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Roadside Use of Native Plants

Choosing Non-Invasive Plant Species — When is it safe to use non-native plants?

John M. Randall, The Nature Conservancy, and Sarah Reichard; Center for Urban Horticulture, University of Washington

Working to protect our natural areas, parks and preserves by halting the use of invasive non-native plants does not mean you need to avoid using all non- native plants. Invasive plants are those that spread by seed or vegetative growth into natural areas where they often dominate and degrade the native vegetation. non- native plants that will not spread from plantings into natural or semi-natural vegetation, on the other hand, are safe to use in roadside and ornamental plantings. Special care should be taken for plantings adjacent to or near nature preserves, parks, riversides and other areas with natural vegetation. In urban areas with no natural vegetation somewhat less caution is needed but even here, invasive plants whose seeds may be carried great distances by birds or the wind should not be used.

Lists of invasive non-native species are available for several areas of the U.S. and should be consulted before plant materials are selected. Before planting, consult lists from surrounding areas and areas with similar climate areas too, because the best way we now have of predicting whether a species is likely to become invasive in the future is by determining whether it is invasive somewhere else. For example, if you work in California, check lists for Florida and the Pacific Northwest and carefully consider whether species on these lists could survive without cultivation in their area.

Unfortunately, lists of non-invasive plant species are harder to come by, although several groups around the nation are now working to compile lists for their regions. Some rules of thumb may be used to distinguish those species least likely to be invasive. Sterile cultivars and hybrids are among the safest materials to use. For example, Syringa xpersica. Unfortunately, some cultivars of known invasive species that were thought to be sterile proved to be fully or partially fertile on careful examination. For example, 20 cultivars of purple loosestrife, including 'Morden Pink' and others widely advertised as sterile, produce fertile seeds, pollen or both. Dioecious species for which only males are available are likewise safe to use. Ginko biloba is a well-known example.

Species, cultivars, and hybrids that have been widely used and never reported as spreading or invasive in natural or semi-natural vegetation are likely to be safe to use. For example, Xylosma (Xylosma congestum), an evergreen or deciduous shrub, commonly used for ornamental and hedge plantings in California and other western states is not known to spread. Lilacs (Syringa spp.) have also been widely planted throughout much of the U.S., particularly in cooler climates and do not become invasive.

Non-invasive groundcovers include: Cotoneaster adpressus, Hypericum calycinum, and Lamium maculatum.

You can also help protect natural areas by reporting any 'new' species you find spreading into native vegetation — whether they are from plantings, accidental introductions or unknown sources. Reports may be made to nearby natural area managers, state Native Plant Societies, or Exotic Pest Plant Councils. If the species has not been reported growing in the state or region outside cultivation it should also be recorded in a note to a regional botanical journal. Early detection of new invaders provides natural area managers with the best chance of eliminating or containing invaders that may have the potential to do great damage.

Additional Information

Some of the best known invasive non- native species used for roadside and other horticultural plantings are listed in:

Randall, J.M. and J. Marinelli (eds.) 1996. Invasive plants: weeds of the global garden. Handbook #149, Brooklyn Botanic Garden, New York.

Other lists of known invasive non-native species which should be avoided (these lists also include many species which that were accidentally introduced and are rarely if ever cultivated):

Exotic Pest Plants of Greatest Ecological Concern in California (1996) — For copies, contact Sally Davis, 32912 Calle del Tesoro, San Juan Capistrano, CA 92675 or via email:
Florida Exotic Pest Plant Council's List of Florida's most Invasive Species. For copies, contact: Amy Ferriter, South Florida Water Management District, P.O. Box 24680, West Palm Beach, FL 33416 or via email:
Exotics of Illinois Forests. by John Schwegman 1994. Erigenia 13:6567.
Invasive Exotic Plants that Threaten Native Species and Natural Habitats in Maryland. For copies contact: Maryland Department of Natural Resources, Maryland Natural Heritage Program, Tawes State Office Building, E-1, 580 Taylor Avenue, Annapolis, Maryland 21401.
Oregon & Washington
non-native Pest Plants of Greatest Concern in Oregon and Washington as of August 1997. For copies contact: Dr. Sarah Reicllard, University of Washington, Center for Urban Horticulture, Box 354115, Seattle, WA 98195.
Rhode Island
Plants Invasive in Rhode Island. by Lisa L. Gould and Irene H. Stuckey. 1992. Rhode Island Wild Plant Society Newsletter 6(2):1–6.
Exotic Pest Plants in Tennessee: Threats to our Native Ecosystems. For copies, contact Brian Bowen or Andrea Shea, Department of Environment and Conservation, Division of Natural Heritage, 8th Floor, L&C Tower, 401 Church Street, Nashville, TN 37243-0447 or via email:;
Invasive Alien Plants of Virginia. For copies contact: Virginia Native Plant Society, P.O. Box 844, Annandale, VA 22003 OR Department of Conservation and Recreation, Division of Natural Heritage, 1500 East Main Street, Suite 312, Richmond, VA 23219.
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