Walnuts need careful pruning as they heal slowly (if at all) after bad tree work. The best time to prune is in late July when the tree’s defences against infection are their best and the chance of good regrowth over the cuts is good. Lopping or pollarding by simply reducing the length of all the branches is disastrous. They lead to massive and vigorous new growth that makes the canopy denser and is particularly ugly in winter.
‘Crown thinning’ removes branches back to their point of origin and reduces the density of the canopy without stimulating dense clusters of new shoots. Percentages, such as 30%, are used to specify the extent of the work. ‘Crown reduction’ takes this process a stage further and takes out whole branches and reduces the overall canopy size.
A tree surgeon should be fully qualified and experienced and do all the work in accordance with British Standard 3998:2010, Tree work – Recommendations. One particular need is to not cut the branches off flush with the trunk but to leave the ‘collar’ (a small ridge around where the branch meets the trunk) intact without leaving a long branch stub. The tree has natural defenses in this area that deter the fungal and bacterial infection that cause the wood to decay.
Many people say ‘my tree needs pruning’. Well, it does not – unless it is for safety reasons or to remove dead wood. Pruning is possible if it is done well, especially with a more vulnerable species such as walnut.