A large group of deciduous, broad conical trees, which thrive best in a moist situation. Most species develop into very large trees and therefore not recommended for small gardens.
Family: Betulaceae (birch family)
Botanical Name: Alnus
Common Names: Alder
Foliage: Deciduous, bright green, rounded leaves.
Bark: Grey or brown rough bark that develops shallow fissures as the tree develops.
Blossom: Grey-purple buds in spring followed by catkins. Both male and female flowers appear together on the same tree. Male catkins are long pendulous and yellowish. Female catkins are oval and green.
Flowering Period: Winter.
Soil: Moist, well-drained or poorly drained soil (chalk, clay or loam). Acid, alkaline or neutral pH.
Conditions: Full sun, in any aspect.
Habit: Tall, columnar, upright.
Type: Tree or very large shrub.
Origin: Europe to North Africa.
Hardiness: Fully hardy in the UK.
Alders thrive in poor, wet soils and look very attractive near open water. They are useful plants for dense hedging and tall screens in boggy ground. They are often grown as a pioneer tree, breaking up compacted soils to allow other species to establish. Alders fix nitrogen in the soil, so they can grow in less fertile areas.
Easy to grow and low maintenance.
Not necessary except to remove any crossing, dead or diseased branches. Prune in late winter or early spring.
Can be affected by alder sucker and leaf-mining sawflies.
Increase from seed or take hardwood cuttings.
Alnus cordata (Italian Alder) is a stately tree that bears heart-shaped leaves. RHS Award of Garden Merit. Native to southern Mediterranean regions. Height: 30 to 50ft.
Alnus glutinosa (Common Alder, black Alder) forms a very large tree. Native to the UK. The cultivar 'Imperialis' has deeply cut leaves but can be rather ungainly in habit. Height to 40ft (12m), spread to 25ft.
Alnus incana (Grey Alder) a very hardy a tree that is useful for planting in cold wet places. Native to Europe. There are many cultivars the best of which is 'Aurea' with golden leaves and shoots. Height: 20 to 35ft.
Alnus rhombifolia (White Alder) a vigorous tree reaching a height of 30ft or more. Good bark color ranging from white to gray.
Alnus viridis (Green Alder) is native to the Alps and south-east Europe. A compact species producing a large shrub or small tree with shiny green leaves that have a yellow hue. Height: 6 to 14ft.