Alnus glutinosa Incisa - Black Alder

Alnus glutinosa 'Incisa', also known as the Common Alder or Black Alder, is a variety with unusual foliage. Its tiny, elliptical leaves, measuring 2 to 4 cm in length, resemble those of hawthorn. Its brown branches are adorned, from March onwards and before the foliage appears, with numerous pendulous catkins in shades of yellow-green to brown, which are highly decorative. Suitable for small gardens, this small and very elegant tree has very slow growth and also withstands diseases and pests very well. It thrives in waterlogged and poor soils, where it is sometimes the only plant that can grow.

In harsh climates and poor, marshy, and oxygen-deprived soils, the Common Alder, with its rot-resistant and lightweight wood, reigns supreme. This deciduous tree belongs to the birch family and is widespread in the native flora of Europe, up to Scandinavia. It is sometimes called the alder or verne. The 'Incisa' variety is mainly distinguished by its small leaves, similar to those of hawthorn. It is a small tree with a pyramidal and airy habit, reaching only 3 to 4 m in height at maturity, with a spread of about 2 m. Its growth is rather slow. Its foliage is light green and vibrant, even lighter underneath, and slightly sticky to the touch, especially on juvenile leaves. The buds are purple and also sticky, particularly in winter. The spring inflorescence is a small cone called a strobilus, measuring 2 cm in length, containing tiny winged fruits that will be dispersed by the wind. This tree retains its foliage for a long time.

As a pioneer species, thriving in water regardless of its source, the Common Alder is used as a standalone tree, in groves, or to retain banks along streams, natural ponds, or in waterlogged and acidic soils in damp understories. It should be kept away from foundations and buildings, as its root system, which can reach up to 4 m in depth, is capable of obstructing drinking water pipes. Plant the 'Incisa' Common Alder with shrubs or trees that have similar growing conditions, such as Nyssa sylvatica or the Bald Cypress. At its base, plant ferns such as Onoclea sensibilis, in neutral to acidic and moist soil, in full sun or partial shade. An elegant small tree, ideal for river or marsh scenes, in dappled shade or full sun.

The Common Alder is also a useful tree, used in forestry to prepare and improve the soil before planting poplars. Considered a green manure by foresters, it cleanses and enriches the soil in which it is planted, as its roots convert atmospheric nitrogen into plant proteins. Its wood, almost rot-resistant in water, was once used for the construction of underground water pipes and channels. A significant part of the city of Venice is said to have been built on alder wood piles.