Despite its attractive form and flowers, and its popularity among homeowners, the USDA lists the mimosa, or silk tree (Albizia julibrissin) as an invasive plant, and the U.S. Forest Service discourages its planting. It is a leguminous plant (in the pea family), which grows at an extraordinary rate, and seeds prolifically. In fact, according to the University of Alabama in Huntsville, mimosa requires little short of waste ground to successfully, and rampantly reproduce.
The chief feature of this small- to medium-size tree (which usually grows to about 20 to 25 feet but might reach heights of 40 feet under ideal conditions) is the beautiful and lightly fragrant flowers--whose clusters of long stamens resemble, as stated by the University of Alabama in Huntsville, a mass of "fiber optics". They are white to pink-violet in color and slightly fragrant. The tree spreads in an umbrella fashion and is similar in form to the African acacias, though they are unrelated trees. Leaves are compound and pinnately divided (many small leaflets comprise the larger leaf), creating a ferny and tropical look.
The common name mimosa is confusing. It includes not only the alternatively named "silk tree" described here, but the true mimosas as well (Mimosa species--herbaceous plants, seldom growing larger than small shrubs). Both Albizia and the Mimosas are members of the sub-family Mimosoideae under the broad family Fabaceae (alternately called Leguminosae) which includes the acacias, another plant commonly mistaken for, and called mimosa. All members of the family share similar characteristics, including leaf and flower shapes--that contribute to the confusion of identities in this large group.
The Fabaceae family includes members from nearly every continent on Earth. The plant we call the mimosa tree, Albizia julibrissin, though originally from the area between Japan and Iran, is what botanists refer to as a "pantropical" species--one which spans many tropical regions of the planet. Of all the Albizias, it is the most cold tolerant, which explains its overwhelming success in northern temperate climates. Its introduction into the United States was in 1745, according to Plant Conservation Alliance's Alien Plant Working Group.
Mimosa trees will grow in a wide range of soils, including those of high alkalinity. Though it is better with regular watering, it will tolerate dry conditions as well. It is easy to plant, requires little maintenance and will self-sow in waste ground. It needs little or no fertilizer as it fixes nitrogen on its roots, as do other members of the pea family. It is shade tolerant, but prefers full sun.
Interested gardeners should know that mimosa trees are messy plants that produce thousands of tiny seedlings to litter the lawn, flowerbeds and even the cracks in the walkways and the gutters of houses. Beyond pure aesthetics, mimosas produce little of real use--the wood being weak and of no value. While they are attractive to hummingbirds and bear seed pods that many birds will eat, they are prone to disease and infestations of bagworm. Because they reproduce so rapidly, they also choke out valuable native plants.