Bartlett Tree Experts: Common Walnut

Juglans regia

This deciduous tree has bark which changes from green brown when young to pale and fissured when mature. Walnut was introduced by Romans; the food of their god Jupiter. Prized for woodworking the wood at the base of the tree especially valued for its beautiful swirling grain patterns. The leaves and mature fruit yield a dark brown dye.

Culture
Concerns
Management

Culture for Common Walnut

Grows slowly to over 12m, the canopy can be wider than 8m. Low maintenance and frost hardy. Grows best in full sun, on well drained, fertile soils. Tolerant of most soil types and pHs, but prefers slightly alkaline soils. Walnuts naturally release chemicals that can hamper the growth of other surrounding plants.

Concerns about Common Walnut

Aphids can be an issue. Susceptible to Honey Fungus (Armillaria). Walnut leaf blotch causes small brown spots that join to form blotches, sometimes with yellow margins. Sunken blotches may also appear on fruit and petioles. Fruit can shrivel and severely affected leaves will drop. Coral spot, a fungus, causes branch die back followed by pink blisters on bark. Thousand Canker Disease, a beetle, and fungus combination is a potential threat but not yet present in the UK.

Management Practices for Common Walnut

Inspect the base of the base of the tree for peeling bark with mycelial fans/rhizomorphs beneath. If Honey Fungus is present ensure the tree is unstressed to reduce susceptibility and remove any sources of infection as far as possible. Buried root collars should be excavated. Check for aphid symptoms: curling leaves and honeydew. For leaf spot, remove and destroy fallen leaves. In severe cases apply an appropriate fungicide/insecticide. Apply caution if fruit is present and likely to be consumed. A treatment plan of 3 to 4 sprays at regular intervals throughout the growing season is recommended. Prune out and destroy coral spot material, sterilizing equipment between branches.