The Tree of Heaven is a non-native tree of the UK and is occasionally planted in parks and green spaces. There aren't many of these trees across in Exeter so if you find one - you're quite lucky! There are two fine specimens at St Thomas pleasure grounds and one at Northernhay Garden - all are female. I have yet to find a male Tree of Heaven in Exeter - please let me know if you do find one! The main reason the female trees are planted and not the male is mainly down to the smell. During flowering season the smell of the male flowers has quite a strong odour - some people say it smells like old gym socks!
This tree is native to China and was introduced into the UK in 1751. It has also been introduced into other parts of Europe and the U.S.A. It is only planted as an ornamental tree as no parts of the tree provide any edible use. From a distance it can be easy to mistake this for a Common ash - but it is very different. Other than the leaf arrangement and buds being very different the Tree of Heaven releases toxins into the ground which prevent other trees (and plants) from growing nearby. This is called allelopathy and only a handful of trees do this very unique 'natural defence'. The tree is also very fast growing and has extremely invasive roots - easily ripping up pavements. The seed production is also quite heavy and can easily spread new trees nearby (where both male and female trees are present). Due to the these characteristics - it is now classed as a 'weed tree' and not very popular amongst some gardeners. 
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Each leaflet is 5–18 cm (2-7 inches) long and 2.5–5 cm (1-2 inches) wide
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The twigs are quite chunky and the bus are round and reddish
Tree of Heaven can reach heights of 15 m (50 feet) in 25 years - that's pretty fast! However, like most fast growing deciduous trees they are quite short lived - around 50 years but in some cases up to 100 years. The leaves are very pretty and are arranged alternately on the stem and each leaf can have anything from 11 to 40 or more leaflets. They are pinnately arranged  and can range in size from 30 to 90 cm (1 to 3 feet) in length. When this tree is in full leaf it is particularly stunning when being underneath it's canopy. The leaf glands are also quite easy to spot on the leaflets - making it easier to identify this tree. The leaf scars are very interesting during the winter time and look like green shields.
The flowers are yellow / green and pollinated by insects
The young samaras start off light green and as they mature turn a light brown colour and become papery
In most cases the Tree of Heaven is dioecious which means there are separate male and female trees. The male trees produce many more flowers than the female trees and also have the rather 'unpleasant' smell. The individual flowers are yellowish green to reddish in colour each with five petals and sepals They are in large panicles (clusters) up to 50 cm (20 in) and are insect pollinated.  In Exeter they are best seen for their flowers between June and July. The female flowers develop into papery winged fruits called samaras. Each samara will hold one seed but there will be hundreds of samaras per cluster. The samaras (when in seed stage) are distributed by the wind and can easily be carried from the parent tree to a good 300 metres away. 
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The bark is smooth and light grey, often becoming somewhat rougher with light tan fissures as the tree ages
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The leaf scars are bright green and look a bit like shields
In China this tree is used in traditional medicine - including the roots and bark. The tree has been grown extensively both in China and abroad as a host plant for the ailanthus silk-moth, a moth involved in silk production. Due to some of the undesirable characteristics of this tree it is also known as the "tree of hell" among gardeners and conservationists. In the Eastern part of the U.S.A it is sometimes know as the Ghetto Palm. This tree should never be planted in a street or avenue or anywhere near pavements or roads - it will easily damage underground pipe systems. If they are planted in large parks and green spaces and given plenty of room to grow then they are fine.
OTHER USEFUL DATA
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The Biodiversity Heritage Library
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Pierre-Joseph Redouté, Public domain, via Wikimedia Commons
OTHER USEFUL RESOURCES
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www.inaturalist.org
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www.tree-guide.com
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https://powo.science.kew.org
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www.treesandshrubsonline.org
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https://pfaf.org
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www.vdberk.co.uk
QUICK FACTS
  • The Tree of Heaven is native to both northeast and central China, as well as Taiwan
  • Other common names are ailanthus, varnish tree, or in Chinese as chouchun ('foul smelling tree')
  • It is a  deciduous tree which means it sheds it leaves during the winter
  • It is a fast growing tree - reaching heights of 15 m (50 feet) in 25 years
  • Most trees will live up to 50 years of age but some specimen trees will reach up to 100 years
  • In most countries it It is considered a noxious weed and vigorous invasive species
  • It was introduced to the UK in 1751 and was widely planted
  • In most cases the Tree of Heaven is dioecious which means there are separate male and female trees
  • The male flowers can be quite smelly and for this reason it is the female trees that are mostly planted
  • The flowers are insect pollinated and the seeds are dispersed by the wind
  • The tree of Heaven is allelopathic and will release toxins into the ground to stop the growth of other trees and plants
  • The root system is very invasive making this a very poor decision as a street tree
  • They can send out many underground suckers which can soon lead to new trees growing
  • In China this tree is used in traditional medicine - including the roots and bark
  • A Latin superlative meaning very high or tallest; altissima: from alta, high
PLEASE LEAF ME ANY FEEDBACK / COMMENTS
If there is anything out of place or wrong please contact me. Equally if there is anything you wish to add please let me know. The more information we have about Tree of Heaven the better. Many thanks!