The common alder is a water-loving tree that hides a secret nitrogen-fixing engine.

Gardening · Published 3 October 2026 Swipe more like this →

In short: Thriving in swamps and riverbanks, the common alder is a master of survival in poor soils. It forms a unique symbiotic partnership with bacteria to pull nitrogen from the air, while its water-resistant wood has supported the foundations of historic cathedrals and the city of Venice for centuries.

The common alder, Alnus glutinosa, is a resilient pioneer species native to Europe, southwest Asia, and northern Africa. It is uniquely adapted to wet, nutrient-poor environments, largely due to its symbiotic relationship with the bacterium Frankia alni. These bacteria inhabit nodules on the tree's roots, absorbing nitrogen from the atmosphere and providing it to the tree in exchange for carbon products. This process enriches the soil, allowing the alder to thrive where other species might struggle and making it a vital component in ecological restoration.

Beyond its ecological role, the tree has a long history of human utility. Its wood is remarkably durable when submerged, a property that led to its use in the foundations of medieval cathedrals and the piles beneath the Rialto in Venice. Vitruvius, the Roman architect, noted its use in constructing causeways across marshlands. Today, the wood remains valued for its flexibility and ease of working in joinery, carving, and the production of paper and fibreboard. Historically, the bark and twigs were used for tanning and dyeing, producing various shades of brown and black.

The tree is also a subject of modern scientific interest. Research has indicated that extracts from its seeds show activity against pathogenic bacteria, including methicillin-resistant Staphylococcus aureus (MRSA), suggesting potential for future drug development. However, the species faces threats, including the pathogen Phytophthora alni, which causes extensive mortality in parts of Europe. While it is a beneficial native species in its home range, it has become an invasive environmental weed in places like New Zealand and parts of the United States, where its rapid growth and nitrogen-fixing capabilities can disrupt local ecosystems.

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