How to Grow Black Walnut
Juglans nigra
Black walnut (Juglans nigra) is a majestic North American timber tree that also produces richly flavoured nuts with very hard shells. CRITICAL GARDEN PLANNING CONSIDERATION - it is strongly allelopathic, producing juglone from roots, leaves, and husks that kills or suppresses many common garden plants including tomatoes, peppers, azaleas, blueberries, and apple trees. Much larger than English walnut (25-35m vs 15m). In UK gardens, only plant as a specimen tree with enormous space. Prune only in late summer to mid-autumn to avoid sap bleeding.
Yearly Lifecycle
Care Requirements
Light
Full sun; open position
Full sun for the best growth and nut production. Tolerates light shade when young but needs an open position at maturity.
Watering
Water young trees; established ones rarely need it
Water young trees regularly during the first few years. Established trees are deep-rooted and rarely need watering except in severe drought.
Fertilizing
Spring compost mulch when young; none needed once established
Mulch with compost in spring when young. Established trees on reasonable soil need no feeding. The tree feeds itself through its extensive root system.
Pruning
Late summer only; minimal pruning
Prune only in late summer to mid-autumn to avoid heavy sap bleeding. Keep pruning minimal - remove dead or crossing branches only. Black walnut bleeds more than English walnut.
Harvesting
Collect fallen nuts; husks stain permanently
Collect fallen nuts in autumn. Husks stain everything dark brown permanently - wear old gloves. Crack with a hammer. Dry kernels for 2-3 weeks.
Growing Tips
Allelopathy is the defining characteristic
Black walnut produces far more juglone than English walnut. The toxin persists in soil for years after removal. Keep ALL sensitive plants (tomatoes, peppers, potatoes, azaleas, blueberries, apples, birch) well beyond the canopy drip line - ideally 15-20m from the trunk.
Only for very large gardens
This is a 25-35m tree with a spread of 15m or more. It is primarily a timber tree. Only plant if you have the space of a small park. Consider the allelopathic impact on the entire surrounding area.
Prune at the right time
Only prune from late summer to mid-autumn. Black walnut bleeds sap even more heavily than English walnut if cut at other times.
Nuts are harder to crack
Black walnut shells are extremely hard - you need a hammer or dedicated nutcracker. The flavour is richer and more intense than English walnut, prized in baking and ice cream.
Spacing & Planting
| Plant spacing | 1200 cm |
| Row spacing | 1500 cm |
| Mature height | 2500 cm |
| Mature spread | 1500 cm |
Very large tree (25-35m). Strongly allelopathic - keep well away from sensitive plants. Specimen tree for large gardens only.
