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Black Walnut

Black walnut (Juglans nigra), a rapidly growing tree common in all of Ohio, is most common in moist bottomlands and open fields, but is found everywhere due to squirrels burying its nuts. Its beautiful, fine-grained, chocolate-brown, relatively lightweight heartwood is the ultimate choice for making solid wood furniture, interior trim, gunstocks, and high-quality veneer. The large nut contained beneath the husks of black walnut is round and can be cracked open to expose the bittersweet, oily, and highly nutritious kernel. A native of the Eastern, Midwestern, and Great Plains regions of the United States, black walnut is a pioneer invader tree in open fields or cut-over woodlots and grows rapidly in youth. It displays an irregular and open growth habit when young, dividing into several spreading branches that give it an upright rounded shape as it matures. Its bold winter texture makes it an outstanding tree to observe during the dormant season. This tree may easily grow to 70 feet tall by 70 feet wide when it is found in the open. As a member of the walnut family, it is related to other walnuts and to the hickories. Black walnut prefers deep, moist, rich, well-drained soils under sunny conditions, especially the bottomlands of rivers and streams. It also tolerates relatively dry, poor soils, but with a significantly reduced growth rate. Seedlings and saplings are notorious for having a single, very deep taproot that makes transplanting difficult. Black walnut grows in full sun to partial sun and is found in zones 4 to 9. Aside from leaf spot, black walnut is virtually disease and pest free. However, it is famous for the production by its roots of juglone, a chemical that is toxic to some nearby competitor plants. In a woodland setting, very few plants grow under the canopy of this species. When summer drought occurs, the response of this tree is to begin dropping leaves, in spite of its deep taproot system. In an urban setting, a constant rain of leaflets, rachises, dead twigs, stain-laden whole fruits, and debris from squirrel feeding occurs from mid-summer until late autumn, presenting a constant clean-up chore and mowing hazard. The long leaves of black walnut are alternate and pinnately compound, emerge later in spring than most other trees, and have 11 to 23 leaflets. Each leaflet has fine serrations on its margin, and the terminal leaflet at the end of the long leaf is frequently absent on leaves from mature trees. While most parts of this tree are pungent when rubbed or bruised, its leaves are especially so. Fall color is poor or absent, being faded green to chartreuse at best, with many of the leaves beginning to drop by mid- to late summer as a response to drought. Male and female flowers of black walnut occur on the same tree (and thus it is a monoecious species). Male flowers, known as catkins, droop from the previous year’s twig growth while smaller female flowers occur on the current season’s stem growth. Flowering occurs in mid- to late spring as the foliage is just beginning to emerge. The fruit of black walnut is composed of an inner kernel, surrounded by a hard corrugated round shell composed of two fused halves. This in turn is surrounded by a thick outer husk that is green when immature, and yellow-black when ripe. If ripe fruits are picked up, a brown-black dye will easily seep from the moist husk into the skin of your hand, rendering them stained for a couple of days. The dormant twig of black walnut is characterized by being stout, having large leaf scars without hairy twigs, and with a prominent terminal bud. The bark of black walnut is flaky when young, but usually becomes ridged and deeply furrowed with age, forming a diamondback pattern as the ridges interlace. Sometimes, the bark alternatively becomes platy with age. Bark color is brown-gray to gray-black, and may serve as another reason that contributes to its common name.