Trees to 20 m tall, dioecious. Bark yellowish brown or gray-brown. Branchlets glabrous, those of present year green, older ones yellowish brown; winter buds small, scales 2(or 3) pairs. Leaves deciduous; leaf blade 10-25 cm, papery, pinnate; petiolules 5-7 cm, pubescent, glabrescent; leaflets 3-7(-9) per petiole; leaflet blades ovate or elliptic-lanceolate, 8-10 × 2-4 cm, base rounded or truncate, margin entire or with 3-5 teeth, apex acute. Pistillate inflorescence pendulous, racemose or compound racemose, axillary from leafless buds, 15-50-flowered. Staminate inflorescence usually a cluster of 4 flowers. Flowers 4-merous. Petals and disk absent. Stamens purplish, 4-6. Ovary glabrous. Samaras brownish yellow; nutlets convex, glabrous; wing including nutlet 3-3.5 cm × 8-10 mm, wings spreading acutely or nearly erectly. Fl. Apr, fr. Sep. 2n = 26.
Boxelder (Acer negundo) is one of the most widespread and best known of the maples. Its other common names include ashleaf maple, boxelder maple, Manitoba maple, California boxelder, and western boxelder. Best development of the species is in the bottom-land hardwood stands in the lower Ohio and Mississippi River valleys, although it is of limited commercial importance there. Its greatest value may be in shelterbelt and street plantings in the Great Plains and the West, where it is used because of its drought and cold tolerance.
The chief rot-causing fungi attacking boxelder are Fomitopsis fraxinus, Perrenniporia fraxinophilus, Fomes geotropus, Fomitopsis scutellata, Inonotus glomeratus, and Ustulina vulgaris. Root rots caused by Rhizoctonia crocorum and Phymatotrichum omnivorum have been identified on boxelder, but Armillaria mellea has not been reported on the species, although it is common on other maples (14).
Verticillium wilt (Verticillium albo-atrum) is the only notable killing disease of boxelder. The species is also susceptible to a stem canker caused by Eutypella parasitica.
A red stain in the wood of living trees caused by Fusarium reticulatum var. negundinis apparently is specific to boxelder. The stain regularly is associated with Cerambycid beetles and the galleries of other insects, but itself does no damage to the wood (14).
Insect damage to boxelder is relatively unimportant, but a number of leaf-feeding and scale insects and borers attack it (1). The boxelder bug, Leptocoris trivittatus is a common associate of boxelder throughout most of its range. The nymphs feed mainly on pistillate trees in leaves, fruits, and soft seeds. Although the trees are not greatly damaged, the insect's habits of invading houses in large numbers with the onset of cold weather makes it an important pest. The boxelder aphid, Periphyllus negundinis, and the boxelder gall midge, Contarinia negundifolia, are also common. Other leaf feeders include the Asiatic garden beetle, Maladera castanea, the greenstriped mapleworm, Anisota rubicunda, a leaf-roller, Archips negundana, and the boxelder leafroller, Caloptilia negundella. The scale insects include cottony maple scale, Pulvinaria innumerabilis, and terrapin scale, Mesolecanium nigrofasciatum. Borers include the boxelder twig borer, Proteoteras willingana, and the flatheaded apple tree borer, Chrysobothris femorata.
Ice and wind damage is common in older trees (11) and boxelder is quite susceptible to fire and mechanical damage due to its thin bark.
Boxelder is highly sensitive to 2,4-D. In the northern Great Plains, drift from agricultural spraying operations produced distorted, blighted foliage up to 16 km (10 mi) from the source (20).
Population differences in boxelder have been noted in response to photoperiod (6,28), in seed germination and stratification requirements (29), seed weight (30), tracheid length (31), frost tolerance (5), and in chlorophyll levels (10).
Some 8 to 14 varieties and forms have been described for boxelder, several relating to variegated patterns of the foliage or some other morphological character (2,17,21,23,28). At least two varieties appear to be confined to a definite geographic range: var. arizonicum Sarg. to central and southern Arizona and New Mexico and var. californicum (Torr. and Gray) Sarg. to the Central Valley, Coast Range, and San Bernardino Mountains of California (23).
Acer negundo (boxelder or ashleaf maple, with numerous other common names) is the most widely distributed North American maple species, ranging throughout the U.S. and Canada and south into Mexico and Guatemala. Leaves and form vary considerably across its geographical range; six subspecies are recognized. Boxelder is the only North American maple species with compound leaves.
The common name, “boxelder,” refers to the resemblance of its leaves to elder (Sambucus) and the use of the soft wood for box making. “Ashleaf maple,” refers to the resemblance to ash (Fraxinus).
Boxelder is a small, deciduous, fast-growing, short-lived tree growing to 20 m tall, with a broad rounded crown. It often branches low into multiple trunks, which may grow almost parallel to the ground, and can form dense thickets. The bark is light brown-gray with shallow fissures, becoming deeply furrowed. Twigs are slender, shiny green, usually glabrous (without hairs), and often have a whitish to pink or violet waxy coating (glaucous) when young. The leaves are opposite, pinnately compound with 3 to 9 leaflets, long-pointed, coarsely toothed and often shallowly lobed. The flowers are yellow-green, about 5 mm long; the species is generally dioecious (functional male and female flowers occur on separate trees; Wagner 1975; Fewless 2011). Fruits are long-stalked clusters of winged nutlets (samaras) in a pair, 2.5-4 cm long.
Boxelder is generally a tree of river bottoms and disturbed sites on heavy, wet or seasonally flooded soils, where it usually follows cottonwood and willow species in colonizing alluvial bottoms. Populations in native habitats have decreased because of clearing of bottomland forest for agriculture, but they have greatly increased in urban areas, where it readily colonizes disturbed sites due to its prolific seed production, wide dispersal, ease of germination, tolerance of cold, drought, and low-oxygen conditions, and fast growth on clay or heavy fill. It frequently grows along fencerows, railroad tracks, ditches, and abandoned lots (Michigan Flora Online 2011).
Boxelder was widely planted in the Great Plains as a shelterbelt tree—its shallow, fibrous root system helped reduce wind erosion and dust storms—but shelterbelts have largely been removed. It was also widely planted in the U.S. as a street tree, and ornamental cultivars have been developed (including forms with variegated leaves and without seeds).
It has been planted in Europe, Australia, and South America as a roadside, park, garden, and shelterbelt tree, and has naturalized widely in disturbed areas and along riverbanks. It is considered invasive in Poland, Germany, Austria, Russia, Latvia, and Lithuania (Mędrzycki 2011), as well as Australia, New Zealand, China, and Chile (USFS 2011).
Boxelder is sometimes confused with poison ivy (Toxicodendron radicans), because its compound leaves often have three leaflets. A key distinction is that boxelder leaves are opposite rather than alternate.
The boxelder bug (Boisea trivittata, family Rhopalidae) commonly associates with boxelder. The insects cause little damage to the trees, but are considered a pest species because they invade human habitation, often in large numbers, with the onset of cold weather (Hahn and Ascerno 2007).