The genera are all wind-pollinated, monoecious trees and shrubs, flowering before the leaves are mature. The staminate flowers are in elongate cylindrical aments in all genera. While the leaves are distinctive, they are hard to describe objectively, and the key relies often on reproductive parts.
1. Pistillate flowers in dense elongate aments or cone-like inflorescences, with subtending deciduous or persistent woody bracts or scales less than 1 cm long; nut usually winged; scales of staminate ament somewhat peltate in appearance, usually ± obtuse or several-lobed (from adnate bractlets), with additional perianth parts and/or bractlets evident; anthers mostly glabrous, the separate locules 12 or 24 (the bifid stamens 6 or 12) per scale (in B. pumila often as few as 3 bifid stamens).
2. Pistillate aments clustered (2 or more loosely racemose), old ones remaining on the plant all year, the scales at maturity persistent, woody, ± at right angles to rachis, glabrous, strongly wedge-shaped and truncate or obscurely 5-lobed (more definitely with 5 ± overlapping lobes when young) at apex; nut wingless (widespread species) or winged; staminate aments basically with 4 stamens per flower and 3 flowers per scale, thus with 12 stamens per scale, these slightly bifid, resulting in 24 barely separated anther locules.
2. Pistillate aments solitary, the scales deciduous (or readily dislodged) at maturity, firm or only slightly woody, usually ± strongly ascending, ciliate or hairy (to glabrate in a peatland shrub), strongly 3-lobed; nut with membranous wings; staminate aments basically with 2 stamens per flower and 3 flowers per scale, thus with 6 stamens per scale, these slightly to deeply bifid, resulting in 12 separate locules (in B. pumila sometimes only 3 or 4 stamens, hence 6 or 8 locules per scale).
1. Pistillate flowers in a short head or loose raceme-like ament, with subtending bracts becoming leafy, persistent, at least 1.5 cm long; nut wingless; scales of staminate ament acute to abruptly short-pointed, with or without partly adnate lobe-like bractlets but without additional perianth; anthers with a few hairs at the tip, the separate locules either 8 or 16 or more, on 4 or 8 or more bifid filaments per scale.
3. Leaves with about 8 veins or fewer on each side of midrib, the blades up to twice as long as broad (but mostly ca. 1.5 times), with undulate or shallowly lobed (as well as toothed) margins; scales of staminate aments each with 4 bifid stamens (= 8 separate anther locules) and 2 small partly adnate bractlets; pistillate flowers few, in short (not over 6 mm) ovoid buds, concealed (with villous bractlets) by the bud scales except for protruding red stigmas (2 per flower); flowers appearing well before the leaves; fruit a large (ca. 1 cm) acorn-like nut hidden by involucral bracts.
3. Leaves with about 10 or more veins on each side of midrib, the blades mostly about twice as long as broad or slightly longer, with margins only toothed (not lobed or undulate); scales of staminate aments each with 8 or more bifid stamens (= 16 or more locules) and no bractlets; pistillate flowers in a loose elongate terminal ament or drooping spike; flowers opening at about the same time as the leaf buds or later; fruit a much smaller nut, either enclosed in a sac-like bract or in pairs subtended by a leafy bract.
4. Plant a tall or medium shrub (usually with several crooked stems), the bark smooth, light gray, ± fluted or “muscular” in appearance; staminate aments solitary, covered by bud scales in winter, elongating in spring to 1.2–2 (–2.7) cm long, the scales acute or broadly and obscurely acuminate but without any prolonged tip; nut with several strong ribs and a several-toothed crown at the apex, in pairs subtended by a few-toothed or few-lobed bract that enlarges at maturity to 1.8–3 (–4) cm long; leaves without glands on petioles, the blades with lateral veins only very rarely forked.
4. Plant a small tree with dark scaly bark and single erect trunk; staminate aments mostly in clusters of 2–3, naked in winter, elongating to 1–5 cm at full flowering in spring, the scales acuminate to a short prolonged tip; nut smooth (at most obscurely ribbed toward end), without toothed crown (entire at apex), surrounded by a tubular (fused) bract which enlarges at maturity into an inflated ellipsoid sac 1.5–2.5 cm long; leaves usually with sparse to dense stalked glands on petioles (these sometimes also on young branchlets) and lateral veins often branched (especially beyond the middle).