Walnut trees are much-loved by many growers who recognise their fast growth and high value. However, both the common (often called English) walnut Juglans regia and black walnut J. nigra are quite particular in terms of optimal growing conditions. Hybrid walnuts provide an exciting alternative.
Many trees will naturally hybridise in the wild with similar species in the same family: examples include larch, lime and poplar. It is also possible to encourage and control hybridisation through tree breeding programmes. Hybrid species tend to have greater than either of their parents, and may be more tolerant to a wider range of conditions.
Across Europe several varieties of walnut hybrids have been bred and made commercially available. They include both Juglans nigra x J. regia (e.g. NG23, NG38) and J. major x J. regia (e.g. MJ209) crosses, all of which are vigorous and have good form. Hybrids planted into existing woodlands in continental Europe have grown very well, generally have good apical dominance and fewer branches than either species.
There have been limited plantings of walnut hybrids in the UK. I planted some silvicultural trials, with the Northmoor Trust in the early 2000s. In 2003 I established a hybrid varietal trial with the Northmoor Trust. The hybrid walnuts were planted in a mixture with alder (Alnus glutinosa), silver birch (Betula pendula) and wild cherry (Prunus avium). The alder, as a nitrogen-fixing nurse tree, was included to provide a natural fertilising effect for the walnut.
As the graph shows, once that the walnut trees had become established, growth was very good. The NG38 trees in particular grew phenomenally well; being over 3 metres tall by year 5. One individual NG38 tree grew 2.68m (over 8’6″) during the fifth growing season alone!
Hybrid walnuts have the potential to be a fast-growing home-grown hardwood alternative to timber imported from far away. They do not have the dark heartwood so prized by furniture makers but hybrid walnut timber is easy to work. can be sliced for veneer and is easily stained. Lastly yet importantly, they are likely to benefit from projected climate change.
I recently published an article on this: Clark, J., and Hemery, G. ( 2010 ) Walnut hybrids in the UK: fast growing quality hardwoods. Quarterly Journal of Forestry. 104, 1, 43-46. Read the full article at Forestry Horizons.