A small tree, often semi-shrubby; young shoots covered with short, yellowish down. Leaves 6 to 12 in. long; leaflets thirteen to over twenty, lance-shaped or narrowly ovate, 1 to 3 in. long, 1⁄4 to 3⁄4 in. wide, long and taper-pointed, finely toothed, obliquely rounded at the base; when young both surfaces are covered with minute down, which mostly falls away except on the midrib and chief veins; common stalk downy like the young shoots. Male catkins slender, 2 to 4 in. long. Fruits globose, 1⁄2 to 1 in. in diameter, covered with a thin, smooth husk. Nut deeply grooved.
Native of central and western Texas, western Oklahoma, south-eastern New Mexico, and parts of Arizona; also of northern Mexico; it was discovered by Berlandier in 1835 and described by him under the above name in 1850 (the more familiar name J. rupestris was published in 1853). It was sent to Kew by Prof. Sargent in 1881 and again in 1894. It is a handsome bushy tree, quite distinct from all other cultivated walnuts in its small, narrow, thin leaflets.
A very handsome specimen grows in the University Botanic Garden, Cambridge; planted in 1923 it measures 36 × 33⁄4 ft (1969).
var. major (Torr.) Benson J. rupestris var. major Torr.; J. major (Torr.) Heller – A tree 50 ft high, with larger, fewer leaflets (usually eleven to fifteen). It is also stated that the male flowers have more numerous stamens (thirty to forty) than in J. microcarpa (normally twenty). It should perhaps rank as a species, but the name used for it at that level – J. major (Torr.) Heller – is illegitimate. Probably its correct name as a species should be J. torreyi Dode (the earlier J. elaeopyron Dode has been cited as a synonym of J. major, but judging from the material of the type-collecting at Kew this species agrees better with J. microcarpa).
This walnut has a more western distribution than J. microcarpa and is of a coarser, less interesting appearance.