2. Acer negundo L. (box elder, ash-leaved maple)
Pl. a, b; Map
805
Plants
dioecious, small to medium trees to 25 m tall with ascending to spreading
branches, the bark of young trees smooth and gray to light brown, eventually
becoming separated into a network of long thin ridges on older trees. Twigs
green to olive green, sometimes glaucous and appearing pale purple to purple,
glabrous or densely hairy, the winter buds bluntly pointed at the tip, with 2
or 4 overlapping scales. Leaf blades 7–15 cm long, broadly triangular-ovate to
oblong-ovate in general outline, pinnately compound, those of fertile branches
and seedlings with 3–5 leaflets on a short rachis, those of vigorous vegetative
shoots with 5–9 leaflets. Leaflets 5–10 cm long, oblong to ovate (terminal
leaflet sometimes obovate), angled or tapered to sharply pointed tips, the
upper surface light green, the undersurface pale grayish green, usually
somewhat hairy when young, becoming sparsely hairy or glabrous at maturity, the
margins coarsely and irregularly toothed and occasionally also with a few
shallow lobes. Inflorescences produced before the leaves or during leaf
development, the staminate ones umbellate clusters from buds along the
branches, the individual flowers with long drooping stalks, the pistillate ones
narrow drooping racemes from at or near branch tips. Calyces 1–3 mm long, the
sepals fused only at the very base, the 5 lobes obovate to narrowly
oblong-elliptic, rounded at the tips, yellowish green, hairy. Petals absent.
Staminate flowers with 3–6 stamens and lacking a nectar disk. Pistillate
flowers with the ovary glabrous or less commonly hairy. Fruits dispersing long
after the leaves are mature, often not until the following autumn, the samaras
2.5–4.5 cm long, glabrous or less commonly hairy, the wings 2–4 cm long,
spreading at less than a 90° angle. 2n=26. April–May.
Common nearly
throughout Missouri (U.S., Canada,
Mexico, Guatemala).
Bottomland forests, banks of streams, mesic upland forests in bottoms of
ravines, and bases of bluffs; also shaded ditches and moist roadsides.
Box elder has
fallen into disfavor horticulturally, for although it grows easily under a
variety of site conditions, the branches of older trees break easily during
storms, and the trees are susceptible to a number of fungal diseases and insect
pests, notably the box elder bug, Leptocoris trivittatus (Say) (Wagner,
1975). The leaves turn yellow in the autumn and are shed earlier than those of
most other trees. Unlike other Missouri
species of Acer, which are pollinated by both insects and wind, the
flowers of A. negundo are only wind-pollinated and are a cause of hay
fever during the spring.
Acer negundo has been treated as consisting of a
number of varieties by many botanists (Murray,
1975), each occupying a portion of the species’ overall distribution. Four of
these taxa were accepted as occurring in Missouri
by Steyermark (1963), who nevertheless had difficulties with infraspecific
determinations of some specimens. For convenience, the species is here treated
as a complex of only two varieties, which allows nearly all specimens to be
classified relatively easily.