Plant · P-746

European Alder

Alnus glutinosa

European Alder is a wet-ground pioneer tree that fixes nitrogen through root nodules formed with the bacterium Frankia, among the highest rates recorded for any temperate tree. Its wood stays sound after decades underwater, which made it the traditional choice for pilings, and it coppices hard for poles, charcoal and nitrogen-rich mulch. It thrives on waterlogged ground that kills most other trees but needs the soil to stay reliably moist, since long hot dry summers are what stop it rather than winter cold. Its floating seed drifts far downstream and colonizes open wet ground readily, which is why several US states regulate it as an invasive species.