Two species of walnut are commonly used in British forestry for timber purposes: Common or European walnut (Juglans regia L.) and black or American walnut (J. nigra L.).
About walnut
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.
Why walnut is important
Walnuts are the only trees that can be grown in Britain which produce naturally a dark hardwood timber, making them a sustainable alternative to many tropical timbers. Walnut timber consistently demands the very highest prices due to its fine figure and quality, and rarity. Walnuts are likely to benefit from projected changes in the climate.
Black walnut is economically one of the more productive broadleaved timber species in Britain. However, it is also one of the least planted species. There is insufficient knowledge about the species among foresters and very little, if any, improved material is available.
Many walnut hybrids exist of various crosses and these typically outperform pure species on most sites.
Our achievements so far
We have established progeny and provenance trials for both common and black walnut across several sites, including Lount Wood in the National Forest. However, results have been disappointing due to poor form in all trials attributed to exposure and insufficient hardening in the autumn. Knowing its reputation as a difficult species to establish, silvicultural trials were also established concurrently, and these have yielded startling results. Growing walnut with other species (nurse species) can provide shelter to help the walnut through the tricky establishment phase. Using a faster growing tree nurse such as Alnus cordata and a spreading shrub nurse such as Elaeagnus umbellata, both of which are nitrogen fixing, provide shelter and encourage upward growth of the walnut, dense shade to supress ground vegetation, and also provide the walnut with a fertilising effect.