Black walnut trees, Juglans nigra, is a deciduous tree grown in the East and Midwest. Find facts and info on growing and using black walnut trees in your landscape and the juglone toxin it produces.
Uses of Black Walnut
It is true that black walnut trees are common in the East and Midwest, thanks to squirrels that plant the seeds and to people who farms the tree for its hardwood and edible nuts. When black walnut is raised for wood production, it is grown in competitive, forest-like conditions to encourage columnar trees with long trunks and short crowns. The heartwood, which is warp, shrink, and splinter-free, can touch the ground without rotting. Gum makers use it for strong, smooth stocks and railroad engineers use it for railroad ties. Abrasives made from ground walnut shells are used for blast-cleaning metals, alloys, and plastics without scratching and without mineral and metal contamination.
Growing Black Walnuts
On the other hand, cultivating black walnut for its fruit is best done in a sunny, open space, where the tree develops a short trunk that splits into an upward form with widespread branches that have large nut crops every couple of years. The heaviest fruiting takes place when the tree is about 30 years old. Black walnuts grow 50’ to 75’ h and w., in Zones 4-9. They grow in full sun and some in light shade. It prefers deep, fertile, moist, well-drained soils with neutral pH for maximum growth.
Black walnut fruit (black walnuts) are enclosed in a green, outer husk and an inner, dark-brown shell, which is good to eat, but hard to shell. Black walnut trees bear alternate, deciduous, compound leaves up to 2” long, comprised of 15-23 lance-shaped 3 1/2” x 1 1/2” leaflets arranged featherlike along a midrib. The summer green turns to a yellowish color in fall. Dark blackish gray bark has diamond-shaped ridges surrounded by deep, irregular grooves. Like birch, black walnut produces spring flowers on male catins and female spikes. Rounded, green, 2” fruits appear every 2-3 years, dropping just after the leaves and then turning black. Inside you will find sweet oily, edible nuts in a ridged brown shell.
Walnut trees produce tasty, high-protein nuts for baking, roasting, or eating raw. In addition to their edible hard shelled fruits, they are grown for their powerful winter structure and distinctive, coarse-textured foliage that grows 1’-2’ long in a featherlike pattern of leaflets along a mid-rib.
Toxic Juglone in Black Walnut Trees
To ward off competition from other plants, walnuts secrete juglone, a chemical toxic to many nearby plants. This toxin causes problems with many other plants that grow around this tree, even to the point of killing everything growing under or near the tree.
Caution: Be sure to wear gloves when collecting ripe black nuts off of the ground, since they can stain your skin.
Landscape Uses of Black Walnuts
Grow them as a specimen in large lawns for its fruits and for shade. Plant away from the house because of litter from the nuts and big fallen leaves. If you are growing your trees for the nuts be sure to plant at least two so that the trees will pollinate.