- Name also: Black Alder, European Alder
- Family: Birch Family – Betulaceae
- Growing form and height: Tree with one or several trunks. 5–20 m (15–65 ft.).
- Flower: Small male and female flowers in separate inflorescences. Male flower with four yellowish-green to yellow perianth-segments. Male inflorescence a dense, pendent catkin. Female flower lacking perianth, red to reddish-brown. Female inflorescence a cylindrical and hard, long-stalked catkin. Wind-pollinated.
- Leaves: Broadly obovate, with blunt or notched apex. Margins serrate to shallowly incised, rarely deeply incised, vein pairs 5–7. Dark green, glossy, glabrous or sparsely pubescent.
- Buds: Egg-shaped, glabrous, quite large, dark purple-red, stalked. Scales few, rather large.
- Fruit: Female catkin ripening into a hard, brown cone-like structure. Fruit a very small nut.
- Habitat: Sea and lake shores, streamside swamps, edges of nutrient-rich mires, spruce-broadleaf swamps, moist broadleaf woods. Also a park and forestry tree.
- Flowering time: April. 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.
Originally common alder was a tree of nutrient-rich wetlands and other waterlogged soils. It thrives only on sites with flowing surface or ground water. Its seeds are also dispersed by water. In addition, common alder well tolerates saltiness of seashores. It hybridizes quite commonly with grey alder, in North-Finland hybrids are more common than common alders.
Soft timber of common alder was formerly used for making wooden shoes and moulds in glass factories. It is also suitable for constructions in or near water as it is decay-resistant.