Alnus is one of 15 genera of trees that fix nitrogen but are not in the legume family. The bark is dark green or grey, often with yellowish patches and short, raised lenticels. Open-grown Alnus nepalensis in eastern Nepal. The narrowly cylindrical clusters of tiny flowers, or catkins, occur as male or female separately on the same or different twigs in autumn. Male catkins are yellow, 10-25 cm long, and hang in clusters at the end of twigs.
Female catkins are much shorter, erect and woody, and occur on branching side twigs.
The fruits, which superficially resemble cones of the pine family, are dark brown, upright on short stalks, elliptical, composed of many spreading, hard woody scales. Empty cones may persist on the tree. The seeds are light brown, circular and flat with two broad membranous wings, more than 2 mm across. Seeds ripen from November to March depending on geographical locality. At lower altitudes particularly, utis occurs on moist sites, such as near rivers and in ravines, but it will colonize rocky sites exposed by landslips, or lands abandoned following cultivation.