Our quest for plants from around the globe has enriched our gardens and brought us important food plants. At the same time, we have unwittingly created some devastating ecological consequences. Imported plants have spread pests and disease, including Dutch elm disease and dogwood anthracnose. Ornamentals, such as amur honeysuckle (Lonicera maackii) and oriental bittersweet (Celastrus orbiculatus), threaten native ecosystems. Many have negative effects on animals as well. The Nature Conservancy believes that invasive species contribute to the decline of 49 percent of all threatened and endangered species. This percentage is second only to habitat destruction.
An invasive species—be it plant, animal, or pathogen—is defined by Executive Order 13112 (1999) as a species that is non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm, or harm to human health. While the majority of ornamental plants are not invasive, from time to time a plant adapts too well, escapes cultivation, and becomes established, or naturalized, in the native landscape. The Plant Conservation Alliance has identified approximately 500 species of exotic plants across the country that are competing with native species and altering the structure and function of the ecosystems they invade. Of these 500, the New England Wild Flower Society estimates that horticultural activity is responsible for about 60 percent of invasive species introductions, while conservation activities, such as erosion control, windbreak, and wildlife enhancement introduced about 30 percent. Accidental introductions make up the remaining 10 percent.
Despite these statistics, exotic-plant bashing has become the pastime of zealots who would like to ban all nonnative plants. This is neither necessary nor desirable. Not every naturalized plant poses a threat to native ecosystems. Not every invasive species acts the same in all regions of the country, or in all ecosystems within a region. Buckthorn (Rhamnus cathartica) is a good example of a soil-specific species. This hedging shrub, popular in the Midwest and parts of the East, becomes a major problem on neutral to alkaline glacial soils. It is not a problem in most of Virginia, where acidic soils prevail, but on the limestone bedrock of the Shenandoah Valley, it runs rampant. Other plants are problematic only in ecosystems at specific stages. Tree of heaven (Ailanthus altissima) and princess tree (Paulownia tomentosa) thrive on disturbed sites; linden viburnum (Viburnum dilatatum) thrives in mature forests. Privet (Ligustrum spp.) is specific to riparian systems.
How Problems Start
When a plant escapes cultivation and begins to proliferate, problems arise. Two plants cannot occupy the same spot. When an invasive plant finds a favorable habitat, it can crowd out native species by growing faster or taller, leafing out first, or holding its foliage longer. Once entrenched, exotic species dramatically transform the vertical and horizontal structure of the ecosystem, potentially altering hydrology and corrupting nutrient cycles.
For a plant to escape cultivation, many favorable events must happen in tandem. First, the seed must be dispersed beyond the confines of cultivation, most often by animals, wind, or water. Berries eaten by migrating birds can move many miles. Seeds attached to animal fur travel great distances before dislodging. Wind-dispersed plants usually show up in a pattern on the leeward side of the parent plant, though gusts can carry seeds many miles.
The seed must survive predation and germinate, and the plant must reach reproductive maturity, which may take a season in an annual like Japanese stilt grass (Microstegium vimineum) or more than a decade with some woody plants, such as viburnums.
A seed has a slim chance of falling on a favorable substrate in a favorable niche and surviving to maturity. But if it does, the problem balloons. Each season, dozens (or hundreds or thousands) of seeds fall, as opposed to the random event that introduced the original seed. Free from natural checks and balances, the exotic plant may reproduce exponentially.
Invasive plants can hold staggering economic consequences. Ellen M. Jacquart, director of stewardship with the Indiana chapter of The Nature Conservancy, estimates the total cost of dealing with invasive plants to be $35 billion per year. Many of the well-documented invasives are so thoroughly entrenched that it may be impossible to totally eradicate them. This fact should not be seen as justification for ignoring the problem or for continuing to buy the offending plants. Instead, it points to the importance of early detection of emerging invasives, and the need to eliminate these plants from the palette that we have traditionally relied upon.
The time has come for us to take steps to ensure that our gardens do not contribute to the problem. Do not count on your local nursery or a horticultural consultant to do this for you. While many conscientious nurseries voluntarily remove known invasives from their stock, others remain interested in simply selling plants. Because plants are regionally and ecosystem specific, it may be difficult to predict the next invader, but a few traits should put up red flags. Nonnative species bearing fleshy fruits head the suspect list. Proven culprits include autumn olive (Elaeagnus umbellata), burning bush (Euonymus alatus), and Japanese barberry (Berberis thunbergii). One commitment we can all make is to exclude nonnative plants with fleshy fruits from our gardens. This is a simple way to start protecting the future of our wild lands, which sits in our hands.