Some 35 species of deciduous trees and shrubs having stalked winter buds. Leaves alternate, serrate or dentate, often with prominent veins, straight and parallel. Flowers monoecious, in separate male and female catkins; male catkins longer, narrowly cylindrical, often clustered at shoot tips. Female catkins in clusters of 1-6, short, erect, but later often spreading or pendulous and woody cone-like (but differing structurally from conifer cones). The roots of many species from associations with soil bacteria to fix nitrogen from the air and subsequently enhance the fertility of the soil. Native to the Northern Hemisphere from the subarctic to the Himalayas and Andes, extending into the Southern Hemisphere in Peru.
Alnus: Latin name for alder.