Alnus glutinosa Imperialis

The Alnus glutinosa Imperialis, also known as Common Alder or Black Alder, is distinguished from the common European alder by its beautiful foliage, which is deeply lobed and pointed, resembling that of ferns. Its brown branches are adorned from March onwards with numerous highly decorative yellow-green to brown and pendulous catkins. Adapted to medium-sized gardens, this small tree with a pyramidal habit is very elegant and also highly resistant to diseases and parasites. It thrives in waterlogged and poor soils, where it is sometimes the only species able to grow.

With its rot-resistant and lightweight wood, the Common Alder reigns supreme in harsh climates and poor, marshy, and poorly aerated soils. This deciduous tree from the birch family is widespread among the native flora of Europe, extending up to Scandinavia. It is one of the few species capable of growing from northern to Mediterranean European coasts, always in close proximity to water. It is sometimes referred to as "Speckled Alder". The Imperialis variety, from which it originates, is mainly distinguished by its deeply laciniate foliage. It is a small tree with a pyramidal and airy habit, reaching a height of 8 to 10 m (26 ft 2 in to 32 ft 10 in) at maturity and an approximate spread of 3 m (9 ft 10 in). 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 kind of small cone called a strobilus, measuring 2 cm (0.8 in) long, containing tiny winged fruits that will be dispersed by the wind. 'Imperialis' seems to be less susceptible to alder wasting disease than the typical species. This tree retains its foliage for a long time.

As a pioneering species, the water-loving Common Alder, no matter its source, can be used in isolation, as a lining tree, in hedgerows, or to retain the banks of streams, natural pools, or in waterlogged and slightly acidic soils with damp undergrowth. It should be kept away from foundations and buildings, as its extensively developed root system (up to 4 m (13 ft 1 in) deep) is capable of blocking drinking water supply pipes. For example, it can be combined with Nyssa sylvatica, Bald Cypress (Taxus distichum), which require almost the same growing conditions. Two species of ferns, such as Onoclea sensibilis in neutral to acidic and moist soil, in sun or partial shade, and Dryopteris palustris (Telypteris palustris), can also be planted at its base. An elegant small tree with the appearance of a giant fern, ideal for riverside or marsh settings, 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 purifies and enriches the soil in which it is planted, as its roots convert atmospheric nitrogen into plant proteins. Its almost rot-resistant wood in water was once used for underground water conduits and channels. A significant part of the city of Venice is said to have been built on piles made of Alder wood.