Mimosa, or Silk Tree, is not a native to the United States. It is originally from China and was introduced here in 1745 and cultivated as an ornamental. It is a member of the legume family (peas and beans) family and is capable of fixing nitrogen (pulls nitrogen from the air). Mimosas are a popular ornamental tree in the Bluegrass because of its fragrant and showy flowers.
Mimosa is a small to medium-sized, deciduous (drops its leaves in winter) tree that can grow 20 to 40 feet tall, with the average height being about 25 feet. Typically, younger trees have branches that are lime green in color, but as they mature, the bark turns a light brown. Leaves are alternately arranged and have 20 to 60 leaflets per branch, giving the Mimosa a fern-like appearance. Mimosa flowering occurs from May through July and it is currently in full bloom all over the Bluegrass. The pom-pom flowers grow in clusters at the base of the current year's growth; they are fragrant and pink in color. The seed pods are flat and approximately 6 inches long with the characteristics of many legumes. When the seed pods are dry, they turn straw-colored and contain 5 to 10 light brown oval-shaped seeds about ½ inch in length. The seed pods cling tightly to the branches and can be seen on the bare branches during the winter months.
Mimosa can be reproduced by seedlings, grafting, or by seed. Vegetative reproduction happens when trees are cut back severely, causing quick re-sprouting and regrowth. Young branches of Mimosa can also be grafted (a method of rooting a new plant from a living branch) to make new plants. The Mimosa seeds require scarification (scratching or notching the seed) in order to germinate. The cold weather of winter allows the seeds to freeze and naturally break apart with thawing. Because of the need for scarification, Mimosa seed pods can remain dormant for many years. Most of the time, seeds are sprouted close to the parent plant, but seeds can also travel by water or wildlife.
Mimosa has the ability to grow in various soil types, so you can see them in open areas, like fields and vacant lots, or along forest edges. Mimosa is often seen along roadsides in urban and suburban areas becoming a problem along banks of creeks and rivers, where its seeds are easily transported in water. Because of their ability to produce large amounts of seed and are ability to re-sprout when cut back or damaged, it is a strong competitor for other native plants. Mimosa trees are very dense and they decrease sunlight and nutrients to other plants.
In many states, Mimosa is considered an invasive plant, due to its ability to grow and reproduce along roadways and disturbed areas, and its tendency to readily establish after escaping from cultivation. However, many people love these trees in their gardens, and Mimosa is commonly planted near back patios because they attract hummingbirds. This tree is a great choice where you need a small to medium sized ornamental tree. Mimosas are drought tolerant and can be planted in full sun or partial shade. Although they can tolerate zone 5 and above, Mimosa can suffer dieback when winter temperatures drop below -5 F for an extended period and they die practically to the ground when temperatures are much colder.
Many Mimosa trees in the Bluegrass Region took a major hit during the severe winter of 2013-2014. What until mid-summer before removing a seemingly dead tree; the tree may actually still be alive.