7 Common Invasive Trees in North America

Nearly 250 species of trees are known to be harmful when introduced beyond their natural geographic ranges. The good news is the majority of these, confined to small regions, are of less concern and have a low potential to overtake our fields and forest on a continental scale.

According to a cooperative resource, the Invasive Plant Atlas, an invasive tree is one that has spread into "natural areas in the U.S. and these species are included when they are invasive in areas well outside their known natural ranges, as a result of human activities." These tree species are not native to a particular ecosystem, and have or are likely to cause economic or environmental harm, or harm to human health, and are considered invasive.

Many of these species are also considered alien exotic pests after being introduced from other countries. A few are native trees introduced outside its North American natural range to become problems out of its natural range.

In other words, not every tree you plant or encourage to grow is desirable and can actually be harmful to a particular location. If you see a non-native tree species that is out of its original biological community and whose introduction causes or is likely to cause economic or environmental harm, you have an invasive tree. Humans actions are the primary means of introducing and spreading these invasive species.

01
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Royal Paulownia or Princess Tree

The cluster of pale, nutlike fruit of the princess tree, against heart shaped leaves

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Royal paulownia or Paulownia tomentosa was introduced into the U.S. from China as an ornamental and landscape tree around 1840. The tree has recently been planted as a wood product that, under exacting conditions and management, commands high lumber prices where there is a market.

Paulownia has a rounded crown, heavy, clumsy branches, reaches 50 feet tall, and the trunk can be 2 feet in diameter. The tree is now found in 25 states in the eastern U.S., from Maine to Texas.

Princess tree is an aggressive ornamental tree that grows rapidly in disturbed natural areas, including forests, stream banks, and steep rocky slopes. It easily adapts to disturbed habitats, including previously burned areas and forests defoliated by pests (like gypsy moths).

The tree takes advantages of landslides and road right-of-ways, and can colonize rocky cliffs and scoured riparian zones where it may compete with rare plants in these marginal habitats.

02
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Mimosa or Silk Tree

The distinctive, fluffy, pinkish purple flowers of a silk tree against the fern-like foliage

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Mimosa or Albizia julibrissin was introduced into the United States as an ornamental from Asia and Africa and was first introduced into the U.S. in 1745. It is a flat-topped, thornless, deciduous tree which reaches 50 feet in height on fertile disturbed forest borders. It is usually a smaller tree in the urban lands, often having multiple trunks. It can sometimes be confused with honey locust because of the bipinnate leaves of both. 

It has escaped into fields and waste areas and its distribution in the U.S. is from the mid-Atlantic states south and as far west as Indiana. Once established, mimosa is difficult to remove due to the long-lived seeds and its ability to re-sprout vigorously.

It does not establish in forests but invade riparian areas and spread downstream. It is often injured by severe winters. According to the U.S. National Park Service, "its major negative impact is its improper occurrence in historically accurate landscapes."

03
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Black Locust, Yellow Locust, or Robinia

A branch of the black locust with white flowers in the spring

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Black locust ​or Robinia pseudoacacia is a North American native tree and has been planted extensively for its nitrogen-fixing abilities, as a source of nectar for honeybees, and for fence posts and hardwood lumber. Its commercial value and soil-building properties encourage further transportation outside its natural range.

Black locust is native to the Southern Appalachians and the Southeastern U.S. The tree has been planted in many temperate climates and is naturalized throughout the U.S., within and outside of its historical range, and in some parts of Europe. The tree has spread to and become invasive in other parts of the country.

Once introduced to an area, black locust expands readily into areas where their shade reduces competition from other sun-loving plants. The tree poses a serious threat to native vegetation (especially the Midwest) in dry and sand prairies, oak savannas, and upland forest edges outside of its historic North American range.

04
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Tree-of-Heaven, Ailanthus, or Chinese sumac

Leaves and red seed at tree of heaven or Ailanthus altissima in Bulgaria

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Tree-of-heaven (TOH) or Ailanthus altissima was introduced into the U.S. by a gardener in Philadelphia in 1784. The Asian tree was initially promoted as a host tree for silkmoth production.

The tree rapidly spread because of an ability to grow quickly under adverse conditions. It also produces a poisonous chemical called "ailanthene" in TOH bark and leaves which kills nearby vegetation and helps limit its competition'

TOH now has a wide distribution in the United States, occurring in 42 states, from Maine to Florida and west to California. It grows stout and tall to about 100 feet with a "fern-like" compound leaf that may be 2 to 4 feet long.

Tree-of-Heaven can't handle deep shade and is most commonly found along fence rows, roadsides, and waste areas. It can grow in nearly any environment that is relatively sunny. It can pose a serious threat to natural areas recently opened to sunlight. It has been found growing up to two air miles from the nearest seed source.

05
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Tallow Tree, Chinese Tallow Tree, or Popcorn-tree

The autumn branches of a Chinese tallow tree with changing red, green, and yellow leaves

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The Chinese tallow tree or Triadica sebifera was purposely introduced into the southeastern U.S. via South Carolina in 1776 for ornamental purposes and seed oil production. Popcorn tree is a native of China where it has been cultivated for about 1,500 years as a seed-oil crop.

It is mostly confined to the southern United States and has been associated with ornamental landscapes as it makes a small tree very quickly. The green fruit cluster turns black and splits to show bone white seeds that make a beautiful contrast to its Fall color.

The tree is medium sized, growing to a height of 50 feet, with a broad pyramidal, open crown. Most of the plant is poisonous, but not to touch. The leaves somewhat resemble a "leg of mutton" in shape and turn red in autumn.

The tree is a fast grower with insect inhibiting properties. It takes advantage of both of these properties to colonize grasslands and prairies to the detriment of native botanicals. They rapidly turn these open areas into single species forests.

06
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Chinaberrytree, China Tree, or Umbrella Tree

The poisonous fruit of Melia azedarach, called the Chinaberry

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Chinaberry or Melia azedarach is native to Southeast Asia and northern Australia. It was introduced into the United States in the mid-1800s for ornamental purposes. 

The Asian Chinaberry is a small tree, 20 to 40 feet tall with a spreading crown. The tree has become naturalized in the southeastern United States where it was extensively used as an ornamental around old southern homes.

The large leaves are alternate, bipinnately compound, 1 to 2 feet in length, and turn golden-yellow in fall. Its fruit is hard, yellow, marble-sized, stalked berries that can be dangerous on sidewalks and other walkways.

It has managed to spread by root sprouts and an abundant seed crop. It is a close relative of the neem tree and in the mahogany family.

Chinaberry's fast-growth and rapidly spreading thickets make it a significant pest plant in the U.S. Even so, it continues to be sold at some nurseries. Chinaberry outgrows, shades-out and displaces native vegetation; its bark and leaves and seeds are poisonous to farm and domestic animals.

07
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White Poplar or Silver Poplar

Silver poplar with yellow leaves in autumn against the blue sky

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White poplar or Populus alba was first introduced to North America in 1748 from Eurasia and has a long history of cultivation. It is chiefly planted as an ornamental for its attractive leaves. It has escaped and spread widely from many original planting sites. White poplar is found in 43 states throughout the contiguous U.S.

White poplar out-competes many native tree and shrub species in mostly sunny areas such as forest edges and fields, and interferes with the normal progress of natural community succession.

It is an especially strong competitor because it can grow in a variety of soils, produce large seed crops, and resprouts easily in response to damage. Dense stands of white poplar prevent other plants from coexisting by reducing the amount of sunlight, nutrients, water, and space available.