What Tree? Tree of Heaven

Tree of Heaven (Ailanthus altissima)

This non-native species was introduced to the UK from China in 1751, and has gained its name from the speed at which it grows.  Its botanical name ‘Ailanthus’ is derived from Indonesian word ai lanit which means a tree reaching for the sky.   However, it is considered to be an invasive species, growing very rapidly and able to easily propagate while being very hard to control.  One of its strategies to reduce competition and improve its chances of survival is allelopathy, a process whereby the tree secretes toxins into the ground around it to prevent other species growing.  As would be expected of a species that uses this technique to reduce competition, it is a very hardy tree, tolerant of pollution and ground disturbance.  The survivial strategy of this tree has implications for biodiversity in the UK as it could harm habitats and upset the delicate balance in the ecosystem.

Appearance and Identification

The Tree of Heaven can achieve heights of up to 28m and is frequently described as having a robust look with stout branching.  Mitchell describes the tree as “Usually a good straight cylindrical bole, then stout, strongly ascending branches bearing a tall, irregular dome.” (Trees of Britain and Northern Ireland)

The leaves, bark and buds of this tree share the appearance of more commonly found native species such as Ash and Walnut.  The leaves of the trees are pinnate (like ash), consisting of 12-25 leaflets.  The overall length of the leaf can be up to 60cm and like ash, the leaves flush late.  However, the similarity with ash ends here as the leaves are considerably larger and are red when flushing, and when bruised the leaves exude an unpleasant smell.  This comes from a small gland at the base of each leaflet that is thought to be a defence mechanism of the tree.  When leaf eating insects attack the leaflets, the glands release a nectar to attract ants which in turn will eat the insects.  There are reports of a physiological reaction in humans to the smell through headaches and nausea.  This tree has other names that allude to the smell including Stinking Sumac and the Stink Tree.

The flowers of this tree appear in mid-summer, with the theme of the unpleasant smell continuing through the male line.  The flowers hang in large bunches which turn an apricot colour in the autumn months.

The bark and winter buds of the tree can be mistaken, on first glance, for walnut.  The bark is often smooth and dark grey with slightly corky ridges that give a slightly snake like appearance, but it is the winter buds that can cause the most confusion.  The heart shaped leaf scars have a very strong resemblance to walnut leaf scars, with a large heart shaped  scar (also described as looking like a monkey face) on the twig.  Small reddish domes buds grow just above this scar.

Propagation & Control

The tree is dioecious meaning that each tree is either male or female but never both.  It requires both sexes for the seed to be fertile and although it will not self propagate, it does sucker vigourously when cut back making it very hard to control.  If the tree can be identified at a young stage, it should be removed ensuring the tap root is taken at the same time.  For more established trees, the stump of the tree should be removed where possible or frequent cutting back will be required where it suckers.