General
Alder (Alnus spp.) includes species such as black alder (Alnus glutinosa) and grey alder (Alnus incana). These are fast-growing pioneer trees well adapted to poor and disturbed soils. They are especially known for their ability to fix nitrogen through a symbiotic relationship with Frankia bacteria in root nodules. This makes them valuable ecological engineers that enrich soil, improve structure, and support succession.
Light
Full sun to partial shade; grows fastest in full sun.
Soil type
Moist to wet soils; tolerates clay and poor soils, including degraded or compacted ground.
Water requirement
Medium to high; especially black alder prefers consistently moist or wet soil.
Nutrient requirement
Low; improves soil fertility through nitrogen fixation.
Precultivation
Seeds are small and benefit from a short cold stratification period. Sow on the surface in moist conditions with good light. Germination is usually quick once conditions are suitable. Natural sowing outdoors in autumn often gives best results.
Planting
Plant in spring or autumn. Alder tolerates a wide range of soils, including compacted or degraded land. It establishes quickly and can be used to stabilize soil, improve fertility, and prepare sites for more demanding species. Spacing depends on purpose—closer spacing for biomass or wider for tree growth.
Pruning
Alder tolerates heavy pruning and responds well to coppicing and pollarding. This makes it useful in managed systems where biomass production or size control is desired.
Maintenance
Very low maintenance once established. Water during establishment, especially in dry soils. It can thrive in poor soils without fertilization. Regular coppicing can maintain productivity and prevent shading of other plants.
Harvest
Not typically grown for harvest, but wood can be used for fuel, crafts, or small timber. Leaves and prunings are valuable as mulch or compost material.
Common issues
Generally resilient, but can suffer from fungal diseases in poorly ventilated sites. Some species can spread aggressively in favorable conditions.
Pollination
Wind-pollinated (catkins).
Companion plants
Willow, birch, hazel, currants, woodland perennials, and most fruit trees that benefit from improved soil fertility.
Incompatible plants
Plants requiring very dry, nutrient-poor soils; may outcompete slow-growing species if unmanaged.
