Alder is a deciduous tree which can grow up to 30m tall. It is prolific in wet woodlands and streamsides, and is able to grow in these conditions because of the presence of bacteria in its root nodules which extract nitrogen from the air and fix it for the tree, making up for the lack of nitrogen in the waterlogged soil. It can be a pest in managed wetlands as the seeds can germinate prolifically for up to 20 years.
The long drooping male catkins appear in February, initially appearing purple then becoming yellow with pollen, then turning orange. The female catkins are green and egg-shaped, and in fruit become woody, like small pine cones. Both first and second-year female fruits are often seen on the same tree.

Early male catkins (purple, long), female fruits (green, rounder) and mature fruits (brown, woody)

Alder leaves and early fruits		Alder fruits: first-year green fruits, and second-year brown cones

Early catkins (autumn)		Catkins with pollen (Feb-March)		Mature orange catkins