Alder – Alnus Glutinosa

Common name: Alder  

Scientific name: Alnus glutinosa 

Introduction: Alder is a deciduous tree species that is native to the UK. It is the only British native deciduous tree to produce cones and has a fast growth rate.   

Identification

Leaves: Leaves are long, dark green and ovate. They are irregularly toothed (have a serrated edge) and are typically indented. The leaves on alder typically remain until late autumn and have a minimal change in colour before falling in the later autumnal months. 

[Top: Young Alder leaves. Bottom: Alder leaves with catkins]

Buds: Are a greyish-purple colour on long stems.  

Bark: The bark of an Alder tree is a dark-grey and is often fissured. Typically, you may also find that it is covered in lichen. 

[Image: The bark of an Alder]

Form: Conical shape.

Reproduction

Alder trees are monoecious, meaning that both male and female flowers are produced on the same tree. The flowers of an Alder (both male and female), typically form between February and April. Male catkins turn to yellow and have a droopy appearance.  Female catkins, however, are ovate and green, forming in clusters of 3-8 per stalk. They then get pollinated by the wind and develop into small, cone-like fruits. Eventually, these ‘cones’ open up and release their seeds which are then dispersed by the wind and rain.  

Growth Habit

Relatively fast growth rate but have a relatively short lifespan.

Ecological Importance

Alder trees are linked to a nitrogen–fixing bacteria called Frankia alni which not only helps them to grow in soil that has fewer nutrients but also to condition weaker soil, helping to improve its fertility. The leaves also provide an essential food source to a variety of caterpillar species. The catkins of an Alder tree help to provide an early source of nectar and pollen to beers due to their early formations. Female catkins that develop into cone-like fruits eventually release their seeds which also provides a food source to birds such as goldfinches. Additionally, due to Alder trees thriving in wet conditions, they also provide an ideal environment for mosses, lichens and fungi to grow.

Maintenance

Pruning and Pruning Qualities: Alders tend to require different methods of pruning depending on the age and maturity of the individual tree. Typically, young trees benefit from formative pruning which can be carried in winter when the tree is in its dormant period. Pruning in a trees dormant periods helps to prevent sap bleeding, which ordinarily attracts pests that can lead to disease. Mature trees typically require less pruning, but when pruning is conducted it should be to aid its appearance and maintain its health. 

Similarly, both young and mature trees may benefit from reducing congestion of branches. This is because it encourages greater air circulation and exposure to light which aids in the tree’s health maintenance. Like most trees, you should also remove any dead or diseased branches as soon as they are spotted to prevent the further spread of disease. Alders also respond well to coppicing. If you are ever in doubt about how you should prune your tree, no question is too big or small for a qualified arborist. Find your local tree surgeon here. 

Growth Rate after Pruning: Helps to encourage a maintained and controlled appearance.