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Alnus serrulata (Tag Alder)
Male catkins on right,
mature female catkins left
Johnsonville, South Carolina
Scientific classification


About 20-30 species, see text.

Common alder in Holma, Sweden

Alder is the common name for about 30 kinds of trees and shrubs of the Alnus genus. They are a part of the birch family (Betulaceae). Most of them are smaller in size. Leaves are mostly deciduous, only very few alders are evergreen.

Alder trees are found in all parts of the northern hemisphere and they are often found on poor and/or wet soil. Alders have alternate simple leaves with a toothed edge. The male flowers are in the form of hanging catkins and the female flowers are much smaller catkin-like structures (see image). The pollen is transferred from the male to the female flower by the wind. The female flower parts develop into little, green, rounded cone-like fruits which turn brown in autumn and open to release the seeds.

The best known species are the Black Alder, which can be seen throughout Europe. The largest is probably the Red Alder, native to North America. It reaches about 32–35 metres in height. The widespread Green Alder is a shrub, rarely more than 5m high. The commonest alder in the UK is the Black Alder, also called the Common Alder or European Alder.

Alder is a preferred tree for bees, especially in spring. It is also used to make charcoal.