The mimosa tree, also known as the silky acacia or silktree, is an introduced tree species from Asia that is now an invasive plant in the United States. The tree has certain qualities that allow it to flourish in specific areas and crowd out other native trees and plants. A member of the pea family, the mimosa tree is a naturalized species in a large portion of the nation now after its introduction to the U.S. in 1745.
The typical mimosa tree will be between 20 and 40 feet high and have leaves that resemble those of a fern. The leaves are from 5 to 8 inches in length and 3 to 4 inches in diameter. The flowers of a mimosa tree are an inch and a half long before developing into 6-inch long pods encompassing the half-inch long seeds.
The bark of a mimosa tree is a light shade of brown and it is smooth; the younger developing stems will be greenish before they mature and turn brown as well. The leaves are compound, with as many as 60 leaflets on a branch that can be 20 inches long. These compound leaves are what make the mimosa a desired ornamental tree as they help give the mimosa the appearance that it is feathery. The flowers look like little pink pom-poms, another feature that enhances the tree's looks, with a strong fragrant smell. The brown seedpods hold as many as 10 tan seeds and stay on a mimosa well into the winter.
The mimosa's native range includes much of Asia from Iran eastward to Japan. The mimosa tree came to America as an ornamental species but "escaped" into the wild via its hardy seeds, taking over from New Jersey southward along the Atlantic Coast states into the Deep South as far west as Louisiana. Mimosa trees also grow in California.
The problem with mimosa trees is that they will significantly reduce the amount of sunlight beneath them as well as deplete the soil of nutrients. This results in native plants and trees having little chance to compete with a mimosa. They grow in nearly every type of soil and especially near waterways where they can form dense thickets. The seeds have the ability to lie in a dormant state for years and still germinate, with the Plant Conservation Alliance website stating that after five years, 90 percent of the seeds are still able to produce a seedling. All of this makes the mimosa an invasive species despite its pleasing appearance.
Mimosas require an area where they will receive a full measure of sun, so they do not grow deep in the forest. The tree is sensitive to extreme cold so it has not advanced into the northern tier of states or into high elevations. The fact that the many seeds mimosa trees produce often find their way into a river or stream allows the plant to establish itself on the banks of these systems. Vacant lots and along the side of the road are other spots where mimosas thrive.