Silviculture

Black walnut is a light demanding species that is intolerant of competition whether from ground vegetation or from other trees. It develops a long tap root which is easily damaged when seedlings are lifted from bare-root nursery beds and can result in delayed establishment. Seedlings grown in deep containers are preferable planting stock. The long ‘carrot shaped’ tap root requires careful site preparation and potentially pit planting followed by several years of rigorous weed control if establishment is to be successful. Planting density is often around 1000 to 1500 stems ha^-1 wherever quality sawtimber is an objective. Young trees can be cut back to the base (‘stumped’) and resultant regrowth singled to improve growth and form.

In its native range, black walnut typically grows as individual trees or small groups in mixture with a range of other broadleaves. This approach can be followed in Britain by creating mixtures with other broadleaved species like lime, wild cherry or silver birch that can provide shelter for the young walnuts. Nitrogen fixing shrubs such as autumn olive can also be helpful. To ensure sustained growth it is essential to maintain an open crown (< 25% of the crown with any competition) so regular thinning of mixtures is critical. Selected walnut stems should be pruned to about 6 m height to increase the outturn of valuable timber. A typical rotation will be 60 to 80 years for trees grown for sawn timber, when diameter at breast height (DBH) should be 40 cm or more.

The species starts to flower early, and regular seed production can occur from 15-20 years onwards. However, natural regeneration is rare in Britain and could be due to browsing or vegetation competition. Thus, planting is the most reliable method of establishment. In North America, the species is grown in agroforestry systems where it provides both nut production and an eventual timber crop.