Ailanthus (/əˈlænθəs/; derived from ailanto, an Ambonese word probably meaning “tree of the gods” or “tree of heaven”) is a genus of trees belonging to the family Simaroubaceae, in the order Sapindales (formerly Rutales or Geraniales). The genus is native from east Asia south to northern Australasia.
The number of living species is disputed, with some authorities accepting up to ten species, while others accept six or fewer. Species include:
- Ailanthus altissima (Mill.) Swingle (Tree of Heaven, syn. A. vilmoriniana Dode) – northern and central mainland China, Taiwan, considered invasive in North America and Britain.
- Ailanthus excelsa Roxb. – India and Sri Lanka
- Ailanthus fordii Noot. – China
- Ailanthus integrifolia Lam. – New Guinea and Queensland, Australia
- Ailanthus triphysa (Dennst.) Alston (White Siris syn. A. malabarica) – northern and eastern Australia.
- Ailanthus vietnamensis H.V.Sam & Noot. – Vietnam
There is a good fossil record of Ailanthus with many species names based on their geographic occurrence, but almost all of these have very similar morphology and have been grouped as a single species among the three species recognized:
- Ailanthus tardensis Hably – From a single locality in Hungary
- Ailanthus confucii Unger – Tertiary period, Europe, Asia, and North America
- Ailanthus gigas Unger – From a single locality in Slovenia
The Ailanthus normally referred to as Tree of Heaven is Ailanthus altissima
Listed in flowering trees and shrubs / hardy ornamentals by A D Webster, the date 1751 is the date he lists as the introduction to Britain.
AILANTHUS .—Tree of Heaven. China, 1751. A handsome, fast-growing tree, with large pinnate leaves that are often fully three feet long, and terminal erect clusters of not very showy greenish-white flowers that exhale a rather disagreeable odour. It is one of the most distinct and imposing of pinnate-leaved trees, and forms a neat specimen for the lawn or park. Light loam or a gravelly subsoil suits it well. (Webster)
It is not considered an invasive species in Britain. But the plant has been spread to many other areas beyond its native range. In a number of these, it has become an invasive species due to its ability both to colonise disturbed areas quickly and to suppress competition with allelopathic chemicals. It is considered a noxious weed in Australia, the United States, New Zealand and many countries of central, eastern and southern Europe.
The tree also re-sprouts vigorously when cut, making its eradication difficult and time consuming. In many urban areas, it has acquired the derisive nicknames of “ghetto palm”, “stink tree”, and “tree of Hell” due to the disagreeable odour of its blossoms and the habit it has of spreading like a weed. The problem of odour was previously avoided by only selling pistillate plants since only males produce the smell, but a higher seed production also results. Michael Dirr, a noted American horticulturalist and professor at the University of Georgia, reported meeting, in 1982, a grower who could not find any buyers. He further writes (his emphasis):
For most landscaping conditions, it has no value as there are too many trees of superior quality; for impossible conditions this tree has a place; selection could be made for good habit, strong wood and better foliage which would make the tree more satisfactory; I once talked with an architect who tried to buy Ailanthus for use along polluted highways but could not find an adequate supply […]— Michael A. Dirr, Manual of Woody Landscape Plants
It is especially common in London squares, streets, and parks, and is also frequently found in gardens of southern England and East Anglia. It becomes rare in the north.