Common alder (Alnus glutinosa), also known as black alder, European alder or just alder, is a tree that can grow up to c. 15m high. It thrives in wet locations where its association with the bacterium Frankia alni enables it to grow in poor quality soils. The bark of young trees is smooth, glossy and greenish-brown while in older trees it is dark grey and fissured. The branches are smooth and somewhat sticky, being scattered with resinous warts. The buds are purplish-brown and have short stalks. Both male and female catkins form in the autumn and remain dormant during the winter. The leaves are short-stalked, rounded, up to 10cm long with a slightly wedge-shaped base and a wavy, serrated margin. They have a glossy dark green upper surface and paler green underside with rusty-brown hairs in the angles of the veins, and remain green late into autumn.