Juglans nigra aka Black Walnut

Black walnut, Juglans nigra, is a large deciduous tree in the walnut family, Juglandaceae. It is native to central and eastern North America. Its range runs from southeast Canada and New England west to the Dakotas and Minnesota. It extends south to Texas and Florida. Other common names include American walnut. The species name means black and refers to the dark bark and the stain from its nuts.

In gardens the tree usually reaches 50 to 75 feet (15 to 23 metres) tall. Wild trees can reach 90 feet, and records note heights up to 125 feet (38 metres). The trunk may grow 3 to 6 feet across. The crown is oval to rounded and open, with a coarse texture. Older trees shed their lower limbs, so branching often starts about 20 feet up. The bark is dark brown to greyish black. Deep, narrow furrows give it a rough diamond pattern.

The leaves are alternate and pinnate, 12 to 24 inches (30 to 60 cm) long. Each carries 15 to 23 finely toothed, lance-shaped leaflets. The foliage smells when crushed. Leaves appear late in spring and turn clear yellow before dropping early in autumn. Stout twigs show a chambered, buff pith. Leaf scars have three lobes and look like a small monkey face.

Each tree bears both male and female flowers. Green male catkins hang 2.5 to 5.5 inches long in April to June. The small female flowers sit in short clusters at shoot tips. Wind carries the pollen. By October the tree bears round green fruits about 2 inches (5 cm) wide. A firm husk encloses a dark, hard-shelled nut. Small crops can start after six years, but good crops often take about twenty.

Black walnut grows best in full sun and deep, rich, moist soil that drains well. It tolerates light shade, clay, sand and a wide pH range. It copes with heat, drought and compacted ground. Poor sites slow its growth. Hardiness covers USDA zones 4 to 9, and the RHS rates it H6. A deep taproot makes moving the tree difficult. Gardeners raise it from seed or by grafting.

The tree suits parks, meadows and large landscapes as a shade tree, but not streets. Its timber ranks among the most prized hardwoods for furniture, cabinets and veneer. Logging has greatly reduced wild stands. The husks, nuts and bark yield brown or yellow dyes. Caterpillars of the luna moth, hickory horndevil and banded hairstreak feed on the leaves. Squirrels eat the nuts.

The roots, leaves and stems release juglone, a chemical that harms many nearby plants. Tomatoes, potatoes, peppers, blueberries, azaleas and peonies suffer most. Possible diseases include anthracnose, leaf spot, canker, root rot and honey fungus. Thousand cankers disease threatens trees in some regions. Pests include aphids, fall webworm and walnut blister mite. Falling nuts make a mess, and the husks stain skin, clothes and paving.

The nut kernels taste sweet and rich but are hard to extract. People use them in baking and sweets. The seeds also give an edible oil, which spoils fast. The sap is sweet and drinkable. The tree is toxic to dogs and horses. Mouldy hulls can cause seizures in dogs, while horses may develop colic or laminitis. Contact can occasionally cause skin irritation in people.