Alnus glutinosa is a tree that thrives in moist soils, and grows under favourable circumstances to a height of 20–30 m, exceptionally up to 37 m, though often less. It is characterized by its 5–10 cm short-stalked rounded leaves, becoming wedge-shaped at the base and with a slightly toothed margin. When young they are somewhat glutinous, whence the specific name, becoming later a glossy dark green. As with some other plants growing near water it keeps its leaves longer than do trees in drier situations, the glossy green foliage lasting after other trees have put on the red or brown of autumn, which renders it valuable for landscape effect. As the Latin name glutinosa implies, the buds and young leaves are slightly sticky with a resinous gum.
Various species feed on Alder, including several moths.