Invading our native space

Noxious weeds or nonnative invasive plants may appear pretty and harmless, but they pose serious environmental threats across the country and in America’s Crossroads, one of six designated regions in the NWTF’s America’s Bix Six of Wildlife Conservation plan.

So what is an invasive plant species? It is defined as a species that is nonnative (or alien) to the ecosystem under consideration, and whose introduction causes or is likely to cause economic or environmental harm or harm to human health.

What makes nonnative invasive plants so successful? For starters, nonnative invasive plants produce large quantities of seed and thrive in disturbed soil. Invasive plant seeds are often distributed by birds, wind or people unknowingly transferring them. Some nonnative invasive species have aggressive root systems that spread long distances from a single plant. Those root systems often grow so densely that they smother the root systems of surrounding vegetation, choking out the competition. And, some nonnative plants produce chemicals in their leaves or root systems, which will inhibit the growth of the native plants around them.


According the USDA Forest Service, nonnative invasive species have contributed to the decline of 42 percent of U.S. endangered and threatened species; and for 18 percent of  these species, nonnative invasive species are the main cause of their decline.

For example, nonnative invasive species compete directly with native species for moisture, sunlight, nutrients and space, leading to a decrease in overall plant diversity. That reduction in diversity and the establishment and spread of nonnative invasive species degrade wildlife habitat by creating single stands of nonnative vegetation and can ultimately reduce water quantity and quality and increase soil erosion.  

Tree of Heaven

Appearance: Tree of Heaven (Ailanthus altissima) is a rapidly growing, typically small tree up to 80 feet in height and 6 feet in diameter. It has large leaf scars on the twigs.

Foliage: The leaves are 1-4 feet in length with 10-41 leaflets. Tree of Heaven resembles native sumac and hickory species, but the notched base on each leaflet easily distinguishes it.

Flowers: Has male and female reproductive systems on separate plants, and flowering occurs in early summer when large clusters of yellow flowers develop above the foliage.

Fruit: Fruit produced on female plants are tan to reddish, single winged and can be wind- or water-dispersed.

Ecological Threat: Tree of Heaven forms dense thickets that displace native species and can rapidly invade fields, meadows and harvested forests. This invasive tree is extremely tolerant of poor soil conditions and can even grow in cement cracks. Not shade tolerant, but easily invades disturbed forests or forest edges causing habitat damage. Introduced as an ornamental, it was widely planted in cities because of its ability to grow in poor conditions. Management and control efforts for this species continue across the U.S. at great economic cost.

Garlic Mustard

Appearance: Garlic Mustard (Alliaria petiolate) is an herbaceous, biennial forb. First-year plants are basal rosettes that bolt and flower in the second year. Recognized by a garlic odor that is present when any part of the plant is crushed.

Foliage: Foliage on first year rosettes is green, heart shaped, 1- to 6-inch leaves. Foliage becomes more triangular and strongly toothed as the plant matures.

Flowers: Second-year plants produce a 1- to 4-foot flowering stalk. Each flower has four small, white petals in the early spring.

Fruit: Mature seeds are shiny black and produced in erect, slender green pods that turn pale brown when mature.

Ecological Threat: Garlic Mustard is an aggressive invader of wooded areas throughout the eastern and central U.S. A high shade tolerance allows this plant to invade quality, mature woodlands, where it can form dense stands. These stands not only shade out native understory vegetation but also produce compounds that inhibit seed germination of other species. It is native to Europe and was first introduced during the 1800s for medicinal and culinary purposes.

Multiflora Rose

Appearance: Multiflora Rose (Rosa multiflora) is a multi-stemmed, thorny, perennial shrub that grows up to 15 feet tall. The stems are green to red arching canes that are round in cross section and have stiff, curved thorns.

Foliage: Leaves grow on each side of a stem with seven to nine leaflets. Leaflets are oblong, 1-1.5 inches long and have serrated edges. Its fringed stalks usually distinguish it from most other rose species.

Flowers: Small, white to pinkish, five-petaled flowers occur abundantly in clusters on the plant in the spring.

Fruit: Small, red rose hips that remain on the plant throughout the winter. Birds and other wildlife eat the fruit and disperse the seeds.

Ecological Threat: Multiflora Rose forms impenetrable thickets in pastures, fields and forest edges. It restricts human, livestock and wildlife movement and displaces native vegetation. It tolerates a wide range of conditions, allowing it to invade habitats across the country. Native to Asia and first introduced to North America in 1866 as rootstock for ornamental roses. During the mid-1900s, it was widely planted as a “living fence” for livestock control.

Leafy Spurge

Appearance: Leafy Spurge (Euphorbia esula) is an erect, perennial, herbaceous plant that grows from 2-3.5 feet tall. The stem is smooth and bluish-green. The plant produces a milky sap if stem is broken or a leaf is removed.

Foliage: Leaves are lance shaped, smooth and 1-4 inches. They are arranged alternately along the stem, becoming shorter and more oval-shaped toward the top of the stem.

Flowers: Yellow flowers develop in clusters at the apex of the plant in June.

Fruit: Fruits are three lobed capsules that explode when mature, propelling brown mottled, egg-shaped seeds up to 15 feet away.

Ecological Threat: Large infestations of leafy spurge give the landscape a yellowish tinge due to the yellow bracts. It invades prairies, pastures and other open areas. It is a major pest of national parks and nature preserves in the West. It can completely overtake large areas of land and displace native vegetation. This plant is native to Europe and was introduced accidentally into North America in the early 1800s as a seed contaminate.

Mark Hatfield, NWTF director of conservation administration

Article Categories