Invaders TitleNYSM home


Landscaping with ornamental plants is a tradition dating back thousands of years, in many cultures.
When the early colonists came to North America, they brought their plants: the useful ones, the weeds, and the ornamentals. Many introductions of ornamental plants to North America took place in the 1800s when large numbers of immigrants began arriving. During the Victorian era, landscaping with ornamentals became quite popular.

New plants continue to arrive for use in landscaping. Some non-native plants are great garden successes, but others become problems. What characteristics make a plant potentially invasive? Some questions to ask are: Is the plant a problem somewhere else? Does it grow in a variety of conditions? Does it produce seeds or fruits that are easily dispersed by wind, water, birds, or other animals? Is it a perennial that spreads easily? Can a piece of the plant be moved and grow in another place? Does it compete with other species? These are all qualities you should be concerned about.

Plant native species.

If you do plant a nonnative species, consider the consequences. Take steps to ensure it won’t spread beyond the borders of your garden.
Robert M. Bennett, Chancellor

Japanese Barberry

Berberis thunbergiifera
Japanese Barberry is a shrub native to Asia. It was introduced to North America in 1875 for use as an ornamental. The shrub is commonly planted in groups and often escapes cultivation. Research has shown that it can alter soil chemistry and biological activity. It has become an invasive problem throughout the East and Midwest.

Japanese Barberry produces a large number of seeds in small red fruits that are eaten and dispersed by birds and other animals. It can form dense stands in the understory of forests, old fields, or other habitats where it shades and displaces native plants. This reduces wildlife habitat and forage. White-tailed Deer avoid browsing on barberry, giving the plant a competitive advantage over native plants.

Recent research on Japanese Barberry has shown that some cultivars may have less of a potential to become invasive. Research is also being done to try to create sterile plants. These may become valuable tools to help combat the invasiveness of new plantings of this popular shrub in the future. Unfortunately, this will not diminish the negative effects of the plants that exist in the wild.
James R. Allison, Georgia Department of Natural Resources,

Oriental Bittersweet

Celastrus orbiculatus
Oriental Bittersweet is native to eastern Asia. It was introduced to North America in the 1860s as an ornamental vine. It is often found near abandoned homesites. It reproduces rapidly by seed, which is easily transported by many wildlife species. It also reproduces by root suckers. Although it is an invasive species, Oriental Bittersweet is still planted as an ornamental. This also encourages its spread.

Oriental Bittersweet grows vigorously and can grow over the top of other vegetation. It can grow so densely that it causes the death of other plants from excessive shading, breakage, or eventual uprooting. It is also threatening native American Bittersweet through competition and hybridization.


Oriental Bittersweet can be confused with the less common native American Bittersweet. The flowers and fruits of American Bittersweet occur only at the ends of the branches and usually have more than seven flowers and fruits in a cluster.
Hilary Oles, Adirondack Park Invasive Plant Program

Water Chestnut

Trapa natans
Water Chestnut is an annual aquatic plant, native to Europe, Asia, and Africa. It was brought to northeastern North America as an ornamental water garden plant in the late 1800s. It escaped into the wild in several locations in the Northeast. Water Chestnut spreads rapidly and is now found in seven northeastern states and in Quebec.

Water Chestnut can replace native aquatic plant communities. It forms dense mats over the surface of lakes and slow-moving waters, blocking almost all incoming sunlight. It does, however, provide habitat for a variety of aquatic insects and fishes. The mats can impede fishing, hunting, swimming, boating, and even commercial navigation. They can also increase sedimentation and reduce available oxygen.

The control measures necessary to clear waterways can be expensive and usually need to be undertaken annually. Landowners can learn to recognize Water Chestnut and hand-pull it from their ponds several years in a row before it becomes too abundant to control. On the Hudson River, however, the largest mats of Water Chestnut are typically too extensive for that approach.
Cornell University Cooperative Extension of Allegany and Cattaraugus Counties

Giant Hogweed

Heracleum mantegazzianum
Giant Hogweed is native to the Caucasus Mountains of eastern Europe and western Asia. It was introduced in North America in 1917 for use as an ornamental plant. It flowers from late spring to mid-summer, with small white flowers clustered in a large umbrella-shaped head. It is a member of the Parsley (Carrot) family. Its most impressive characteristic is its massive size. It can grow from eight to fifteen feet and resembles a towering Queen Anne’s Lace. Giant Hogweed can grow in dense stands and crowd out native species.

Giant Hogweed is a public hazard on the federal noxious weed list. It produces a toxic sap that can cause phytophotodermatitis. Contact with the sap can prevent your skin from filtering the sun’s harmful ultra-violet rays. This causes severe burning blisters.


Several species are similar in appearance to Giant Hogweed. Do not touch if you are uncertain.
broadhead planarian

Fire Ant
Manuel Morales, Williams College

Japanese Knotweed

Fallopia japonica
Japanese Knotweed is native to eastern Asia. It was introduced in North America in the late 1800s for use as an ornamental plant and for erosion control. It grows quickly in a variety of conditions, which helps make it a successful invader. Japanese Knotweed can grow as tall as fifteen feet and form dense stands that compete with native plants. Native plants are rarer in these dense stands, and few animals eat the leaves.

Japanese Knotweed can easily sprout from small fragments of roots or stems and spread rapidly along rivers. Its presence may even facilitate the invasions of other non-native species. Japanese Knotweed secretes sugary nectar that attracts ant “plant guards.” Ongoing research suggests the non-native European Fire Ant, Myrmica rubra, is particularly common in knotweed stands.

This invasive ant is often the only ant that visits the nectaries in invaded sites. These ants may be “guarding” the nectar and preventing other animals from using this important source of food. Knotweed may provide the fuel that drives this second invasion.
John D. Byrd, Mississippi State University,

Purple Loosestrife

Lythrum salicaria
Purple Loosestrife is a perennial wetland plant native to Europe and Asia. It was introduced in North America in the 1800s as an ornamental, for medicinal uses, and from the release of ship ballast containing seeds.

Purple Loosestrife often forms dense thickets in wetlands, which can displace native plants and animals. Some of the many native species affected include the endangered Bog Turtle and Black Tern and the endangered plant Lesser Fringed Gentian. Purple Loosestrife can also reduce survival and growth rates of American Toad tadpoles. The flowers, however, provide an abundant source of nectar for insects, including bees. Common native species, such as the Red-winged Blackbird and American Goldfinch, use loosestrife stands for nesting.

In 1992, a biological control program was started in New York state. Several species of specialized insects that feed on loosestrife in its native range were introduced. These included two leaf beetles, Galerucella calmariensis and G. pusilla. They were released at hundreds of sites in New York. In many locations, such as in Montezuma National Wildlife Refuge, feeding by these insects has greatly reduced Purple Loosestrife and allowed native species to return. These beetles will eat leaves of two native plants, Winged Loosestrife and Water-willow. However, the beetles need Purple Loosestrife to complete their life cycle.

To date, no significant negative effects to the populations of the native plants have been detected. Researchers expect that Purple Loosestrife will continue to decline in the state as these insects spread.
tree of heaven

Tree of Heaven

Ailanthus altissima
Tree-of-heaven is native to China and was introduced into North America as an ornamental in the late 1700s. By the mid-1800s, it was commonly available in nurseries. It is fast-growing and can reach a height of more than eighty feet (24.4 m). It also is allelopathic, which means it produces chemicals that inhibit the growth of other nearby plants. Some native shrubs, such as Staghorn Sumac, can be confused with Tree-of-heaven.