Alder leaves are deciduous (not evergreen), alternate, simple, and serrated. The flowers are called catkins and have elongated male catkins on the same plant as the shorter female catkins, often before the leaves appear. They are mainly wind-pollinated, but bees and other insects also play a small role in this. They differ from the birches in that the female catkins are woody and do not disintegrate at maturity, opening to release the seeds in a similar manner to many conifer cones.
The largest species are Red Alder on the west coast of North America and the Black Alder, native to most of Europe and widely introduced elsewhere, both reaching over 30 metres in height. By contrast, the widespread Green Alder is rarely more than a 5 metre tall shrub.
The Alder is an unusual tree, it is water loving yet is also highly combustible, making it very sacred as it combines the elements water and fire. Because of its fierce flame it is sometimes known as the warrior tree, its symbolism being that of strength, tenacity and determination. Because of its resistance to water it can be used to hold water elementals and negative spirits. Dyes can be made from its bark, flowers and twigs, one of which was red which the druids used to dye their faces during rituals.