Walnut is a deciduous broadleaf tree which can grow to 35 metres in height. Native from south-east Europe through to south-west China, walnut trees typically have a short trunk and a broad crown though they can be narrower if grown in a woodland situation. Walnut has been widely planted throughout the UK and has naturalised in lowland Britain in secondary woodland and hedgerows as well as on river banks, field borders and roadsides, helped in no small measure by squirrels hoarding the walnuts.

The leaves are the foodplant for caterpillars of a number of micro moths, and the nuts are eaten not just by humans but by mammals including mice and squirrels.