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. 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.
Tall colonial shrub; younger parts ±glutinous; winter buds virtually sessile, acuminate, with ca 5 imbricate scales; lf-bearing stems usually with both long shoots and short spurs, the latter bearing the lvs (unique among our spp. in this regard, the other spp. usually not forming both long and short shoots); lvs glutinous, especially beneath and when young, broadly elliptic to ovate, broadly obtuse to rounded or subcordate at base, with mostly 6-9 main veins on each side, green and shiny (not glaucous) beneath, sometimes sparsely velvety-rusty, finely and sharply toothed, or almost laciniate; catkins appearing with the lvs, the pistillate ones in infls with 2-3 lvs at the base, at maturity 1-1.5(-2) cm, evidently slender-pedunculate; filaments 0.5-1 mm; fr 2-3 mm, the body only 1-2 mm wide and surrounded by a broad, pale membranous wing; 2n=28. Bogs, shores, and cold woods; nearly circumboreal, in Am. s. to Mass., N.Y., Wis., Minn., Ida., and Calif., and at disjunct stations in Pa. and on Roan Mt. (N.C.-Tenn.). Boreal Amer. and e. Amer. pls, as here described, are var. crispa (Michx.) House (A. alnobetula; A. crispa; A. mollis, the pubescent extreme)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.