Alder tree (Alnus glutinosa) is a native tree throughout Europe, including Britain and Ireland. It grows to a maximum height of 25m over around 50 years, and is short-lived, with a life expectancy of just 60 years. It grows in a variety of habitats and soil types but especially in areas with damp soil, including wet woodlands (carrs), marshes, and on the banks of rivers and streams. It makes a fantastic garden tree, particularly in areas of damp or poor soil, such as new housing estates. Alder tree roots lock nitrogen into the soil so can tolerate, and even improve poor soils.
How to identify an alder tree
Alder trees are broadly conical in shape, and have dark and fissured bark. Twigs are light brown with orange spots, and young twigs are sticky (as implied by the glutinosa part of alder's botanical name). Leaves grow from purple or grey leaf buds and are fresh green and rounded.
In spring, look out for pendulous male, yellow catkins, and green, oval female catkins. Over summer the male catkins drop off and the female catkins harden and become reddish brown, and look like small cones. They open to release their seeds, which are carried by wind and water.
Alder trees and wildlife
Alder trees support a wider range of wildlife, including the spectacular yellow-and-black-striped caterpillar of the alder moth (Acronicta alni). Its catkins provide nectar and pollen for bees, and the seeds are eaten by finches.
How to grow alder trees
Alder trees will grow well in a variety of locations, but do best in moist soil and full sun. Keep the tree well watered until it's established.
How to plant alder trees
Dig a square hole that's wider, but no deeper than the bare roots or potted rootball of your alder. Plant it at the same depth it was in the pot (or where the soil 'tide mark' suggests it was planted previously), and fill around the roots with soil. Firm gently and water well. Stake the tree for around two years after planting, so its roots firmly anchor the tree into the soil.
How to care for alder tree
Alder trees need very little care. Keep well watered in the first few weeks after planting and ensure the stake is secure and the tree tie is not cutting into the bark. There's no need to prune alder. There's also no need to fertilise alder, as it makes its own nitrogen.
Growing alder: pests and diseases
Fluffy threads around the base of leaf stems are alder sucker. They pose no threat to alder trees and should be left alone
Tunnels in the leaves could be leaf mining flies or moths. Again, these pose no harm to the tree and contribute to the biodiversity of the garden.
Advice on buying alder trees
- You can buy bare-root alder trees from November to March, or in pots throughout the year. Hedging whips are also available
- Ensure you have the right growing conditions – alder tolerates a wide variety of soil types and will grow on poor soils, but it does best in a moist environment
- Always check plants for signs of disease or damage before planting