What is an Invasive Exotic?
There are hundreds of species of flora and fauna known to be harmful when introduced beyond their natural ranges. Seven trees in the United States are considered invasive and listed as an alien exotic pest.
According to Invasive.org , "any species, including its seeds, eggs, spores, or other biological material capable of propagating that species, that is not native to that ecosystem; and whose introduction does or is likely to cause economic or environmental harm or harm to human health" should be considered an invasive.
Not every tree you plant 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 or harm to human health you have an invasive tree. Interestingly, humans actions are the primary means introducing these invasive species.
The Unwanted Tree List
You need to be extremely careful when planting these species outside their natural habitats:
Black Locust ( Robinia pseudoacacia L.) yellow locust, locust
Black locust has been planted extensively for its nitrogen fixing abilities, as a source of nectar for honeybees, and for fenceposts and hardwood lumber. Its commercial value and soil building properties encourages further transportation outside it's natural range.
Black locust is native to the Southern Appalachians and the Southeastern United States. The tree has been planted in many temperate climates and is naturalized throughout the United States, within and outside of its historical range, and in some parts of Europe.
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 in dry and sand prairies, oak savannas and upland forest edges, outside of its historic North American range.
Chinaberrytree ( Melia azedarach L.) China Tree, Pride of India, Umbrella Tree
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. 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.
Mimosa ( Albizia julibrissin Durazz. L.) silk tree
Mimosa was introduced into the United States as an ornamental from Asia and Africa. It has escaped into fields and waste areas and its distribution in the United States is from the mid-Atlantic states south and as far west as Indiana. It is a flat-topped, thornless, deciduous tree which reaches 35' in height. It can sometimes be confused with honey locust because of the bi-pinnate leaves of both.
It does not establish in forests, but commonly occurs on forest borders. It can also 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."
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