Boxelder is a medium sized native deciduous tree, growing 30 to 60 feet high with a short trunk (frequently branched) and a broad open crown when growing unobstructed, but in close quarters it will be irregular and main branches will twist and turn to achieve light.
The bark is fairly thin, gray to light-brown with many narrow ridges and fissures that with age are more deeply furrowed and the ridges become interlacing.
Twigs are green, often with purplish tones and when very young, whitish, with white lenticels, mostly hairless, ringed at the leaf nodes from leaf scars that meet at a raised point. Terminal buds are light colored with hair, lateral buds are tight to the twig.
Leaves are opposite, stalked, pinnately compound with 3 to 5 leaflets (sometimes 7), up to 6 inches long, light green above, paler below. The individual lateral leaflets have short stalks. They are variable in shape, ovate to elliptical with sharp pointed tips, somewhat rounded at the base, and coarsely saw-toothed, sometimes with lobes. Leaf stalks are often reddish later in the season, but green early in the season. Leaves are yellow in fall with the red leaf stalks. Box elder is the only maple with divided leaves. See the leaf comparison at bottom of the page.
Flowers: The tree is dioecious, that is the male and female flowers are on separate trees. Male flowers (staminate) are yellow-green and appear in spring just before the leaves, or with the young leaves, on slender stalks from a common point, near the end of twigs. The calyx of the male flower is yellow-green with 5 very short sepals and no petals and 5 protruding stamens that have long anthers that are dark red initially. Each flower is on a long stalk. The female flowers (pistillate) are more reddish-green with the same 5 sepals and a pistil and a divided style and develop long hairy stalks. They hang in drooping racemes.
Seed: Female flowers mature to a one-seeded samara with a broad curved finely veined wing, 1 to 1-1/2 inches long that is paired with another, green initially and turning brown. The young samaras can vary in color with the local ecotype of the tree. The angle of joining of the pair is very acute. These mature in the fall and often persist on the tree into winter. They are wind dispersed. Trees require about 8 to 11 years to produce a quantity of seed.
Habitat: The tree grows from a vigorous shallow and spreading root system but is somewhat short-lived, but fast growing. In a native environment it is found in moist bottom-lands but can quickly colonize disturbed sites. Seedlings need sun for the tree to continue to grow, which can result in twisted shapes on trees that found a light source. Damage to the trunk will result in new sprouts and the tree re-sprouts from cut stumps. The nymph of the common boxelder bug feeds on the female flowers and young seeds but does not seem to greatly damage the tree.
Names: The genus, Acer, is the Latin word for 'maple'. The species name, negundo, is an old reference to an Asian species, Vitex negundo, that resembles this species. The alternate common name of Ash-leaved Maple refers to the leaves resembling the ash tree compound leaves. In older literature the tree was classified as Negundo fraxinifolium. The author name for the plant classification, 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. Botanists have recently move the maples into the Sapindaceae family from the older Aceraceae family.
Above: The Box Elder will frequently be found with an irregular shape as it is often found nestled in with other species. Note that the samaras are retained even after the leaves have dropped.
Below: Twigs often have a purplish tone. 1st photo - a fall twig, note the tight lateral buds and the leaf scar at the bottom that almost encircles the twig. 2nd photo - spring buds with fine hair on the bud scales.
Below: 1st photo - Young bark is fairly thin, gray to light-brown with many narrow ridges and fissures that with age (2nd photo) becomes deeply furrowed, but interlaced - no diamond pattern.
Below: The compound leaves have variable shapes, with 3 to 5 leaflets - the only maple with divided leaves. 3rd photo - The developing winged samaras; below - mature samaras in the fall. Note the very acute angle between the pair.
Below: 1st & 2nd photos - Female flowers developing. Flower stems are hairy and the styles are very long. 3rd photo - Clusters of male flowers before elongation.
Below: 1st photo - A cluster of female flowers. 2nd photo - a group of male flowers.
Below: A leaf comparison of the common maples. Images not to scale.
Notes: Box Elder is indigenous to the Garden, Eloise Butler catalogued it on April 29, 1907. This is the most widely distributed North American Maple. It is found in all the lower 48 states and all the lower Canadian provinces except Labrador and Newfoundland. Within Minnesota there are only 16 widely scattered counties where it has not been collected.
Eight species of Maple are found in the wild in Minnesota: A. negundo, Box Elder; A. nigrum, Black Maple; A rubrum, Red Maple; A. saccharinum, Silver Maple; A. saccharum, Sugar Maple; A. spicatum, Mountain Maple; A. ginnala, Amur Maple and A. platanoides, Norway Maple. The latter two are not native but introductions that have naturalized.
Toxicity: It has recently been documented that ingestion of Box Elder seeds by horses in pastures causes a deadly muscle disease known as seasonal pasture myopathy. Under research is the question of how much material is a deadly dose. This research was reported by the University of Minnesota Equine Center in the Nov. 2012 issue of Equine Veterinary Journal.
Uses: Box elder wood is soft and weak and not used for construction or fine work. Because of the weak wood it is not suited for use as an ornamental tree. Snow, wind and ice frequently break off branches. The sap has a high sugar content. In Canada where the Sugar Maple (Acer saccharum) is less wide-spread, the early explorers produced many reports of using this tree to make a very good maple syrup. Examples would be Richardson in 1840 [Sir John Richardson, British Arctic Searching Expedition] and Palliser [John Palliser, leader of the British North American Exploring Expedition] who in 1857 wrote "Everything is commencing to wear a spring aspect; the women of the fort are scattered along the banks of the river, busy gleaning their annual harvest of maple sugar. The tree from which they obtain sugar is not the true maple. It is the Negundo fraxinifolium." [That is the old scientific name for Box Elder.] According to Richardson the resulting syrup was a darker color than that from the Sugar Maple. (Ref.#6). Harrington (Ref. #9) also reports of this use in areas outside the range of the Sugar Maple.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"