Updated: August 2008
|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
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
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.
Produce large numbers of new plants each season.
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.
Invasive Plants are a Problem Throughout the Country
Here are some of the most commonly encountered invasive plants and the areas where they are a problem:
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
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
More information on invasive plants may be found at the following links:
Last Updated December 4, 2009 4:33 PM
URL = http://www.usna.usda.gov/Gardens/invasives.html