The common walnut (Juglans regia) was introduced to Britain by the Romans, and the black walnut (J. nigra) introduced from North America sometime in the early 17th Century. 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.
The species thrives best on well-drained and fertile soils, and is a rare component in British and Irish woodlands. It has the widest crown diameter at any given age compared to all other common trees and therefore is more often grown in pure plantations. Research led by the Northmoor Trust is testing across several sites in Britain, including the National Forest, the combining of common walnut with tree and shrub nurses. Where the nurses are nitrogen-fixing benefits have been dramatic, with improvements in vigour and form (Clark et al., 2008).
A large collection of 350 progenies was supported by the Northmoor Trust in the late 1990s (Hemery 2000). The majority were collected from the walnut-fruit forests of Kyrgyzstan in Central Asia, where there is high genetic diversity and excellent tree form. Additional collections were amassed from France, Germany, Iran, Romania, Slovakia, and Turkey. The combined provenance and progeny trials, planted across three sites in southern England in 1996, also act as an ex situ genetic conservation collection.
Five year results were published in 2005 (Hemery et al.). Damage from Spring frosts is a major issue for walnut health and quality. Much of the breeding work will focus on selections that are "late-flushing", and therefore more resistant to frost damage.
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. An international research programme was initiated jointly by Northmoor Trust and HRI in 2001 to address these issues. A range-wide collection of seeds was undertaken from plus trees and populations in the USA (13 states) and Europe (7 countries). Field trials were established at two sites in Britain during 2003, 2004 and 2005 (Clark et al., 2006).
Adequate trials exist for walnut, and one of the major findings has been to discover that shelter during the early years after planting is much more critical for this species than any other. Investigations into the most suitable silvicultural techniques will continue, mostly by the Earth Trust at Little Wittenham.
- Clark, J., Hemery, G. & Savill, P. Early growth and form of common walnut (Juglans regia L.) in mixture with tree and shrub nurse species in southern England. Forestry 81, 631-644 (2008).
- Clark, J., Hemery, G.E., Russell, K. & Williams, H. The future of black walnut in Britain. Quarterly Journal of Forestry 99, 207-212 (2005).
- Hemery, G.E., Savill, P. & Thakur, A. Height growth and flushing in common walnut (Juglans regia L.): 5-year results from provenance trials in Great Britain. Forestry 78, 121-133 (2005).
- Hemery, G.E. Juglans regia L: genetic variation and provenance performance. (2000).