Large, spreading trees suitable for big properties. All produce oval or round, edible nuts in fleshy husks. Nuts of native species have a wild flavor; those of English walnut (J. regia) are the ones sold commercially. These trees have shallow, competitive root systems (a chemical released by the roots, fallen leaves, and nuts of black walnut trees can kill other plants), and their pollen can cause an allergic reaction. They tend to be out of leaf for a long time and can be messy when in leaf. Worth growing if you like the nuts or need large trees that thrive in adverse conditions.
J. cinerea. BUTTERNUT. Zones US, MS; USDA 6-7. Native from New Brunswick to Georgia, west to Arkansas and North Dakota. To 5060 ft. (even 100 ft.) tall, with broad, spreading canopy 4050 ft. across. Resembles J. nigra but is smaller, with 11 to 19 leaflets per leaf and fewer nuts, which are oval or elongated rather than round. Flavor is good, but shells are thick and hard to crack. Tolerates alkaline soil.
J. microcarpa. RIVER WALNUT. Zones US, MS, LS, CS; USDA 6-9. Native to Oklahoma, Texas, New Mexico, and northwest Mexico. Grows to about 20 ft. (rarely 30 ft.) tall and wide; often multistemmed and more shrubby than treelike. Dark green leaves have 15 to 23 leaflets, each about 3 in. long. Small (-in.) nuts are mostly eaten by wildlife. Thrives in dry, rocky, limy soil.
J. nigra. BLACK WALNUT. Zones US, MS, LS, CS; USDA 6-9. Native from Massachusetts to Florida, west to Texas and Minnesota. A huge tree that can reach 150 ft. high, though it's usually closer to 5075 ft. tall and wide in gardens. High-branched, oval- to round-headed habit. Furrowed, blackish brown bark. Leaves have 11 to 23 dark green leaflets, each 25 in. long. Nuts are 11 in. across, with a rich, distinctive flavor and very thick, hard shells (some types, such as 'Thomas Myers' and 'Surprise', are easier to crack than others). Wood is highly prized for furniture, cabinets.
J. regia. ENGLISH WALNUT, PERSIAN WALNUT, CARPATHIAN WALNUT. Zones US, MS, LS, CS; USDA 6-9. Native to south- west Asia, southeastern Europe. To 60 ft. high, with equal spread. Fast growing, especially when young. Trunk and heavy, horizontal or upward-angled branches are covered with smooth, gray bark. Leaves are medium to dark green, with five to seven (occasionally more) 3- to 6-in.-long leaflets. Roundish nuts to 2 in. long, with ridged shells and sweet, rich flavor. Husks open in fall, dropping nuts to the ground; to hasten drop, knock nuts from tree. Gather fallen nuts immediately, remove any adhering husks, and dry in a single layer in airy shade until kernels are brittle (crack a nut open to test); then store.