Ailanthus is a family of about ten fast growing, deciduous trees in the Simaroubaceae genus. They are native to Asia and Australia but have been imported to many other parts of the world. They have spread to the point where they are considered a noxious, invasive species in both Europe and North America. These trees are identified by their groups of long, pinnate leaves that grow alternately on long stems, much like a fern or sumac in appearance. Clusters of tiny yellow green or reddish flowers appear in late spring or summer depending on their location.
Also known as tree of heaven, white siris, or tree of the gods, this tree is short lived, usually only surviving for about 50 years. It can grow extremely tall, reaching up to 90 feet (27 m). Only one species, Ailanthus altissima, grows in temperate climates and survive cold winters. It is this variety that is found in the U.S. and northern Europe. All other varieties are tender perennials and grow only in tropical or subtropical areas.
Ailanthus are not particular about growing conditions and will thrive nearly anywhere. They spread rapidly by suckers that can damage pavement, foundations, and walls. Since they grow very tall, they tend to crowd out native trees. These invasive trees can be very hard to remove; even when they are cut down or burned, the stumps and roots continue to produce new sprouts for years. It is seldom grown as a landscape tree because the ragged appearance of its fruit is considered unattractive.
Another undesirable feature of the ailanthus tree is its fetid odor, which has been compared to the scent of male cat urine. The smell is strongest within the leaves and can be overpowering when they are crushed. This persistent odor can adhere to the hands and does not wash off easily.
One single mature female ailanthus tree can produce as many as 350,000 seeds which have a high rate of germination. These seeds are carried through the wind in small winged pods, and will sprout wherever they land, usually in an area where they are not wanted. Wherever possible, seedlings should be removed while they are very small, and when the ground is moist, to remove the entire long tap root.
The bark of the ailanthus tree has been used in the past in Chinese medicine to treat many different conditions including mental illnesses, intestinal problems, and baldness; today it is used mainly for its astringent properties. Ailanthus trees are home to the ailanthus silkworm in China, which feeds on the leaves and makes its cocoons within the trees. A type of Shantung silk is made from the unwound cocoons and is said to be crude but durable.