These are alders - excellent trees or shrubs, almost all deciduous, for growing in damp places, typically along the banks of rivers. They are related to the birches, Betula, and originate from the temperature regions of the northern hemisphere. They have long male catkins and shorter female catkins, both kinds on the same plant. The female ones remain after flowering, becoming woody and brown, and open when they are ripe to release the small seeds.

Alnus sieboldiana is very attractive, with broad, roughly ribbed, serrated leaves. It has haging, yellow catkins, and these are followed by quite large ovoid cones, greenish yellow at first but later becoming brown.