Alnus glutinosa, often referred to as the Black Alder or European Alder, is a native British tree frequently found around wetlands, floodplains, and riparian zones across Europe. This medium-sized deciduous tree is notable for its adaptability, ecological contributions, and interesting characteristics.
Black Alder trees typically reach heights of 20-30 metres and have a lifespan of up to 60 years, although some have been known to live longer under favourable conditions. The tree’s growth rate is considered moderate to fast, generally maturing within 20-40 years. The bark of the Black Alder is dark grey to blackish and fissured, lending to its common name. It features rounded, dark green leaves that turn yellow before falling in autumn, offering seasonal interest.
This tree is monoecious, producing both male and female flowers on the same plant. The male flowers appear as hanging yellow catkins, while the female flowers are smaller, upright, and green. They bloom between February and April, and following wind pollination, the female catkins mature into small woody cones. These cones, persistent through winter, provide both visual interest and a valuable food source for wildlife.
What sets the Black Alder apart from other trees in England is its ability to thrive in wet, poorly drained conditions. This adaptation is enabled by a symbiotic relationship with nitrogen-fixing bacteria, Frankia alni. These bacteria reside in the tree’s root nodules and convert atmospheric nitrogen into a form the tree can use, allowing it to survive and enrich nutrient-poor, waterlogged soils.
In terms of British garden design, the Black Alder is an excellent choice for wet, marshy areas or along ponds and streams, where other trees may struggle. The tree’s lush summer foliage and autumn colour can provide a verdant backdrop or visual anchor in the landscape. Its responsive growth also makes it suitable for coppicing or pollarding, offering flexibility in managing its size.
From an ecological perspective, the Black Alder is invaluable. The early flowering provides vital pollen for bees and other insects, while the cones’ seeds are a crucial food source for birds such as siskins, redpolls, and goldfinches. In aquatic environments, its root system provides excellent cover for fish, and the fallen leaves serve as food for water-dwelling insects.
An interesting fact about Black Alder is the water-resistant quality of its timber, which has been historically used for underwater foundations, including parts of Venice, and for crafting durable water pipes in the medieval era.
In conclusion, Alnus glutinosa, the Black Alder, is a robust, ecologically beneficial tree that not only enhances landscape aesthetics but also plays a vital role in supporting wildlife. Its unique adaptability to waterlogged conditions and the dynamic it introduces in the garden landscape make it a commendable addition to British gardens.