It is a medium-sized tree growing up to 25 m (80 feet) tall (exceptionally to 28 m or 90 feet), with a trunk up to 70–100 centimetres (30–40 in) in diameter. The leaves are deciduous but with a very long season in leaf, from April to December in the Northern Hemisphere; they are alternate, cordate (heart-shaped), rich glossy green, 5–12 cm (2–4 3⁄4 inches) long, with a finely serrated margin.
The slender cylindrical male catkins are pendulous, reddish and up to 10 cm (4 inches) long; pollination is in early spring, before the leaves emerge. The female catkins are ovoid, when mature in autumn 2–3 cm (3⁄4–1 1⁄8 inches) long and 1.5–2 cm (5⁄8–3⁄4 inch) broad, dark green to brown, hard, woody, and superficially similar to some conifer cones. The small winged seeds disperse through the winter, leaving the old woody, blackish ‘cones’ on the tree for up to a year after.
Like other alders, it is able to improve soil fertility through symbiotic nitrogen fixation with the bacteria Actinomyces alni (Frankia alni). It thrives on much drier soils than most other alders, and grows rapidly even under very unfavourable circumstances, which renders it extremely valuable for landscape planting on difficult sites such as mining spoil heaps and heavily compacted urban sites. It is commonly grown as a windbreak.