When gardeners head to nurseries in the coming weeks, a significant number of the ornamental plants on display won’t be native to North America.
At least 50 percent of woody plant species from U.S. wholesale growers are not native to North America, according to a report by Virginia Cooperative Extension. On retail nursery benches, however, the ratio of non-natives to natives is more of a mystery.
“There is no number like that I am aware of,” Joe Bischoff, former director of government relations with the American Nursery and Landscape Association (now known as AmericanHort), told MNN in 2013.
“The industry doesn’t keep those kinds of records,” agrees John Peter Thompson, a consultant on invasive species and bio-economic policy in Maryland. Yet despite the lack of data, Thompson and many other experts say the numbers are high.
Consider, for example, the U.S. abundance of these non-native plants:
- boxwoods and ivy from England
- hollies from Japan and China
- hostas from Korea, China and Japan
- dogwoods from China
- Norway maples, native to eastern and central Europe and southwest Asia
- Bradford pears, native to China (though U.S. hybrids were later developed)
Non-natives are the backbone of the nursery industry, Thompson says. He and others who study non-natives and their impact on American ecosystems have a term for non-indigenous plants destined for U.S. gardens: exotics.
Japanese stiltgrass was introduced to the U.S. in 1919 and now grows wild in at least 16 states. It crowds out native understory vegetation, like this infestation at a mesic forest in Maryland. (Photo: Michael Ellis/Wikimedia Commons)
Exotic doesn’t only mean tropical, says National Park Service biologist Jil Swearingen, a specialist in invasive species and integrated pest management. “Exotic refers to a plant or animal that people have moved to a place where it did not previously occur and where it has not been dispersed to by natural means like birds, wind or water,” she says. “For example, if someone takes a plant that is native only to China or Florida or California and relocates it to Maryland, the plant is an exotic in Maryland.”
Many people see exotic plants as a good way to beautify the home landscape — or, as Thompson notes, reduce the need for pesticides, since exotics often lack natural enemies in North America. And, in some ways, they're right. Exotics can be easy eye candy, offering vivid flowers, attractive shapes and rapid growth rates. Unfortunately, some also beautify the yard next door, the one next to that, the meadow a few miles away, the understory of national forests and countless other places.
“There are about 5,000 exotic plant species that have escaped from cultivation in the continental United States, and about 1,500 plant species are reported to be invasive in natural areas," Swearingen says. “A number of the invasive species, such as Japanese stiltgrass, have been introduced by accident rather than intentionally.”
Some exotics are more likely to become invasive than others, and these are the plants home gardeners should be most wary of, Thompson explains. Once free, they can wreak havoc on ecosystems since the herbivores, parasites, pathogens or predators that control them back home don't exist in American landscapes.
Without these limiting factors to keep invasive exotics in check, they outcompete native species for limited resources like sunlight, water, nutrients, soil and space. In time, they can form dense, single-species stands that dominate and displace existing native vegetation, causing a loss of biodiversity that alters the natural ecosystem.
Three “bad actors” in particular have become poster children for invasive plants in the U.S., Thompson says, and each one escaped into wild American habitats from home gardens: English ivy, Japanese barberry and purple loosestrife.
“English ivy is to the shade as kudzu is to the sun — unstoppable,” Thompson says. Native to Western Europe and Asia, this common ivy (Hedera helix) is an evergreen climbing vine that can reach 100 feet. It has small, rootlike structures that help it adhere to trees, brickwork and other surfaces. “Nothing beats a well-maintained driveway lined with beds of English ivy, until the maintenance stops and the ivy reaches the rooftops and begins to pull down the garage, house and trees,” Thompson says. "Because it needs no weeding, feeding, spraying or mowing and resists deer, mowers and car traffic with ease, people often think of it as the perfect ground cover — until they try to get rid of it.” It's documented in 675 U.S. counties, occurring throughout the eastern states from Texas to Massachusetts, and is a problem in Arizona, California, Oregon, Washington and Hawaii.
Introduced to the U.S. in 1875, Japanese barberry can displace native plants and alter soil pH. (Photo: Jamie Richmond/Flickr)
Many people like Japanese barberry (Berberis thunbergii) because it's a shade-tolerant plant with gorgeous flowers that stay in bloom from mid-spring to summer, Thompson says. Native to Asia, it's popular as an ornamental hedge. Birds, however, have helped spread seeds in 754 U.S. counties, primarily in the Northeast and the Great Lakes area. As a result, the understory of many woodlands and natural areas in the affected areas is crowded with thorny thickets of barberry that Thompson describes as too tangled and dangerous to walk through.
A wetland plant from Eurasia, purple loosestrife chokes out native vegetation in many American marshes and lake shores, while offering little food value to native animals. (Photo: Jay Cross/Flickr)
Purple loosestrife (Lythrum salicaria) “combines beauty and attraction with establishment and destruction,” as Thompson puts it. Native to Europe and Asia, this water-loving perennial has red, maroon and pink flowers and grows to 10 feet. Motorists driving along the New Jersey Turnpike can see its flowers for miles from mid-summer until frost, Thompson says. “It even has the advantage of growing faster than Japanese beetles can eat it,” he adds. “I once thought, ‘How great is that?’” Not very great at all, it turns out. One plant can produce up to 2 million seeds annually, making it such a serious invader of wetlands that it's now in 1,392 counties across the top half of the country. Its sale is banned in 24 states, according to Thompson.
"Once these and other invasive exotics become established, they are unstoppable until something, like an herbivore or pathogen, comes along to slow them down,” Swearingen says. That, she adds, could take thousands of years or longer.
On top of displacing native vegetation, these alien plants also change ecosystems in other ways. “Native insects find them as interesting as plastic plants because they haven’t evolved with them, are not attracted to feed on them and the immature stages (caterpillars) can't feed and develop on them,” Swearingen says. “If the caterpillars don't survive, neither does their next generation.” The food web of ecosystems, Swearingen points out, begins with insects.
Invasive plant references
Several online tools can help U.S. gardeners identify and manage invasive plants.
One is the Invasive Plant Atlas Of the United States. This collaborative project between the National Park Service (NPS), the University of Georgia Center for Invasive Species and Ecosystem Health, the Invasive Plant Atlas of New England and the Lady Bird Johnson Wildflower Center focuses on non-native invasive species that cause trouble in natural areas. The site’s "All Species" button is especially helpful.
Another is the National Association of Exotic Pest Plant Councils website. Its key feature is a U.S. map divided into color-coded regions. Mousing over the regions will bring up a link to locally invasive plants. People who live in states not colored in who want information about invasive exotic plants should contact their county government weed control boards.
How gardeners can help
The Be PlantWise program — a partnership between the NPS, Lady Bird Johnson Wildflower Center, the Garden Club of America and the National Invasive Species Council — offered home gardeners these 10 basic tips about invasive plants:
1. Know your plants (including where they're from and how they affect their habitat).
2. Use non-invasive alternatives, preferably species native to your area.
3. Watch out for invasive plant hitchhikers.
4. Be careful about which plants you share with gardening friends.
5. Use only seed mixes that are invasive plant-free.
6. Use weed-free soil and mulch mix.
7. Be especially careful with aquatic plants.
8. Keep an eye on new sprouts and volunteers.
9. Dispose of invasive plants carefully.
10. If you can't part with your invasive plant, remember to contain, control or cage it.
For more details about these suggestions, see the program's brochure.
Gardening with native plants is especially appealing, Swearingen says. “There are about about 17,000 plants native to the United States, with hundreds being grown and sold for use in our home landscapes,” she says. “They are delightful and diverse and provide nectar, pollen, foliage, fruits and seeds that native wildlife depend on. These plants are wonderful choices for our yards. But the natives haven’t received nearly enough attention. The horticultural features that make exotics appealing to the nursery trade and gardeners — they are hardy in lots of conditions, they grow fast and produce lots of flowers and seeds — are the same attributes that make them successful invaders and why we are battling against them in wild areas.”