable disagreement over why and what kind of threat they pose; this, too, is a subject for another volume.
The problem is surprisingly widespread. Hawaii, California and Florida appear to be the hardest hit, but few—if any—regions of the U.S. and southern Canada are without non-native pests. According to The Flora of North America, the most comprehensive reference on this continent's plants, one-fifth to one-third of all species growing north of Mexico have come from other continents.
This ground-breaking handbook focuses on 80 of the invasive plants used horticulturally. These include the most serious invasives, such as purple loosestrife, which are so widespread that they'd be found on any "most wanted list" of plant invaders. Others, such as baby's breath, are problems in a geographically limited area-in this case, Great Lakes dune systems-but threaten unique habitats or rare plants. A few, like dame's rocket, right now appear to be only slightly invasive but are on weed experts' "to watch" lists because it often takes decades for a plant to begin spreading out of control—and once invasives are well established they are extremely difficult to manage.
In the chapter that follows, co-editor John Randall, who regularly criss-crosses the country to study invasives for The Nature Conservancy, explains how problem plants damage natural areas and why some introduced species become pests while others don't. Next is a section on the tools and techniques to control invasive plants, from hand-pulling to prescribed burning to, as a last resort, chemical herbicides.
The core of the book is an encyclopedia of invasive species used horticulturally, which was written by many of North America's leading weed experts and natural areas managers from 20 states and the District of Columbia. The encyclopedia is organized by plant type: trees, shrubs, annuals and perennials, grasses, vines and aquatic plants. To find out which ones are problems in your area, look in the index and in individual encyclopedia entries under the section "Where has it spread?" Each plant write-up will not only help you identify invasives and understand their ecological impacts but also tell you which to avoid planting and how to control plants already on your property that threaten nearby natural areas.
Scientists don't yet know whether some invasives represent permanent threats to the Earth's biodiversity, or whether they will become less dominant over the long haul as the plant communities they've invaded change and mature. In any event, the most prudent course of action is to avoid planting these species—because when it comes to invasive plants, as land managers have learned the hard way, an ounce of prevention is worth a pound of cure.
Brooklyn Botanic Garden Director of Publishing Janet Marinelli is editor of BBG's renowned series of quarterly gardening handbooks and the author of Your Natural Home and The Naturally Elegant Home. Janet is a champion of the gardener's role in the preservation of the planet, a philosophy that informs her P&G News column, "Down to Earth." It's a philosophy that also serves as the bedrock for her latest book, Stalking the Wild Amaranth: Gardening in the Age of Extinction. In Stalking the Wild Amaranth, Janet tells of her quest for a landscape art that protects disappearing species, both flora and fauna. It's a gardening journey marked by humor—ecologically sensitive gardening needn't be a dreary affair, Janet insists. "We can do our part," she says, "and still have flair and fun."