The common alder is a smallish, often rather inconspicuous and overlooked tree. It is very widespread in Europe, Asia and North Africa and common throughout Ireland in damp and moist habitats. It often grows on stream, river and lake shores and banks, thriving in boggy soils and in moist woodlands.

As a timber tree it is little used now. In Britain it was used to make clogs as its timber is a poor conductor of heat. Its wood is useful and long-lasting in moist situations: indeed much of the city of Venice rests on alder piles.

Its leaves are dark green, round or broadly oval with bluntly toothed edges, four to seven centimetres across. Separate male and female catkins are produced in early spring. The long male catkins are dark purple in colour and drooping in clusters generally of three to five. The female cones open later and are short and ovoid, dark brown and woody when ripe. These female catkins persist on the tree after the leaves have dropped, which themselves can stay on the tree until winter. The alder is not a very remarkable tree and in Ireland was often regarded as unlucky. Nevertheless, it has a quiet, self-assured dignity and grows well even in the most apparently unfavourable situations.

A close relative, although not a native, the grey alder, Alnus incana (Fearnóg liath), is occasionally planted and gone wild in a few areas. It has toothed and pointed leaves that are paler underneath than those of A. glutinosa.