The first juvenile leaves on seedlings are simple, and rare mutant trees may bear simple leaves. In all of our species, the flowers appear before the leaves, on twigs of the previous year. Since the inconspicuous flowers are often unisexual (plants dioecious or polygamous), vegetative features are stressed in the key. Mites often distort the inflorescences, producing persistent galls. The fruit is conspicuously winged, a samara much like that of the maples but not in pairs.
Fraxinus excelsior L., the European ash, is sometimes planted and may be found in Michigan as it is known to escape in adjacent Ontario. It has a broadly winged linear-oblong fruit like that of F. quadrangulata and F. nigra, and likewise lacks a calyx; it has sessile leaflets, like those of F. nigra, but the rachis of the leaf is glabrous where the leaflets attach, not with the characteristic tomentum of F. nigra. The terminal buds are broad and the terete greenish twigs contrast strongly with the black buds.
Fraxinus in Michigan and surrounding areas are being devastated by the emerald ash borer (Agrilus planipennis), an Asian insect first discovered in North America in southeastern Michigan in 2002.
1. Branchlets ± sharply 4-angled or even slightly winged, with flattish sides.
1. Branchlets (at least internodes) essentially terete.
2. Lateral leaflets definitely sessile, 6–10; flowers without a calyx; samara strongly flattened, scarcely if at all thickened over the seeds, ca. (4.5–) 6–8 (–10) mm broad across the middle of the seed, winged to the base.
2. Lateral leaflets (at least the lower ones) petiolulate (with at least a short though sometimes winged stalk), 4–8; flowers with a calyx (persistent at base of fruit); samara distinctly thickened over the mature seed, ca. 1.3–7 mm broad across the middle of the seed, very narrowly if at all winged at the base.
3. Leaflets whitish beneath and minutely papillose (or densely pitted in appearance) [30–40×], the lateral ones on petiolules often 5–10 mm long; leaves and young twigs glabrous or nearly so (sometimes with hairs along main veins of leaflets beneath); samaras with distended portion over the seed ca. 4–6.5 (–7) times as long as wide (measured at middle), winged only along apical half or less of the seed.
3. Leaflets green beneath, not papillose, the lateral ones on petiolules rarely as long as 5 mm (usually shorter and often winged to the base, but much longer in the rare F. profunda); leaves and young twigs densely pubescent to glabrous; samaras with distended portion ca. (6.5–) 7.5–12 times as long as wide, often winged (very narrowly) to below the middle of the seed.
4. Calyx at base of fruit ca. (2.2–) 3–5 mm long; wing of samara ca. (6.5–) 7–10 mm broad; leaflets pubescent beneath.
4. Calyx at base of fruit ca. 0.5–2 (–2.5) mm long; wing of samara (4–) 4.5–6.5 (–8.5) mm broad; leaflets glabrous or pubescent beneath.