- Family: Birch Family – Betulaceae
- Growing form and height: Tree with one or several trunks. 3–15 m (10–50 ft.).
- Flower: Small, yellowish-green to yellow. Male and female flowers in separate inflorescences. Male flowers form a pendent catkin. Female catkins erect, cylindrical, and red. Female flowers lack perianth. Wind-pollinated.
- Leaves: Elliptic to roundish, with a tapering tip, at least slightly pubescent. Margins doubly dentate, rarely incised. Veins 7–11. Upper side matt green, underside greyish.
- Buds: Egg-shaped, hairy, quite large, dark purple-red, stalked. Scales few, rather large.
- Fruit: Small, nut. Width of wing varies.
- Habitat: Moist spruce and mixed forests, lush broadleaf woods, spruce swamps, shores, roadsides, margins of fields and meadows, burned-over areas.
- Flowering time: April–May. Flowers before coming into leaf.
The genus Alnus comprises approx. 30 species of trees or large shrubs which flower before coming into leaf in the spring. They have root nodules which contain nitrogen-fixing bacteria. Thus they improve the soil quality. The genus is best represented in North America and East Asia.
Grey alder flowers before coming into leaf in spring. It reproduces and disperses efficiently, but favours areas with rich soil. Seeds are dispersed by water. Grey alder is a fast-growing but short-lived tree. It is a very variable species. The Kola grey alder that dominates in Northern Finland is a subspecies of the grey alder (ssp. kolaënsis). Forms with variously incised or hairy leaves are also known.