Walnut family (Juglandaceae)
Description: This native tree is 80-120' tall at maturity, forming an ovoid crown and tall trunk. On mature trees, the trunk is 3-6' across, developing a deeply furrowed bark that is gray to nearly black. The bark of branches is more smooth and gray. Alternate compound leaves are 1-2' long and ½' across; their structure is odd-pinnate, although the terminal leaflet is often deformed or missing. Individual leaflets are about 3" long and 1" across; they are lanceolate or ovate-lanceolate, serrated along their margins, and nearly sessile. The upper leaflet surface is medium green and glabrous, while the lower surface is pale green and short-pubescent along the ribs of the veins. The rachis (central stalk) of each compound leaf is light green to pale brown and short-pubescent. The rather stout petioles of the compound leaves are 3-6" long, light green to greenish brown, and pubescent.
Black Walnut is monoecious with individual trees bearing either all male (staminate) or all female (pistillate) florets. The male florets occur on dangling cylindrical catkins about 3-6" long. Individual male florets are about 1/8" across, consisting of a short tubular calyx with 2-6 lobes and 15-40 stamens. The female florets occur on short spikes in groups of 2-5. Individual female florets are about 1/8" across, consisting of a short tubular calyx with 4 lobes and a pistil with a pair of styles. The calyxes of both male and female florets are short-pubescent. The blooming period occurs during late spring as the leaves begin to develop. The florets are cross-pollinated by the wind, although female florets are often self-fertile should cross-pollination fail to occur. Fertile female florets develop into ovoid-globoid nuts about 2½" long and 2" across that occur either individually or in pairs. These nuts are covered with thick green husks that later become yellowish brown and finally black. The husks are covered with a pimply surface and they are aromatic. When the nuts are harvested before their husks blacken, the shells of the nuts are light brown and wrinkled. Each nut is incompletely 4-celled and contains edible meat that is sweet and slightly bitter. The root system consists of a deep taproot and widely spreading lateral roots. This tree reproduces by reseeding itself. The deciduous leaves turn yellow and fall to the ground rather early during the autumn.
Cultivation: The preference is full or partial sun, moist conditions, and deep soil containing loam, silty loam, or sandy loam. The pH of the soil should be slightly acid to neutral. This tree tolerates spring flooding if it is temporary. Because of the deep taproot, it is difficult to transplant successfully. To germinate the nuts, they must be subjected to cold winter temperatures for 90-120 days. The resulting seedlings may produce nuts as early as 5 years, but significant production of nuts doesn't occur until they become 10-15 years old.
Range & Habitat: In Illinois, Black Walnut is a common tree that is probably found in every county (see Distribution Map). Habitats consist of rich mesic woodlands, moist bottomland woodlands in valleys and along rivers, and the bases or lower slopes of bluffs. Black Walnut is found in deciduous woodlands with moisture-loving maple, hickory, oak, and ash trees. Sometimes this tree is planted deliberately because of its edible nuts and valuable wood.
Faunal Associations: Miscellaneous insects use Black Walnut as a food source, particularly the caterpillars of many moths (see Moth Table). Other insect feeders include the wood-boring larvae of long-horned beetles, leaf beetles, stink bugs, plant bugs, aphids, and species from other families (see Insect Table). The nuts are an important food source for the Gray Squirrel, Fox Squirrel, Red Squirrel, and Eastern Chipmunk; these animals bury the nuts in secret caches and help to distribute them to new locations. White-Footed Mice also feed on the nuts, while rabbits sometimes gnaw on the stems of young trees during the winter.
Photographic Location: Meadowbrook Park in Urbana, Illinois.
Comments: Because of its nuts and high quality wood, Black Walnut is a valuable tree, although a rather messy one. The rotting husks of the nuts can stain side sidewalks and driveways a brown-black color that is difficult to remove. Even immature green husks can stain your fingers yellow when they are handled. The closest relative of Black Walnut in Illinois is the less common Juglans cinerea (Butternut), which has been declining in numbers because of disease. The nut husks of Butternut are more smooth and pubescent than those of Black Walnut, and its nuts are a little smaller in size and more narrowly ovoid in shape. The young branchlets of Butternut are downy, while those of Black Walnut are smooth. In general, both Butternut and Black Walnut differ from the related Carya spp. (Hickories) by their unsegmented nut husks and the chambered (or partitioned) pith of their twigs.