Photographer: Charles T. Bryson Affiliation: USDA Agricultural Research Service Source: Bugwood.org Copyright: (CC BY 3.0)
The silk tree (Albizia julibrissin) is a deciduous tree that grows from 10 to 50 feet tall. It can have one or more trunks and the canopy is shaped like an umbrella. The bark is light brown to gray and have lenticles that look like corky dots. The wood is weak and brittle and the doubly compound leaves alternate on the stem. The leaves are 6 to 20 inches in length and are almost fern-like and feathery in appearance. There can be 20 to 60 leaflets on one compound leaf with each being .5 inches long. The sweetly fragrant light to dark pink, flowers are showy and wispy; and one of the major reasons for its importation as an ornamental plant. The 6 inch seed pods are flat and linear and contain 5-10 light brown oval seeds within. They split to release the seeds and the empty pods remain on the trees throughout winter.
Ecological Threat: The wide environmental tolerance of the silk tree allows it to outcompete a wide variety of native trees and shrubs in disturbed areas. Also, because it can grow in several types of soil, reproduce profusely and re-sprout after damage it has a significant advantage over other plants. By reproducing quickly, silk trees can form in dense stands that shade out native plants and deprive them of nutrients from the soil.
Biology: Albizia julibrissin produces a very large number of seeds but also can spread vegetatively. Most seeds fall near the parent plant but they can also be dispersed by animals or water. Seeds have a very long seed bank and can remain viable for 50 years. Damage to the top of the tree can trigger regrowth from the roots and those sprouts can grow over 3 feet in one year.
History: The silk tree was introduced into the United States back in 1745 as an ornamental because of its showy and fragrant flowers that are attracts for hummingbirds. After its introduction, the trees were used to reclaim disturbed areas in riparian sites and it is still available for purchase at nurseries.
Native Origin: Iran, India, China and Japan
U.S. Habitat: The silk tree invades disturbed habitats including roadsides or open areas in urban or suburban areas. It is tolerant of several soil Types, drought, wind and moderate salt spray. However, it is rare under a full forest canopy and is limited by cold winters so it doesn’t grow about 3,000 feet in elevation.
U.S. Present: AL, AR, AZ, CA, CT, DC, DE, FL, GA, IL, IN, KY, LA, MA, MD, MO, MS, NC, NJ, NM, NY, OH, OK, PA, SC, TN, TX, UT, VA and WV
Albizia julibrissin resembles the non-native species, Woman’s tongue (Albezia lebbeck). However, unlike the silk tree, the seed pods do not split open once ripe and the leaflets are more rounded and do not appear as feathery.
The best way to manage this tree is to not plant it and to remove it from any landscape, when possible. However, you have to be sure to remove all the roots or those will cause new sprouts. Mid-size trees can be cut to ground level, preferably before the tree flowers and seeds are set. Herbicide works on re-sprouts and on cut tree stumps. Herbicide application should be done within one minute of cutting the tree stump. Very large tree can be injected with imazapyr or triclopyr. There is one potential biological control, the Mimosa wilt (Fusarium oxysporum f. perniciosum) can infect the root system and be fatal. It is not used though because more research is needed to confirm its effectiveness. There are 3 natural enemies of the silk tree that have been found in the United States. The first has been found from Mississippi to Florida and up to North Carolina is the chrysomelid beetle Bruchidius terrenus. The other beetle is a buprestid beetle, Agrilus subrobustus, has been spotted in Georgia in 2006. The last is a psyllid insect called Acizzia jamatonica that was also spotted in Georgia in 2006, and surveys in 2007 and 2008 showed it to spread to five other states. Even though much research has not been performed; some studies have shown these insects to be viable seed and silk tree predators. However, their effectiveness as silk tree pests and biological controls is not yet confirmed.
Gogue, G. J.; Emino, E. R. 1979. Seed coat scarification of Albizia julibrissin Durazz by natural mechanisms. Journal of the American Society for Horticultural Science. 104(3):421-423.
Hoebeke, E. R., Wheeler Jr, A. G., Kingsolver, J. M., & Stephan, D. L. 2009. First North American records of the east palearctic seed beetle Bruchidius terrenus (Coleoptera: Chrysomelidae: Bruchinae), a specialist on mimosa (Albizia julibrissin, Fabaceae). Florida Entomologist 92(3):434-440.
Pardini, E. A.; Hamrick, J. L. 2008. Inferring recruitment history from spatial genetic structure within populations of the colonizing tree Albizia julibrissin (Fabaceae). Molecular Ecology. 17(12):2865-2879.
Woodward, Susan L., and Joyce Ann. Quinn. 2011. Silk Tree. Encyclopedia of Invasive Species: From Africanized Honey Bees to Zebra Mussels. Santa Barbara, CA: Greenwood. 572-575. Print.