1. Juglans cinerea L. (butternut, white walnut)
Pl. 431 d–f; Map
1932
Plants trees to
30 m tall. Bark light gray or grayish brown, shallowly divided into smooth or
scaly plates. Pith dark brown. Terminal buds 12–18 mm long, conic, flattened.
Leaf scars with the upper margin straight or nearly so, bordered by a
well-defined, tan to gray, velvety ridge. Leaves 30–60 cm long, the petiole
3.5–12.0 cm long, glandular-hairy, with (7–)11–17 leaflets, including usually a
large terminal leaflet. Leaflets (2.5–)5.0–11.0(–17.5) cm long, 1.5–6.5 cm
wide, ovate to lanceolate or oblong-lanceolate, more or less symmetrical (not
appearing asymmetrically tapered), mostly rounded at the base, tapered at the
tip, the margins finely toothed, yellowish green, the upper surface with
scattered fasciculate hairs or becoming nearly glabrous at maturity, the
undersurface with abundant 4–8-branched hairs (the branches appearing
fasciculate) and yellowish scales, sometimes also with gland-tipped hairs, the
axils of the secondary veins with prominent tufts of fasciculate hairs.
Staminate catkins 6–14 cm long, the staminate flowers with 7–15 stamens, the
anthers 0.8–1.2 mm long. Fruits usually in clusters of 3–5, 4–8 cm long,
ellipsoid to ovoid or more or less cylindrical, the husk smooth, densely
covered with gland-tipped hairs, becoming slightly wrinkled and papery with age.
Nuts 3–6 cm long, ellipsoid to ovoid or more or less cylindrical, with about 8
high, narrow, irregular, longitudinal ridges, the surface between the main
ridges with narrower, interrupted, longitudinal ridges or folds. 2n=32.
April–May.
Formerly scattered
nearly throughout the eastern 2/3 of Missouri, becoming increasingly uncommon
(eastern U.S. west to Minnesota and Arkansas; Canada). Bottomland forests,
mesic upland forests in ravines, bases of bluffs, and banks of streams and
rivers.
Butternuts are
difficult trees from a silvicultural perspective. They never form large stands
but instead occur as scattered trees within their habitats. They are relatively
intolerant of human disturbance, but they do not reproduce well in
closed-canopy forests (Ostry et al., 1994). In recent decades, many trees have
been killed by butternut canker, Sirococcus clavigignenti-juglandacearum
Nair, Kostichka & Kuntz, a fungus that kills trees by girdling the limbs
and trunk (Ostry et al., 1994). As a result, this imperiled species is becoming
very uncommon throughout its range. Some of the butternuts in the horticultural
trade apparently are hybrids between J. cinerea and the closely related
Japanese walnut, J. ailanthifolia Carrière, which have improced
resistence to the pathogen. These are not yet known to have escaped into the
wild in Missouri, but are nearly impossible to distinguish from genetically
pure butternut trees morphologically.