Our native alder is a medium sized tree with a narrow crown and short, spreading branches. It grows extensively in damp places alongside streams, rivers, ponds and lakes as well as marshy areas.
There are a number at Upton Country Park where the water is, of course saline, so it seems to be tolerant of salt. In some boggy areas it grows in great perfusion and forms the habitat commonly called alder carr. Alder is rarely planted as it has little forestry value although wood turners quite like it because the wood is both strong yet easily worked.
The tree has both sexes of flowers on it. There are the male catkins that resemble hazel catkins but are longer. The female flowers develop a little later and are smaller, cylindrical and are purplish brown in colour. The flowers are wind pollinated.
Fertile female flowers develop in to small cones and they will often stay on the tree all winter, long after the seeds have dropped. The seeds themselves are distributed generally by floating on the water until they reach land. These cones, which are quite unique for a deciduous tree, are quite often the defining feature in winter.
The alder bears on its roots little nodules that contain a live bacterium which enable it to make soluble nitrogen salts out of the inert nitrogen of the air. Consequently, the soil on which alder grows is remarkably fertile.