|Conditions:||Adaptable, but prefers wet soils|
Acer negundo is a species of maple (Acer) that grows throughout much of North America. It is called Manitoba Maple in Canada, and Ash-leaved Maple (also Ashleaf Maple, Ash Maple) or Box Elder (also Boxelder, California Boxelder, Western Boxelder, Inland Boxelder, Boxelder Maple) in the United States. The American common names come from the pinnately compound leaves of Acer negundo that are similar to those on the elder (Sambucus) and some species of ash (Fraxinus). The "box" in the name is thought to be because this maple's wood superficially resembles that of the box hedge (Buxus sempervirens). Additional common names include Cut-leaved Maple (because of the compound, or fully dissected, leaves), or Three-leaved Maple (because all new leaves from overwintering buds have 3 leaflets), or Sugar Ash (because the leaves resemble the ash, but it is a source of maple syrup). Its numerous and diverse common names attest to its familiarity to many peoples over a great geographic range.
This maple is a small, usually fairly short-lived tree that grows up to 10–20 m tall, with a trunk diameter of 30–50 cm, rarely up to 1 m diameter. The shoots are green, often with a whitish to pink or violet wax coating when young. Unlike most other maples (which usually have palmate leaves), it has pinnate leaves with usually five (sometimes three or seven) leaflets, resembling an ash or an elderberry, hence the two alternative common names; the leaves are 12–25 cm long, with each leaflet 6–10 cm long and 3–7 cm wide. No other maple ever has more than three leaflets. The leaves have large serrations and are inequalateral at the base of the leaf. Twigs under one year old appear purple or green with a smooth whitish bloom. The flowers are small, lacking petals, borne in clusters of 10-30 together on drooping racemes 10–20 cm long, produced in early spring. The seeds are paired samaras, each seed slender, 1–2 cm long, with a 2–3 cm incurved wing; they are wind-dispersed and drop in autumn, or may persist through winter. Unlike most other maples, the tree is dioecious, so each tree grows only one gender of flower and both a male and female tree are needed for either to reproduce.
A few botanists treat it in its own distinct genus, as Negundo aceroides, but this is not widely followed.
There are two subspecies:
- The typical subspecies Acer negundo subsp. negundo, with hairless shoots and leaves, in most of its area: southeast British Columbia east to southern Ontario and New Hampshire, and south to eastern Nevada, Arizona, northernmost Mexico and southeast to Florida. Some authors further subdivide subsp. negundo into a number of regional varieties, but these intergrade and their maintenance as distinct taxa is disputed by many.
- The western subspecies Acer negundo subsp. californicum, with pubescent (finely hairy) shoots and leaves; a disjunct population in the Central Valley area of California.
The species is invasive under some conditions, and can quickly colonize abandoned farmland, old roadways, and uncultivated fence lines, both rural and urban. The range is therefore expanding.
This species prefers bright sunlight. It often grows on floodplains and other disturbed areas with ample water supply, such as riparian habitats. Human influence has greatly favoured this species; it grows around houses and in hedges, as well as on disturbed ground and vacant lots. Although native to North America, it is considered to be an invasive species in some areas of the continent.
Several birds and some squirrels feed on the seeds; notably the Evening Grosbeak uses them extensively. The Boxelder Bug (also known as Maple bug) lays its eggs on all maples, but prefers this species.
Acer negundo is rarely planted as a landscape tree; its weak wood, irregular form, and prolific seeding make it a poor choice for most areas. It often is found in landscapes in some areas due to it reproducing from the many seeds produced.
There are some cultivars, including some variegated forms, that are available in the nursery trade.
Although its wood is considered undesirable for most uses, this tree has been considered as a commercial source of wood fibre, for use in fiberboard.