Common walnut was introduced to Britain by the Romans. It is an economically important species cultivated worldwide for its wood and nuts. It is generally accepted that J. regia survived and grew spontaneously in almost completely isolated stands in its Asian native range after the Last Glacial Maximum. Despite its natural geographic isolation, J. regia evolved over many centuries under the influence of human management and exploitation and spread throughout Europe via ancient trade routes such as the Persian Royal Road and Silk Road which enabled long-distance dispersal from Iran and Trans-Caucasus to Central Asia, and from Western to Eastern China.
Common walnut is a light-demanding species and has a larger crown diameter in relation to any given stem diameter than any other broadleaved species used in European forestry. It is also intolerant of shading and for these reasons is traditionally planted at very wide spacing. This creates a problem of delayed self-pruning, resulting in heavy branches low on the stem, which potentially reduce the value of the timber. Walnut is a relatively nitrogen demanding species. Although they root deeply, walnut species are susceptible to water stress and this can be associated with decreased availability of nitrogen. They come in to leaf late in the season, and are late to harden off in the autumn, so winter cold damage is often a problem. For these reasons, walnut has a reputation for being tricky to grow well and only really flourishes on deep free draining soils where exposure is not an issue.
On more challenging sites greater success is often obtained by using black walnut which appears to be more cold hardy. Black walnut (J. nigra) was introduced from North America sometime in the early 17th Century.
Black walnut trees at the entrance to Jaguar Lount Wood