Shrubs, spreading to compact, to 10 m. Bark smooth; lenticels scattered, conspicuous to inconspicuous, small, mostly unenlarged. Winter buds nearly sessile, ovoid, apex acuminate; stalks usually not over 1 mm; scales 4–6, unequal, imbricate. Leaf blade broadly to narrowly ovate or elliptic, 3–11 × 3–8 cm, base rounded, obtuse, or cuneate, sometimes nearly cordate, margins serrulate to coarsely doubly serrate, apex acute to rounded; surfaces abaxially glabrous to tomentose, lightly to heavily resin-coated. Inflorescences: staminate catkins in 1 cluster of 2–4, formed late in growing season before flowering and exposed during winter; pistillate catkins in 1 or more clusters of 2–10, formed season before blooming, enclosed in buds during winter, exposed with new growth in spring. Flowering with new growth in spring. Infructescences ovoid to ellipsoid or nearly cylindric; peduncles relatively long, thin. Samaras elliptic to obovate, wings wider than body, membranaceous.
Alta., B.C., Man., N.B., N.S., N.W.T., Nfld. and Labr. (Nfld.), Ont., P.E.I., Que., Sask., Yukon, Alaska, Calif., Idaho, Maine, Mass., Mich., Minn., Mont., N.C., N.H., N.Y., Oreg., Pa., Tenn., Vt., Wash., Wis., Wyo., Southern arctic, subarctic, and n mountainous regions, North America and Asia.
Subspecies 4 (3 in the flora).
Alnus viridis is distinctive among the alders in its essentially sessile buds with several imbricate scales and in its relatively long, thin, infructescence peduncles. Like the birches, only the staminate catkins are exposed during the winter prior to blooming.
|1||Leaf blade coarsely doubly serrate, thin, light or yellowish green, glabrous to sparsely pubescent; mountainous nw United States, Alaska, and Canada.||Alnus viridis subsp. sinuata|
|1||Leaf blade serrulate to finely and densely serrate or doubly serrate, firm, dark green, sometimes abaxially sparsely to densely pubescent.||> 2|
|2||Leaf blade broadly to narrowly ovate or elliptic, margins serrulate or finely serrate, apex obtuse to acute; e United States, n Canada, Alaska, and s Greenland.||Alnus viridis subsp. crispa|
|2||Leaf blade broadly ovate, margins sharply and densely doubly serrate, apex acute to short-acuminate; w coastal North America, adjacent subarctic Asia.||Alnus viridis subsp. fruticosa|