Common Walnut Silviculture Trial
Walnut in Britain is usually grown in pure stands or as individual trees, rather than in mixed plantations. Research conducted in the USA and Italy has indicated that walnuts can benefit from being planted in mixed stands, particularly with nitrogen-fixing species.
Advantages may include increased growth rates and improvement of tree form, combined with protection from frost damage and reduced weed competition. Three field trials were established in southern England during spring 2000, to test the effects of growing walnut (Juglans regia) with a number of companion species, under the auspices of the British & Irish Hardwoods Improvement Programme (BIHIP).
Most forestry literature regarding walnut silvicultural methods suggest that walnut (Juglans regia) must be grown in open conditions due to the species' intolerance to competition. Most other hardwood tree species however can benefit from using a suitable nurse species, including Juglans nigra, which has been shown to have improved growth when grown with nitrogen-fixing nurse species.
More recently there has been some experimentation on suitable nurse species with black walnut (Juglans nigra) in the USA. In the USA black walnut has been planted with black locust (Robinia pseudocacia L.), 'autumn-olive' (Elaeagnus umbellata Thunb.) which we know as spreading oleaster, and common alder (Alnus glutinosa). These species were chosen for their nitrogen-fixing capabilities. They found that all nurse species increased walnut growth, but only on certain sites.
The Elaeagnus nurse promoted walnut height increases of up to 351% over non-nursed walnuts. Problems with rapid growth from the black locust necessitated severe control of nurse height (coppicing or ring-barking). The alder nurse had a high mortality after 5 years due to an allelopathic reaction with the walnut.
The planting pattern within the multi-site trials used a triangular spacing for the walnut trees. Tree nurses are planted on the opposite pattern to give the walnuts early growing room but also to provide micro-climatic shelter and to encourage vertical growth. The shrub species surround the bases of the walnut trees providing shade and therefore weed control and, with some species, a nitrogen-fixing capability.
| Timber main crop | Walnut Juglans regia |
| Tree nurses | Alder Alnus cordata |
| Birch Betula pendula | |
| Cherry Prunus avium | |
| Western Red cedar Thuja plicata | |
| Shrub nurses | Elder Sambucus nigra |
| Hazel Corylus avellana | |
| Oleaster Elaeagnus umbellata |
Hemery, G. E. (2001).Growing walnut in mixed stands.
