Most people who garden have heard of invasive plants, but many of us aren’t sure how to identify and eliminate them, or how invasives differ from basic weeds. It’s possible that you’ve even seen the effects of invasives. Maybe you’re in Michigan and have noticed purple loosestrife choking out wetland plants and animals along the shores of rivers and lakes. Or perhaps English ivy has overtaken your flower garden in Oregon. If you live in the South, you’ve no doubt seen acres of land swallowed up by kudzu.
Invasive plants are more than just weeds—weeds are simply plants that interfere with a gardener’s management plan for a particular site (think clover that has sneaked from your yard into your flower bed). Invasive species, according to the USDA, are non-native plants, animals and other organisms spread mostly by human actions and whose introduction is harmful in some way. These aggressive invaders can affect the ecological balance of entire ecosystems as they outcompete native species for food and other resources, spread non-native diseases, eat native species, or kill the young of native species. As a result, they can not only wipe out native flora and fauna, but in some cases also eliminate biodiversity in the landscape.
According to the U.S. Geological Survey, more than 6,500 foreign species of flora and fauna have made themselves at home in the United States in the last century, and some of them are aggressive enough to be considered invasive. One prime example is kudzu. An Asian native, it was planted as an erosion controller in the 1870s (the U.S. Soil Conservation Service promoted its use for erosion control from the 1930s through 1950s). Since then, it has taken over thousands of acres in the East, South and lower Midwest. Kudzu thrives in temperate climates, where it grows at an alarming rate of 50 to 60 feet per growing season, and can climb 100 feet. Its dense coverage enables it to choke out light, water and nutrients from other species, and its height enables it to cover trees as well as low-growing plants.
How Do Invasives Get Here?
Invasive species take many routes into American ecosystems. Leigh Greenwood, outreach program director with The Nature Conservancy, says they travel as contaminants in seed, grain or hay carried from state to state or country to country. They’ve also migrated in the ballast of ships, and some have been intentionally introduced as ornamental plants or to serve practical purposes, such as providing windbreaks or erosion control.
Gardeners can unintentionally contribute to the spread of invasives, says Lili Singer, director of special projects and adult education with the Theodore Payne Foundation for Wildflowers and Native Plants in Sun Valley, California. Nurseries sometimes sell plants that qualify as invasive species, and gardeners buy them without realizing the potential negative impact. The English ivy adopted by American gardeners is a good example. It may look lovely growing up the side of a house, but it can also choke out native plants and overtake ornamental gardens.
What Can I Do?
It’s easy for us all to help stop the spread of invasives. First, avoid planting them in your garden. Let’s go back to the English ivy example: If you’re looking for a ground cover, you might read that it’s an excellent choice. But knowing it’s invasive, you might instead use the North American native creeping phlox or choose from a variety of native ferns. You can find a list of plants to avoid and better replacements from the Natural Arboretum: usna.usda.gov/Gardens/faqs/InvasivesAlternatives.html.
Most good advice on plants is based on what region you’re in, so if you’re not sure if a species in your garden is invasive, and what native plants might be good to plant in its place, check with local sources such as your cooperative extension office. By choosing natives, we can help strengthen and enhance the natural ecosystems that have developed for eons in our areas, which will in turn support the wildlife, pollinators, bacteria, fungi and other creatures that have evolved alongside plants. Singer advises gardeners to aim to plant at least 50 percent native plants in their yards and gardens, which has the added benefit of saving on water and fertilizer because native plants have evolved to survive unaided in the environment in which you live.
Gardeners can also help control and irradicate invasive plants by notifying local cooperative extension offices or regional chapters of The Nature Conservancy (find yours: nature.org/about-us/contact/worldwide-and-field-offices.xml) when they identify potential invasives, especially those that have recently spread in a given area. In fact, questions and alerts from ordinary gardeners and homeowners have led to the discovery of most invasive species, Greenwood says. “If you see something that doesn’t look right, say something,” she urges. “Think critically about your environment.”
Can I Only Plant Natives?
You may assume the only way to avoid spreading invasives is to completely exclude non-native plants from your gardens. Such a dramatic response isn’t necessary. Simply evaluate before you plant. Ask yourself what you’re hoping to achieve with a given plant, and whether a native plant might serve the same function.
Next, you might want to reconsider your notion of a perfect garden. Gardeners used to aspire to the traditional English garden, with plants from all over the world arranged in orderly fashion. Increasingly, gardeners are finding natural gardens more appealing and easier to care for. One of the best ways to support healthy ecosystems is to select from the wide array of plants native to our regions, and show off the beauty of native species.
Learn More about Invasive Pests
Invasive species are not limited to just plants.Insects, microbes, fish and more can also wreak havoc on ecosystems, and they can be introduced in a variety of ways. Individuals may unwittingly spread invasive pests by overlooking good practices such as cleaning hiking boots between hikes, thus inadvertently transferring soil matter from one part of the country to another. One of the most notorious forms of transport for invasives is the movement of hay, mulch and firewood, says Leigh Greenwood, outreach program coordinator with The Nature Conservancy. “Even moving firewood from county to county can create a problem,” she says. The emerald ash borer is an example of an invasive pest that has spread this way. This beetle’s larvae (found primarily in the Midwest and East Coast) feed on the inner bark of ash trees, disrupting the trees’ ability to transport nutrients and water. The only organic option for control is to completely remove severely infected trees.
Invasive Plants and How To Get Rid of Them
Here’s a sampling of the many invasives found throughout much of North America. Find more information about invasive plants.
Canadian Thistle
Where: Widespread in the U.S.
What It Does: This noxious weed invades ecosystems such as savannas, glades and prairies where soil disturbance exists. Once established, the thistle spreads rapidly, quickly diminishing and replacing native plants.
How to Eradicate It: Pulling and mowing to weaken roots, burning consecutively for three years, and the use of bud weevil, stem weevil and stem gal fly as biological control methods.
Kudzu
Where: Southern, Eastern and Lower Midwestern United States
What It Does: Originally introduced for erosion control, this Asian vine spreads like wildfire, consuming and choking out all vegetation and even structures in its path.
How to Eradicate It: Repeated mowing, cutting and burning; sheep and goats will also eat it.
Purple loosestrife
Where: All over the United States, most problematic in Northeast and Upper Midwest
What It Does: Can suppress and choke out native plant communities in wetland areas, affecting food sources and cover for wildlife.
How to Eradicate It: Hand-pulling, burning; or use leaf-eating beetles (Galerucella)—contact your Extension Office or Department of Natural Resources to obtain them.
Norway Maple
Where: Northeast
What It Does: Similar-looking to the native sugar maple, it suppresses reproduction of saplings below and eventually crowds out all other forest species, including native maples.
How to Eradicate It: Hand-pulling seedlings, cutting trees to the ground.
Tamarisk (Saltcedar)
Where: Mostly in the West
What It Does: Introduced as a windbreak and ornamental, this plant forms thick stands along streams, restricting water flow and supply to other plants.
How to Eradicate It: Eradication through organic methods is difficult and requires a multiyear commitment. A couple of insects can attack the trees, and flooding is also effective. After saltcedar is removed, a competitive stand of desirable plants must be established to prevent reinvasion. Contact a professional if dealing with this species.
Cheatgrass
Where: Throughout the United States
What It Does: Crowds out native grasses and burns frequently, changing the fire cycle and damaging thousands of acres of private and public land.
How to Eradicate It: Selective grazing by cattle, sheep, etc. in late fall or early spring can allow native species to regain a foothold and crowd it out.
Beneficial Non-Natives
Although some can be harmful, not all introduced species are bad. Plenty of species we value highly today were intentionally introduced from other states or countries and have been living in their newly adopted homes for centuries. Even some species we consider invasive may turn out to have surprising benefits. For example, non-native eucalyptus trees are sometimes considered invasive in California, but they provide a habitat for migrating monarch butterflies, which have taken to overwintering in them. So before you go wild eradicating any plant species, including those considered invasive, do a little research on whether they negatively affect the environment in your area and the most eco-friendly methods for dealing with them.
