How to Grow Alder
Alnus glutinosa
Common Alder thrives in wet or waterlogged soils where most trees fail — stream margins, boggy corners, and pond edges are ideal. It fixes its own nitrogen via root nodules, so skip the fertiliser. Plant in full sun to partial shade, keep the root zone consistently moist, and mulch with bark or wood chip in spring. Prune only in late autumn to midwinter to avoid sap bleed. Site away from drains and foundations — the roots seek moisture aggressively. Hardy throughout the UK with no winter protection needed.
Yearly Lifecycle
Care Essentials
Alder fixes its own nitrogen via root nodules and rarely needs feeding. Mulch with composted bark or wood chip in spring; avoid high-nitrogen fertilisers which push soft growth and reduce nitrogen fixation.
Watch For
- Phytophthora root rot
- Alder dieback (Phytophthora alni)
- Woolly alder aphid
- Alder leaf beetle
Companions
Willows (Salix spp.), Yellow Flag Iris (Iris pseudacorus), Marsh Marigold (Caltha palustris), Meadowsweet (Filipendula ulmaria), Ragged Robin (Lychnis flos-cuculi)
Care Requirements
Light
Full sun to partial shade
Alder grows well in full sun or partial shade. It tolerates more shade than most trees but produces the densest canopy and best growth in an open, sunny position near water.
Watering
Consistently moist to wet soil; tolerates waterlogging
Alder thrives in permanently moist or waterlogged soil. In drier garden soils, water regularly during the first two to three years until established. Once mature, it is largely self-sufficient if planted near water.
Fertilizing
No feeding needed; nitrogen-fixing roots do the work
Root nodules containing Frankia bacteria fix atmospheric nitrogen continuously. Adding fertiliser, especially nitrogen-rich feeds, suppresses this natural process and is counterproductive. A spring bark mulch is sufficient.
Pruning
Minimal; only in full dormancy (Nov-Feb)
Prune only between November and February when fully dormant to prevent sap bleeding. Remove dead, diseased, or crossing branches. Avoid large cuts as wounds heal slowly.
Spacing
6m minimum from buildings and drains
At maturity, alder spreads 8-10m. Allow at least 6m clearance from foundations, drains, paths, and other trees. The roots actively seek moisture and can damage infrastructure.
Support
Stake young trees for the first two years
Newly planted trees benefit from a single stake for the first two years, especially on exposed or windy sites near water. Remove once the tree is well anchored.
Growing Tips
Site near water
Alder is happiest with its roots near a permanent water source. Plant at stream margins, pond edges, or boggy ground. It tolerates brief flooding but also manages in moist but not waterlogged garden soil once established.
Let nitrogen fixation do the work
The root nodules fix atmospheric nitrogen continuously. Avoid adding nitrogen-rich feeds which suppress this natural process and produce weak, disease-prone growth.
Prune only in dormancy
Alder bleeds sap heavily if cut in spring or summer. Do all pruning between November and February while fully dormant. Keep cuts small — large wounds heal slowly and invite disease.
Watch for Phytophthora alni
Alder dieback caused by Phytophthora alni has spread along UK waterways since the 1990s. Signs include bleeding bark, orange-brown wood under the bark, and dying branches. Report to the Forestry Commission if suspected.
Wildlife value
The persistent woody cones provide winter food for siskins and redpolls. The catkins are among the first UK pollen sources in late winter. Plant as part of a wetland wildlife corridor for maximum benefit.
