Common alder
Alnus glutinosa
Botanical family: Betulaceae
Alnus glutinosa, a pioneer tree of Europe's banks and wetlands, is recognisable by its rounded, slightly sticky leaves and its small woody cones that persist through winter. In agroforestry and permaculture, it's readily placed at the edge of a pond or wetland for its role as a nitrogen fixer and bank stabiliser.
Sun to partial shade
Moist, peaty, watercourse edge, tolerates waterlogged soils
Likes permanently moist to waterlogged soils
Very hardy (withstands -25 °C)
Overwinters in your zone
Role in permaculture
A pioneer of wetlands and banks, the common alder captures nitrogen from the air, like the legumes but with a different bacterial partner. It stabilises waterlogged soils, serves as a windbreak by rivers, and durably enriches the poor ground where it establishes.
Insects attracted or useful
Documented synergies
Relationships backed by a named source, with their real scope. A synergy without a named reference has no place here, no “it is said that…”.
The alder fixes atmospheric nitrogen, but not via a rhizobium like the legumes: its partner is Frankia, a filamentous bacterium that forms perennial nodules. The mechanism is certain; the magnitude often overstated: field values range from a few tens to a few hundred kg of nitrogen per hectare per year depending on age and site, on the order of 50 kg on young alders in a Quebec study. A real enricher, not a miraculous one.
institutional source
- · USDA Forest Service, Silvics of North America vol. 2, Alnus glutinosa (Handbook 654)
- · Yuan et al. (2023), Frontiers in Microbiology, DOI 10.3389/fmicb.2023.1230170
A pioneer and nurse tree: the nitrogen it fixes returns to the soil through its rich litter, and the species planted with it grow better, from 11 to 84% taller after ten years in a documented reforestation. But the effect is LOCAL (near the tree) and SLOW (through the litter, over years), measured in a forest plantation. It's not a fast fertiliser for a kitchen-garden bed.
institutional source
- · USDA Forest Service, Silvics of North America vol. 2, Alnus glutinosa (Handbook 654)
A botanical rarity: the alder, a pioneer species, associates both with two types of mycorrhizal fungi (ectomycorrhizae and arbuscular mycorrhizae) and with Frankia. According to the title and abstract of the cited study, carried out on young seedlings in pots, the fungus Gigaspora rosea and Frankia each increase root branching, more so when combined, and reduce the number of root hairs; the phosphorus content of the seedlings does not change significantly, and the abstract says nothing about water. This source therefore does not support better access to water or phosphorus, and no benefit is quantified in the garden: to be seen as a pioneer capacity, not a quantified effect. The content of the reference article (behind a paywall) was not read.
scientific study
- · Orfanoudakis M., Wheeler C.T. & Hooker J.E. (2010), Mycorrhiza 20(2):117-126, DOI 10.1007/s00572-009-0271-0
Nutritional & medicinal properties
For educational purposes. Not a substitute for medical advice. Effects depend on the form, the dose and personal context.
Precautions & contraindications
Its medicinal use has fallen into disuse.
Species & varieties
The best-known species and varieties in France.
Species
Voices of peoples
Paraphrases attributed to their documented sources. Never an invented quote.
Vitruvius, Roman architect and engineer · Rome, around 15 BC
In the De architectura, Vitruvius explains that, when firm ground cannot be reached and the terrain remains soft or marshy, one must drive in, close together, piles of alder, olive, or oak previously charred, then fill the gaps. He singles out the alder for its holding under water: permanently submerged, this wood does not rot. That is the fact attested by the ancient text. The use of alder piles to found Venice on its lagoon is a much later application of it, documented elsewhere, that must not be attributed to Vitruvius himself.
Vitruve, De architectura, livre III, chapitre 4, section 2 (vers 15 av. J.-C.)
The attested fact is limited to this: Vitruvius's text names the alder among the pile woods, for its resistance to immersion. The extension 'Venice rests on alder piles', accurate but from a distinct source belonging to the history of construction, is not merged here into the ancient quotation. It's a precise technical knowledge, with no medicinal claim, and not a vague 'wisdom of the Ancients'.
Key takeaways
- • Prefers sun to partial shade exposure.
- • Ideal soil: moist, peaty, watercourse edge, tolerates waterlogged soils.