Fraxinus excelsior is a species of deciduous broadleaf flowering plants (in the Oleaceae or Olive family), native to Europe including Britain, Asia Minor, the Caucasus and Iran. It is a large tree, which can attain about 40 metres and live for 200 years. It spreads at its crown to a variable extent, forming a graceful open canopy which allows light to reach the undergrowth. In young trees the bark is smooth and light grey, becoming vertically fissured and grey-brown as it ages, and often covered with lichen. The wood has a light colour, low density and is tough, flexible and shock-resistant, and used for furniture and tool handles. Traditionally ash was cultivated by coppicing and pollarding.

In winter, Fraxinus excelsior is often recognisable from a distance by its hanging bunches of brown winged fruit (samaras, called "keys"), which persist from the summer. Close up, it can be recognised in winter from its large black buds, with three buds at the end of a twig. In other ash (Fraxinus) species, the buds are brown, for instance Fraxinus ornus, also present in Whiteknights Park. The buds open in spring, initially looking somewhat like blackberries as the dense bunches of small flowers emerge. The leaves are fairly small and narrow, with a pointed tip and toothed edges, arranged in 3–6 pairs on opposite sides of a stem, with one at the end (botanically, they are leaflets comprising a compound leaf). In autumn they can remain green until they fall, but sometimes turn yellow. From the spring one may also notice shapeless green or brown blobs hanging from the twigs, becoming black in winter; these are "galls", arising from male flowers that have been parasitised by the Aceria fraxiniflora mite.

Fraxinus excelsior is a pioneer species, which colonises open areas. It thrives in moist chalky or limestone soil. It does not grow well in cold conditions, and is therefore not found up mountains in Britain, unlike the the rowan, Sorbus aucuparia, also present in Whiteknights Park, which is also called "mountain ash" because it has somewhat similar leaves. Fraxinus excelsior is the third most common tree in Great Britain, accounting for about 12% of woodland area, but its numbers are being reduced by ash dieback, caused by the fungus Hymenoscyphus fraxineus or Chalara fraxinea, which came from Asia and was first recorded here in 2012. This disease gradually kills the branches and thins the canopy, working downwards in the tree, until eventually it is all dead. The current estimate is that half to three-quarters of all ash trees will be killed over the coming years, but not all individuals are susceptible, and there are indications that resistance is evolving.

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