European alder is a medium-sized tree, suckering and forming thickets; fast-growing when young; usually developing a single trunk with an oval-headed crown. In landscaping, it is sometimes grown as a "feature" plant with multiple trunks.
Leaves alternate, simple; dark glossy green; usually nearly circular, 1½–5 inches long; the tip shallowly notched, appearing cut off (truncate), or sometimes broadly rounded; margins coarsely toothed, sometimes also with small lobes.
Flowers March–April, male and female catkins borne on the same tree; female catkins resembling dark conifer cones.
Fruits mature in autumn and persist into winter; resemble miniature oval pinecones ¾–1¼ inches long; seeds are narrowly winged.
Similar species: Our native species of alder is the common alder, Alnus serrulata, which is usually much smaller and has narrower leaves that have finely toothed (not lobed) edges. In addition to the most typical European alder described here, there are other horticultural varieties that people may plant, featuring yellow leaves, cut leaves, columnar growth, and other unusual characteristics.