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Invasive Plants—What Every Gardener Needs to Know
Plants & Gardens News | Volume 15, Number 1 | Spring 2000
by Janet Marinelli
The conventional wisdom, at least in American gardening circles, used to be that most invasive plants arrived on this continent accidentally—in agricultural seed stocks, say, or even on the bottom of somebody's shoes. So, during the course of researching Brooklyn Botanic Garden's handbook on invasives, we were shocked to discover that about half of the worst invasive plant species degrading natural areas throughout North America were brought here intentionally, for horticultural use.
A bit of background for anyone who's yet to notice purple loosestrife or pampas grass or some other pest species devouring huge chunks of the natural landscape: Invasive plants, the overwhelming majority of which are not native to the region, spread like wildfire across parks, preserves, roadsides, and other wild lands, destroying the ancient native species and biological communities in their path. In fact, most scientists now consider invasive species to be one of the top two threats to this planet's native plant life (the other is habitat loss). Yet invasive plants are still commercially available, and a few of them are wildly popular. Japanese barberry, for example, is one of the hottest-selling plants in the nursery trade, and Norway maple is one of the most widely planted trees in the country.
So what can we gardeners do to avoid introducing invasive species when we choose plants for our properties? Unfortunately, it's not easy to find information on the invasiveness—and especially, the potential invasiveness—of any particular plant we may want to grow. Following is a guide to the kind of information that is currently available, as well as some preliminary guidelines for gardeners who want to avoid introducing new invasive species.
Info on Current Invasives
There is an increasing number of web sites hosted by state, regional, national, and international groups organized to educate the public about invasive plants (see Related Links). These sites include lists of the worst invasive species in the area—plants that gardeners definitely should not grow. For example, the web site of the Invasive Plant Council of New York State is the best place to go for information on the plants that currently pose the greatest threats to the biodiversity of New York's natural areas.
It's much harder to find information on plants that are just beginning to show signs of invasiveness in a region. Yet it's extremely important for us gardeners to be aware of these plants, because identifying and stopping invasives at an early stage is critical. There's only so much that gardeners can do to stop an invasive species that has already swamped natural habitats, but we can play a key role in preventing new invasives from gaining a foothold in the wild. For example, Chinese silver grass (Miscanthus sinensis) has become a very popular ornamental grass in recent years, yet it is becoming invasive in some parts of the eastern United States, including southeast Pennsylvania. For this reason, it is probably not wise for gardeners in the Mid-Atlantic and regions with similar climates and growing conditions to grow this grass.
One good way to check the potential invasiveness of any plant you're thinking of growing is a Nature Conservancy web site that includes a comprehensive list of plants currently causing problems nationwide, as well as "weed alerts" to warn gardeners and others about species that have the potential to become pests. Another great resource is the immense database of invasive weeds around the world compliled by Australian researcher Rod Randall; think twice about growing any plant that has become invasive somewhere else in the world. A list of potential invasive plants in the tri-state New York metropolian area has been compiled by botanists at the Brooklyn Botanic Garden.
Selecting New Plants
It's hard enough to obtain information on plants that are already invasive, or just becoming so. What about the new plants that are constantly being introduced by the nursery trade? This is a very important consideration, because many gardeners now lust after new, exotic plants with a passion that hasn't been seen since the Victorian Era. This wouldn't be worrisome if it weren't so difficult for scientists—no less gardeners—to predict which plants will become invasive if introduced to North America. There are a few computer models that offer some guidance on whether or not a new woody species is likely to become invasive. One of the best models for evaluating the potential invasiveness of woody plants in North America was formulated by Sarah Reichard, a biologist at the University of Washington in Seattle.
How can gardeners utilize information like this? Using Reichard's findings, I've extrapolated some tips for gardeners who want to avoid introducing invasive species when they make woody plant selections:
- Use native plants. It's important to emphasize that the overwhelming majority of invasive plants on this continent are not native to North America. When you choose a plant native to your area, you can be almost totally certain that you won't be unleashing the next invasive menace. Planting regional natives also makes sense because indigenous species have lost—and continue to lose—so much ground, both to development and to non-native plants used horticulturally.
- Plant hybrids. According to Reichard's research, very few—just 1%—of the woody invasives in this country are hybrids. This means that the many showy garden hybrids developed continuously by breeders are safe to grow—good news!
- Use plants native to other regions of North America. Only 3% of the woody invasives were introduced from other parts of North America, according to Reichard. So planting a tree or shrub in New York that's native to, say, the Ozarks is less risky than planting one from temperate woodland areas of China or Japan. (Keep in mind, however, that this is a far from infallible rule of thumb—one of the worst invasives in New York State, black locust, is native to Pennsylvania.)
Also keep in mind that right now there are no good screening models that offer guidelines on the potential invasiveness of herbaceous plants. Therefore, if you do plant a new perennial, biennial, or annual species that is not native to your area, be sure to watch for any tendency it may have to self-seed, especially in areas of the garden that aren't irrigated and therefore more closely approximate conditions in the wild. And if the plant is showing any potential for escaping into the wild, take a deep breath—and throw it on the compost pile.
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."