Common prickly ash is a thicket-forming shrub to 10 feet high, often densely branched above the middle. Occasionally a small tree to 26 feet high. Prickly ash forms thickets by sending up shoots from the underground, creeping stems (rhizomes).
Leaves are alternate, pinnately compound (feather-compound), 4–12 inches long, aromatic, the leaf stalk about 1 inch long; leaflets 5–11; side leaflets without stalks, end leaflet with a short stalk; leaflets get progressively longer from the basal pair, ranging from ¾ to 3 inches long, ⅜–1½ inches wide, egg-shaped or half as broad as long, the tip pointed to blunt, the base rounded, the margin entire or with finely rounded teeth; the upper surface dull deep green, dotted with glands; the lower surface paler, hairy on the veins.
Bark is smooth, gray to dark brown with lighter blotches and scattered, small, light-colored, fairly circular lenticels; bark becomes slightly grooved on old trunks; bark is armed with small, flattened, slightly downward-curving prickles. Wood soft, not strong, light brown.
Twigs are rigid, smooth, dark brown to gray; a pair of prickles at each node, each prickle about ¼ inch long, flat, broad-based, recurved.
Flowers April–May, before the leaves are formed, in small, axillary clusters of 2–10, on twigs of the previous year. Fragrant. Individual flowers are short-stalked. This plant is dioecious, with male (staminate) and female (pistillate) flowers usually on separate plants. Sepals are absent. Petals 4 or 5, broadest at the middle, minute, yellowish green, usually with a fringe of short, crinkly, reddish-brown hairs. Male flowers with 4 or 5 stamens, alternating with the petals; female flowers about ¼ inch long, longer than the male flowers, with 2–5 ovaries per flower.
Fruits June–August, in dense clusters, fruit green to reddish brown, strongly aromatic, about ¼ inch long, globe-shaped, firm, fleshy, surface pitted, splitting down one side; seeds 1 or 2, oval, about ⅛ inch long, finely pitted, glossy black; seed coating oily, aromatic.
Similar species: Prickly ash is the only member of its genus recorded growing naturally in Missouri. A few other shrubs or young trees share some characteristics, however.
- Missouri’s true ash trees (Fraxinus spp.) look rather similar, but their compound leaves are opposite, and they do not have prickles.
- Young specimens of black locust (Robinia pseudoacacia) have spines and pinnately compound leaves, but the leaves typically have 11–19 leaflets, and the leaflets are rounded, lacking pointed tips.
- Honey locust (Gleditsia triacanthos) is common in many of the same habitats as prickly ash. Like prickly ash, it is well-armed, but its spines are much longer, and its spines are often branched. The leaflets on its compound leaves are much smaller.
- Finally, a close relative of prickly ash, Hercules club, or southern prickly ash (Z. clava-herculis), was once reported from southeastern Missouri, but there seems to be no physical evidence to substantiate it. Its native range is to the south of Missouri, from Virginia and Florida to Texas and Oklahoma. Among other differences, its trunk is covered with thick, wartly prickles, it forms its flowers after (not before) the leaves expand, and its leaves have mostly 9–13 (not 5–11) leaflets. You might find this plant growing as an ornamental in a yard or garden.