Invasive Species of Concern in Maryland

Tree-of-Heaven (Ailanthus altissima), also known as ailanthus, stink-tree, stinking or (incorrectly) Chinese sumac or shumac, and by other less than pleasant names, is Maryland’s worst invasive alien tree, and the Maryland Invasive Species Council's September Invader of the Month. Introduced into the U.S. from China in 1784, it was initially valued as a horticultural specimen, and later as an indestructible street and shade tree. It quickly began spreading uncontrollably and has long been established in urban settings, finding a home in back alleys, vacant lots and sidewalk cracks. This is the tree in Betty Smith’s 1943 novel “A Tree Grows in Brooklyn,” with the tree being an analogy for the tough, irrepressible people of the tenements. Rapid-spreading and difficult to control, tree-of-heaven increasingly invades our forests, farms and roadsides. It has little value for wood or wildlife, and often grows in colonies that dominate an area, excluding more desirable native trees and shrubs.

Tree-of-heaven is a fast-growing tree with smooth gray bark that can reach a height of 80 feet and a diameter of more than 3 feet. It has long compound leaves, with 11-25 leaflets on each leaf, the leaflets being smooth-edged except for 1-3 teeth near the base, each with a small, translucent gland on the lower surface. The species is dioecious, with male and females flowers on separate trees. The male flowers and cut or bruised foliage have a strong disagreeable odor sometimes compared to burnt cashews or peanuts. In the winter, the tree is distinguished by the gray stems with stout, blunt brownish twigs, and by the clusters of twisted papery seeds, which often hang on the female trees over winter.

Overall, tree-of-heaven is similar in appearance to walnut and to sumacs. Tree-of-heaven is not a sumac, nor is it closely related. Walnuts have rough, dark bark and leaflets with toothed edges. Sumacs, such as staghorn, smooth and winged sumac, are desirable native shrubs that seldom reach more than 15 feet in height. Unlike tree-of-heaven, sumacs have leaflets with many small teeth on the edge, have clusters of red seeds, and turn a beautiful orange or red in autumn.

While not an especially long-lived tree, once it is established tree-of-heaven usually continues to dominate a site by re-sprouting and root suckering. It also produces a natural herbicide that is toxic to some other species of plants. Whenever the tree is cut it responds by re-sprouting from the stump or root collar. These sprouts can grow at tremendous rates; 10-15 feet per year is common. Once cut or disturbed they also send up hundreds of root suckers, often quite far from the parent tree, which also grow rapidly -- up to 6 feet a year.

The ability to re-sprout and root sucker so well is what makes tree-of-heaven so difficult to control. Seedlings and small saplings can be pulled or dug, but be sure to get the whole root system if possible. Manual or mechanical cutting can be used on larger trees, but unless cutting can be frequently repeated (typically at least monthly for several years), re-growth and spread from the root system usually makes the situation much worse. For this reason judicious application of herbicide is often used.

Tree-of heaven can be controlled by several methods of herbicide application. Foliar spray, basal bark spray, trunk injection, and treatment of a fresh-cut stump are all methods that may be appropriate under various situations. Not all herbicides are suitable, and the type and rate of herbicide used and the timing of application are all important to success. Treatments need to be made during the growing season for best results, and follow-up treatments may be needed to achieve full eradication of the infestation. As always, it is important to read and follow label directions when using a herbicide product.

The relentless spread of tree-of-heaven, once considered by many to be a hopeless situation, is now tempered with a ray of hope. Better information on controlling tree-of-heaven gathered in recent years from research and practical experience is now widely available, and is being put to good use on public and private lands. While tree-of-heaven will continue to be a serious problem, the knowledge is there for those who want to do something about it, and due to increased public awareness of the problem of invasive exotic species more people are taking action.

For more information about other Invasive Species of Concern, visit www.mdinvasivesp.org or call the Maryland Department of Agriculture at 410-841-5920.

For more information on the Internet:

Maryland Department of Natural Resources - Forest Service

Virginia Tech Weed Identification Guide

The Nature Conservancy

Plant Conservation Alliance, Alien Plant Working Group.