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.
Although a 2009 Virginia Cooperative Extension report says that at least 50 percent of woody plant species from U.S. wholesale growers are native to regions outside of North America, no one knows for sure the ratio of all non-natives to natives on retail nursery benches. “There is no number like that I am aware of,” says Joe Bischoff, director of government relations with the American Nursery and Landscape Association in Washington.
“The industry doesn’t keep those kinds of records,” agrees John Peter Thompson, a consultant on invasive species and bio-economic policy in Upper Marlboro, Md. But, the Virginia Extension report and Thompson say the numbers are high. Take for example:
- Boxwoods and ivy from England.
- Hollies from Japan and China.
- Hostas, which are indigenous to Korea, China and Japan.
- Dogwoods from China.
- Norway maples, which are native to eastern and central Europe and southwest Asia.
- Bradford pears, which originated in China (though U.S. hybrids were later developed).
Exotic in this case doesn’t mean tropical, says Jil Swearingen, an integrated pest management and invasive species biologist for the National Park Service in Washington. “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 explained. “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 think that adding exotic plants to home landscapes is an exciting way to beautify their yards or, as Thompson points out, reduce the need for pesticides since many exotics have no natural enemies in North America. And, in some ways, they are right. The exotics, after all, make great eye candy -- they have pretty flowers, attractive shapes and grow fast. Unfortunately, some of them can 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," says Swearingen. “A number of the invasive species, such as Japanese stiltgrass (Microstegium vimineum (Trin.) A. Camus), have been introduced by accident rather than intentionally.”
Alien species on the loose
It’s the exotics that are invasive that are the bad actors home gardeners should be aware of, points out Thompson. Once on the loose, they can wreak havoc on ecosystems because the herbivores, parasites, pathogens or predators that kill or eat them in their native habitats are not present in American landscapes and wild areas.
Without these limiting factors to keep invasive exotics under control, they out-compete native species for such limited resources as sunlight, water, nutrients, soil and space. In time, they can form dense, single-species stands that dominate and displace existing native vegetation, causing a great loss of biodiversity that alters the original natural ecosystem.
Thompson says three “bad actors” make his poster child list of exotic invasive plants. Each, he says, has escaped from home gardens: English ivy, Japanese barberry and purple loosestrife.
“English ivy is to the shade as kudzu is to the sun — unstoppable,” says Thompson. 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 roof tops 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 is in 595 counties, occurring throughout the eastern states from Texas to Massachusetts and is a problem in Arizona, California, Oregon and Washington.
Many people like Japanese barberry (Berberis thunbergii DC) (at right) because it is a shade-tolerant plant with gorgeous flowers that stay in bloom from mid-spring to summer, Thompson says. Native to Asia, it is popular as an ornamental hedge. Birds, however, have helped to spread seeds in 679 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 are crowded with thorny thickets of barberry Thompson describes as too tangled and dangerous to walk through.
Thompson says purple loosestrife (Lythrum salicaria L.) “combines beauty and attraction with establishment and destruction.” Native to Europe and Asia, this aqautic-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,” Thompson added. “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 is in 1,158 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,” says Swearingen. That, she says, could take thousands of years or longer. Not only do these alien plants displace native vegetation, they alter 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.
The ANLA has funded projects to develop tools that help identify potential invasive species before they become a problem. It also is working with the science community, various regulatory agencies and legislative bodies on recognition of work-arounds that breeders are undertaking to produce cultivars of invasive exotics that will speed up the time Mother Nature takes to negate their negative impact. An example of such a cultivar, says Bischoff, who serves on the Federal Invasive Species Advisory Committee, would be one that is low in fertility and reduces the viability of the seeds.
Invasive plant references
There are several online references to help American gardeners identify and manage invasive plants.
One is the Invasive Plant Atlas Of the United States. This collaborative project between the National Park Service, 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 in Austin, Texas, focuses on non-native invasive species that impact 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 National Park Service, Lady Bird Johnson Wildflower Center, The Garden Club of America, and The National Invasive Species Council, offers home gardeners 10 best management suggestions about invasive plants.
1. Know your plants.
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 it, control it or cage it.
More information about these tips is available at: www.wildflower.org/howto/howto_resources/Plantwise_Brochure.pdf.
Swearingen especially likes the idea of gardening with plants native to your region. “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,” she continues. “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.”
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