Alnus glutinosa, more commonly known as the Alder or the Common Alder, is a native British tree predominantly found in wetlands and marshy areas across England and throughout Europe. This species is a medium-sized, deciduous tree that is recognised for its broad ecological value and landscape utility.

The Alder can grow to a height of about 20-30 metres and has a lifespan of up to 60 years, although some specimens can live longer under favourable conditions. Its growth rate is moderate, and the tree reaches maturity in about 20-40 years. The tree has a grey-brown bark and glossy, dark green, rounded leaves with serrated edges that turn yellow before falling in autumn.

One of the most distinctive features of Alnus glutinosa is its flowering and reproduction process. The Alder is monoecious, meaning it carries both male and female flowers on the same tree. It flowers between February and April, with the male flowers appearing as drooping yellow catkins and the female flowers as smaller, upright catkins that develop into small, woody, cone-like fruits. These cones persist on the tree throughout the winter, providing an important food source for wildlife and a unique visual interest.

Unlike most trees found in England, the Alder has a unique ability to thrive in wet and waterlogged conditions, thanks to a symbiotic relationship with a nitrogen-fixing bacterium called Frankia alni. This bacterium lives in nodules on the Alder’s roots and converts atmospheric nitrogen into a form usable by the tree, enabling it to survive in nutrient-poor, waterlogged soils.

In terms of garden design, the Alder offers a lot of value. It provides a lush green canopy in summer and appealing yellow foliage in autumn. Its ability to grow in wet conditions makes it an excellent choice for waterside plantings or rain gardens. The Alder’s form can be manipulated through coppicing or pollarding, allowing gardeners to maintain it at a manageable size if needed.

The Alder tree is vital for wildlife. Its flowers provide early season pollen for bees and other insects. The seeds in its cones are eaten by birds, particularly siskins, redpolls, and goldfinches. Its dense root system provides excellent cover for fish in rivers and lakes, and its leaves, when they fall into the water, are a significant food source for aquatic insects, which in turn provide food for fish and birds.

An interesting fact about the Alder is its historical and cultural significance. Alder wood is resistant to decay when submerged in water, making it a popular material for building foundations, including parts of Venice, and for creating water pipes in the medieval period.

In summary, Alnus glutinosa, the Alder, is a versatile and environmentally beneficial tree. Its unique adaptations, coupled with its value to wildlife and its historical significance, make it a worthy addition to any British garden, particularly those with damp or waterlogged conditions.