Both deciduous trees that are worthy for consideration as shade trees, ash and boxelder trees do not make for an easy, concise comparison. About 65 tree species are known as ash (Fraxinus spp.), but "boxelder" refers to only species of North American maple (Acer negundo). In a widespread generalization, any ash tree species becomes a more ornate specimen tree when compared to the boxelder, which is highly variable and suffers broken limbs.
Ash trees are typically in woodlands on any of their native continents, Europe, Asia and North America. Three examples of North American ash tree species are white ash (Fraxinus americana), green ash (Fraxinus pennsylvanica) and the Arizona ash (Fraxinus velutina). The boxelder tree is native to a vast portion of central North America. In fact, according to the U.S. Department of Agriculture's Forest Service website, it has the largest natural range of all maples on the continent, extending from south-central Canada to Mexico east of the Rocky Mountains.
Ash and boxelder trees are not closely related, even though both are considered flowering plants (angiosperms). Ash trees are in the olive family, Oleaceae. Boxelder is a type of maple, a member of the maple family, Aceraceae. Some taxonomists no longer recognize the maple family but rather consider it a part of the soapberry family, Sapindaceae.
Both ash and boxelder trees have leaves comprising leaflets (a compound leaf) and leaves that are arranged in an alternating pattern on the tree branches. Depending on the species of ash, the compound leaf has from three through 15 leaflets that are narrow, tapering ovals in shape. The boxelder's compound leaves have either three or five leaflets, each tapering ovals with jagged teeth on their edges. Sometimes these leaflets have shallow lobes that somewhat hint of the shape of a typical maple leaf.
In autumn, ash trees' leaves turn anywhere from clear yellow to red or burgundy, depending on species. Boxelder leaves always turn what may be considered an unimpressive, if not unattractive, blend of yellow and green, sometimes becoming brown and curling if frosts occur.
Ash trees, especially male-gendered varieties that do not produce seeds, make exceptional shade and street trees, according to the American Horticultural Society's "A-Z Encyclopedia of Garden Plants." "Boxelder trees have limited ornamental qualities," states American woody plant expert and author Michael Dirr, Ph.D. They possess some merit, however; the Forest Service website mentions that boxelder trees demonstrate exceptional drought and cold tolerances. Historically, they were planted in the Great Plains and American West as a street tree and in windbreaks. Their fibrous root system assists in controlling erosion, too. Mutant selections (cultivars) of boxelders are used in gardens because they display more colorful foliage.
While the precise mature form and habit of an ash tree varies by species, the general characteristic is that it forms a straight, upright trunk. Its canopy possesses many upward-angled branches in a uniform, even pattern, developing a structure that is spreading but rounded and/or upright oval. In contrast, the boxelder tree can be a weedy-looking shrub with many stemlike trunks or an upright tree. Its trunk is often leaning, knobby or irregular with branches that occur all over the place. Damage from wind frequently finds boxelder trees lacking limbs, making their canopy not perfectly oval and upright. According to Dirr, a boxelder tree becomes a broad-rounded tree but has little consistency and can appear as a "biological fright."
Both types of trees have drawbacks, mainly around their numerous susceptibilities to insects and diseases. Fungus, cankers, rot and rust can afflict both trees. Borers, sawflies, larval worms and scale also can be problematic. Boxelder trees, at least the female-gendered ones, produce enough seeds each year that puts even the most productive female-gendered ash tree to shame. Both types are messy due to the fallen seeds and, in the case of the boxelder, the seeds germinate in high numbers all around the landscape, even in soil and conditions you'd think a plant couldn't survive.