Ashes

Missouri has six species of ashes that you might find in natural settings. They have been very popular as shade trees, and their hard, strong, shock-resistant wood is famously useful. Ash trees of all the species in North America are currently being killed by the invasive, nonnative emerald ash borer.

Ash trees in Missouri are usually dioecious, with each tree bearing either male (staminate) flowers or female (pistillate) flowers, but usually not both, meaning that a tree will produce either pollen or fruits, but not both. Note that certain trees may produce some perfect flowers (flowers that contain both pistils and stamens); some trees may change gender, or bear mixed flowers, from one season to the next; and blue ash bears mostly perfect flowers.

Ash leaves are opposite, pinnately compound with mostly 5–11 leaflets. Leaflets are mostly short-stalked, variously shaped, angled or tapered to the sharply pointed tip, rounded or angles at the sometimes asymmetric base, the margins entire or shallowly toothed, the surfaces smooth or hairy, the upper surface medium to dark green, the lower surface pale or lighter green.

Ash flowers are usually either male or female, with the two on separate trees. They form in the leaf axils in many-flowered clusters and are usually yellowish green or sometimes reddish. Flowers develop on one-year-old branches before the leaves or as the leaves are expanding in spring. The flowers are not fragrant or showy. Male (staminate) flowers produce copious amounts of pollen and can be a source of hay fever.

Ash fruits are samaras (like elms and maples, where the nutlet is enclosed in a flattened, papery wing). In ashes, the samaras are slender and elongated, oarlike, usually dangling in a cluster, 1–3 inches long (including the wing), the wing extending fairly straight out from the nutlet; pale green, turning straw-colored to grayish tan or brown with age.

The bark is gray, relatively thick, developing a network of ridges and diamond-shaped furrows, often described as X shapes, sometimes becoming scaly with age.

The twigs are relatively stout, gray to brown, more or less circular (though blue ash is 4-angled) in cross section, with raised leaf scars. The terminal buds are usually closely flanked by the uppermost, shorter pair of axillary buds, with several overlapping, sharply pointed scales. The bud shape and bud-scar shape varies and is useful for species ID.

Experts identify ashes to species by examining details of the twigs, including the shape of the leaf-bud scars, the fruits (length and width of the wings), and various characteristics of the leaves.

There are some 60 to 65 species of ashes worldwide. Six are native to Missouri.

• Green ash (F. pennsylvanica), also called red ash, is common statewide in a wide variety of habitats. Compared to white ash, it is more strongly associated with bottomlands and moister soils. Its leaflets are narrower than white ash’s; they turn bright yellow in autumn. This is one of the most widely planted shade trees around homes and along streets. In the wild, it lives along streams and in low grounds. Sadly, it is one of the trees most vulnerable to the emerald ash borer.
• White ash (F. americana) is common statewide in a wide variety of habitats. It tends to prefer somewhat drier or upland sites than green ash. Its leaflets are more egg-shaped than green ash’s, and they turn yellowish and deep, winey purple in fall, which is one reason it has been a popular landscaping tree. In the past, Sullivan’s ash and Biltmore ash were both considered forms of white ash, but genetic studies have shown that, in addition to the subtle differences in form, the three trees have different numbers of chromosomes, which prevents them from interbreeding, even when they occur nearby one another.
• Blue ash (F. quandrangulata) is scattered to common in the Ozarks and Ozark border and northward locally in the eastern half of northern Missouri. It is primarily an upland species, growing on glades, ledges and tops of bluffs, but also sometimes on banks of streams and rivers, bases of bluffs, and bottomland forests, often on calcareous substrates. It is one of our shorter ashes. This Missouri ash is identified by its twigs, which are strongly four-angled (square) in cross-section, the angles sometimes with narrow, corky wings. The flowers are usually perfect (having both male and female structures), and the fruits are wide (6–12 mm, as opposed to 3–8 mm in most of our other species), looking more oval and less like boat paddles.
• Pumpkin ash (F. profunda), also called red ash, is uncommon in Missouri, restricted to the Bootheel lowlands, where it lives in swamps, bottomland forests, and (uncommonly) in acid seeps; it may also be found on wet roadsides. It grows alongside bald cypress, water elm, and water tupelo. It has the largest leaves and fruits of all Missouri ashes but strongly resembles green ash. The wing of the fruits is 7–13 mm wide. It’s called pumpkin ash because of the swollen base of the trunk, which commonly develops at sites that are flooded for long periods of time.
• Sullivan’s ash (F. smallii) is scattered in the southern half in Missouri, where it grows in a wide variety of habitats, particularly in bottomland or streamside habitats but also in pastures, roadsides, and upland forests. Some botanists have considered it and Biltmore ash forms of white ash. Both are distinguished from white ash by the shape of the leaf scars (more shallowly notched than those of white ash), the color of the winter buds (brown, never black), and longer/larger samaras. Biltmore ash can be distinguished from the very similar Sullivan’s ash by its twigs and leaf stems, which are moderately to densely short-hairy, while the twigs and leaf stems of Sullivan’s ash are glabrous (smooth and hairless). Chromosome studies have shown that Sullivan’s ash is a tetraploid species (having four sets of chromosomes) while white ash is diploid (having two sets of chromosomes).
• Biltmore ash (F. biltmoreana) is uncommon in Missouri, known only from counties of southeastern Missouri, where it has been found in swamps and bottomland forests; its greater range is to the east of our state. Some botanists have considered it and Sullivan’s ash forms of white ash. Both are distinguished from white ash by the shape of the leaf scars, the color of the winter buds (brown, never black), and longer samaras. Biltmore ash can be distinguished from the very similar Sullivan’s ash by its twigs and leaf stems, which are moderately to densely short-hairy, while the twigs and leaf stems of Sullivan’s ash are glabrous (smooth and hairless). Chromosome studies have shown that Biltmore ash is a hexaploid species (having six sets of chromosomes) while white ash is diploid (having two sets of chromosomes).

Similar species: Many native trees have pinnately compound leaves. It helps to remember that ash leaves are opposite, not alternate. Another hint is that unlike trees like hickories and black walnut, the leaves of ashes, when crushed, do not have a spicy or strong fragrance.

• Common prickly ash (Zanthoxylum americanum) is a thicket-forming shrub or small tree. Its compound leaves resemble of those of ash trees, but it’s in a different family. Pairs of stout, curved prickles occur at each node.
• Mountain-ashes, or rowans (Sorbus spp.) are members of the rose family. The leaves look a little like those of ashes, but the fruits are very different; the fruits are tiny pomes (apples).
• Flowering ash, also called manna ash (Fraxinus ornus), is native to Eurasia and is sometimes planted in Missouri for its large, terminal clusters of fragrant flowers with slender, white petals. It is not known to escape from cultivation, however.