Posted: 27/06/2015 at 21:39
To quote Alan Titchmarsh "Do not prune older trees unless absolutely necessary. They are prone to cavities after severe pruning if the collar at the base of the branch is damaged or stumps are left behind. For this reason it is worth raising the canopy when the tree is still young and keeping the stem clear as it grows.".
Joy Bostock of the RHS says "Established walnuts, whether fruiting or ornamental, are best left unpruned. Where pruning is necessary it is important that the work be undertaken between mid-summer and early autumn and that the branch collar be left intact. Flush cuts that remove the branch collar are prone to decay. Hard pruning is not tolerated. Avoid pruning walnuts in late winter or early spring (January-April) as the sap rises early and the pruning cuts will bleed profusely.".
If it has just started fruiting (nutting?) I would just leave it as it is just getting into its stride.