What is an Invasive Plant? An invasive plant has the ability to thrive and spread aggressively outside its natural range.  A naturally aggressive plant may be especially invasive when it is introduced to a new habitat.  An invasive species that colonizes a new area may gain an ecological edge since the insects, diseases, and foraging animals that naturally keep its growth in check in its native range are not present in its new habitat. Some invasive plants are worse than others.  Many invasive plants continue to be admired by gardeners who may not be aware of their weedy nature.  Others are recognized as weeds but property owners fail to do their part in preventing their spread.  Some do not even become invasive until they are neglected for a long time.  Invasive plants are not all equally invasive.  Some only colonize small areas and do not do so aggressively.  Others may spread and come to dominate large areas in just a few years.  Below are some categories to illustrate degree of invasiveness.

Danger! Don't plant it... Purple Loosestrife, Lythrum salicaria, has long been a prized perennial.  Its pinkish-purple flowers appear over a long period in summer.  The seeds of this plant easily wash into waterways, and can be carried in the mud on the feet of waterfowl.  Stands of loosestrife spread exponentially in wetlands and along stream beds.  This plant should be removed by hand only if it is very young.  Attempts to dig it out usually backfire because purple loosestrife resprouts from root fragments; disturbing the soil just provides more room for it to spread.  Cut established plants to the ground periodically to prevent flowering.  Other invasive plants such as Tartarian honeysuckle, Lonicera tatarica; Russian olive, Elaeagnus angustifolia; and Siberian elm, Ulmus pumila, are still available for planting even though they have become invasive over large areas.  This category is threatening because gardeners who are unaware of problems with these plants may still be planting them in areas that have not yet been colonized.

Warning: If you see it, remove it... Tree-of-heaven, Ailanthus altissima, is one of the few trees that can grow in abandoned alleys, gutters, and broken sidewalks, or just about anywhere that is not in shade.  It grows very quickly, and competes aggressively for sunlight in newly developing forests.  Disturbed sites are often dominated by tree-of-heaven.  Pull these seedlings whenever you see them; once they have grown for a few years they are extremely difficult to get rid of.  Reducing the number of trees will reduce the yearly output of seeds.  Other common weeds which are invasive plants are multiflora rose, Rosa multiflora; garlic mustard, Allaria petiolata; and lesser celandine, Ranunculus ficaria.  Although these plants are not often planted intentionally in gardens or offered for sale, they have the ability to spread if not controlled.

Caution: It's not a problem if you manage it wisely... English ivy, Hedera helix, is one of the most popular ground covers in North America.  However, its potential for escape is notorious.  In the Pacific Northwest, English ivy invades the forest floors.  Its evergreen leaves smother other native forest plants by denying them light.  Interestingly, English ivy only reaches maturity and goes to seed after it has grown up a vertical surface.  If you are willing to prune it regularly to contain it, it does not pose a threat.  English ivy is not a good choice, though, if you want a low maintenance garden.  You may want to replace it with native plants such as lowbush blueberry, Vaccinium angustifolium; alum root, Heuchera americana; or partridge berry, Mitchella repens. Some other invasive exotics aside from English ivy that fit this category are common daylily, Hemerocallis fulva; butterfly bush, Buddleia spp.; wintercreeper, Euonymus fortunei; and lilyturf, Liriope muscari.  Although these plants are invasive, they can still be enjoyed by gardeners who want to grow them if they are willing to devote the time and effort to careful stewardship to prevent their spread.

Where are they a problem?

Invasive plants disrupt many natural habitats.  They are most threatening in ecosystems such as wetlands, sand dunes, fire prone areas, and serpentine barrens where rare native plants are found.  Invasive plant species thrive where the continuity of a natural ecosystem is breached and are abundant on disturbed sites like construction areas and road cuts.  Even foot traffic can create a temporary void that is quickly invaded–some national parks have restricted the areas where visitors are allowed to walk with the warning, "we can watch purple loosestrife grow from people's footsteps."
 

Why are they a problem?

It's a matter of ecology.  In many cases, plants from other parts of the world are welcomed, manageable additions to our gardens. However, in some situations these non-native species cause serious ecological disturbances.  In the worst cases, invasive plants like mile-a-minute, purple loosestrife, and kudzu ruthlessly choke out other plant life.  This puts extreme pressure on native plants and animals, and threatened species may succumb to this pressure.  Ultimately, invasive plants alter habitats and reduce biodiversity.
 

Where do they come from?

In some cases, invasive plants arrive purely by accident, as seed in agricultural products, or on shipments from overseas.  In other cases, invasive plants are selected for their horticultural attributes.  Beautiful, unusual, exceptionally hardy, drought-tolerant, or fast-growing plants are sought by gardeners the world over.  Unfortunately, plants selected for their resilience may be invasive because of their adaptable nature.  Plants selected for their aesthetic value may be hard to banish from your garden even after their invasive tendencies are revealed.
 

Invasive Plants:

   Produce large numbers of new plants each season.
   Tolerate many soil types and weather conditions.
   Spread easily and efficiently, usually by wind, water, or animals.
   Grow rapidly, allowing them to displace slower growing plants.
   Spread rampantly when they are free of the natural checks and balances found in their native range.
 

What Can You Do?
 

   Contact your local native plant society or state Department of Natural Resources to find out which plants are invasive in your area.

   Learn to identify locally important invasive plants.

   Remove invasive plants on your property or prevent their spread.

   Only use non-invasive plants when landscaping your property.

   If your property borders a natural area, consider using only native plants in your landscape.

   Find non-invasive or native alternatives for invasive landscape plants.

   Use systemic herbicides carefully as a last resort to remove invasive plants.

   Make others in your neighborhood aware of invasive plants.
 
 

Here are some of the most commonly encountered invasive plants and the areas where they are a problem:

Elaeagnus angustifolia Russian olive  western U.S. Melaleuca quinquenervia Cajeput tree  wetlands, southern Florida Lythrum salicaria  purple loosestrife  wetlands, most of U.S. Ailanthus altissima  tree-of-heaven  most of U.S. Lonicera japonica  Japanese honeysuckle  eastern U.S. to Midwest  Pueraria lobata  kudzu  southeastern U.S. Euphorbia esula  leafy spurge  Midwest to West Polygonum perfoliatum  mile-a-minute  NY to  VA and WV Tamarix ramosissima  tamarisk  western U.S. Imperata cylindrica  cogongrass  Gulf Coast states

So What?

Over $100 million a year is spent in the U.S. combating invasive plants in wetlands alone.  Rich, diverse plant communities can become barren, inhospitable expanses of invasive plants with little value to wildlife.  Invasive plants may even deplete groundwater resources.  Plants introduced to North America from other parts of the world have come to dominate millions of acres of forest, desert, prairie, and wetlands.  Choosing plants wisely and controlling potentially invasive plants in your garden and on your property are the best ways to preserve healthy native plant habitats.  Garden responsibly and control invasive plants while they are still in your garden.
 

Learn More About Invasive Plants

Many states have various government departments or invasive plant workgroups that list invasive plants that are causing problems in your region.  Click on your state for a link to a local resource for information on invasive plants.
 

More information on invasive plants may be found at the following links:

• Agricultural Research Service Research on Invasive Plants
• Alien Plant Invaders of Natural Areas (National Park Service)
• Biological Control of Non-Indigenous Plant Species (Cornell University)
• Center for Aquatic and Invasive Plants (University of Florida)
• Federal Interagency Committee for the Management of Noxious and Exotic Weeds
• Invasive Plants in the Upper Connecticut Valley
• Invasive Plants:  Weeds of the Global Garden (Brooklyn Botanic Garden)
• National Invasive Species Council
• The Nature Conservancy
• New England Wildflower Society
• Southeast Exotic Pest Plant Council
• U.S. Fish and Wildlife Service Invasive Species Program
• U.S. Geological Survey Invasive Plants
• Virginia Natural Heritage Program Fact Sheets and Brochures
• Weeds Gone Wild (Plant Conservation Alliance)

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Last Updated   December 12, 2007 10:10 AM
URL = http://www.usna.usda.gov/Gardens/invasives.html