Sometimes regarded as one of North America's finest native forest trees, the black walnut (Juglans nigra) provides stately shade on a hot afternoon, dark wood to furnish our homes and nutritious nuts to delight our taste buds. Best grown in deep soils that are moist, the tree may not be the best choice for gardeners as some plants fail to prosper around the chemical-emitting roots of the black walnut.
Black walnut is native to the deep, fertile, moist soils of the eastern half of the United States. Its range extends from New England westward across to the Great Lakes, including southernmost Ontario in Canada and Minnesota. It then progresses southward to central Texas and again eastward inland of the Gulf Coast.
This is a large deciduous tree, growing to a mature height of 50 to 75 feet and nearly equal spread in its branched canopy. The leaves, one to two feet in length, are comprised of up to 23 leaflets that are dark yellowish green in color and if crushed emit a skunky odor. The round fruits are bright lime green when developing, becoming slightly larger than golf balls when ripe. The fruits turn black, drop to the ground in late summer and fall, and crack open to reveal edible, oily nut flesh inside.
Association with Other Plants
Across the woodlands of eastern North America, rarely are large groves of black walnuts found. They are usually growing in communion with other deciduous trees, such as basswood, beech, sugar maple, tulip poplar and oak. The U.S. Forestry Service notes that the presence of Kentucky coffeetree often indicates a site favorable for growing black walnut trees.
A toxic substance called juglone is present in black walnuts leaves, stems, bark and roots and acts to retard the growth of many plants close to the tree, an inhibition process called allelopathy. This gives walnut trees a competitive advantage in the woodland ecosystem but may cause plant death in a garden setting. Not all plants are affected by the juglone, but tomatoes, some pine species and apples and crab apples cannot grow when planted too near a black walnut tree.
The nutty fruit of black walnut has a strong, earthy flavor in the oily, brown nutmeat flesh. The flavor is much more intense than that of the English walnuts readily found at the local supermarket. Black walnuts are a high-energy food, rich in fats and proteins and supply minerals such as iron, copper and manganese to the diet.
Unripe fallen green fruits from the tree can stain the skin if handled. The shell or hull protecting the edible flesh is hard and allowing the fruits to dry naturally usually causes the shell to crack open without excessive pressure. Those who harvest fallen fruits use a knife to score the hull in order to access the nutmeat.
The black walnut is an ornamental shade tree, timber tree and nut tree. The wood is prized for furniture, veneer and other small carved items such as handles and kitchen utensils. Although not as easy to harvest as the English walnut, black walnuts can supply nutmeat for the human diet.