Alnus glutinosa

The common alder is a deciduous tree of medium size, emblematic of wetlands and watercourses. It is naturally widespread throughout Europe up to Scandinavia, up to an altitude of 1,200 m (3937 ft), except in the Mediterranean region. Easily regenerating from the stump, it often has several slender trunks with almost horizontal branches, giving it a strikingly pyramidal silhouette. Its charming leaves persist on the tree for a long time before falling in winter. Bright light green in colour, shiny, leathery, and rounded in shape, they are recognisable by their truncate tips. In winter, when most plants remain dormant, the alder illuminates the dark days like an inverted candelabra, with thin and pendulous male catkins that are yellowish when they bloom, measuring 8 cm (3.1 in) in length. The fruits from the previous year form dark-brown, almost black, cones, contrasting with the light shade of its flowers. The alder excels in wet and poor soils, sometimes being the only plant able to grow. It is to be used mainly for revegetation and preventing erosion along riverbanks.

In harsh climates and poor, marshy, and suffocating soils, the Common Alder, with its rot-resistant and lightweight wood, reigns supreme. This deciduous tree from the birch family is very common in the native flora of Europe. It is one of the few species capable of growing from northern France to the shores of the Mediterranean, always close to water. Its growth is rapid when young. At maturity, it reaches 25 m (82 ft) in height and 12 m (39 ft 5 in) in width. Its foliage is lighter on the underside, slightly sticky to the touch, especially on juvenile leaves. The buds are violet in colour and also sticky, particularly in winter. The fruit is a kind of small cone called a strobilus, 2 cm (0.8 in) long, containing tiny winged fruits that will be dispersed by the wind.

As a pioneering species, thriving in water from any source, the Common Alder is used in isolation, as a line tree, in groves, or to maintain banks along streams, natural pools, or in soggy, acidic soils with damp undergrowth. It should be kept away from foundations and buildings, as its root system, which can grow up to 4 m (13 ft 1 in) deep, is capable of clogging water pipes. For example, it can be combined with Nyssa sylvatica or 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 full 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 scenes, in clear shade or in the sun.

The Common Alder is also a multipurpose tree, used in forests to prepare and improve the soil before planting poplars. Considered as 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 wood, which turns blood-orange after cutting and is almost rot-resistant in water, was once used for underground water conduits and channels. A large part of the city of Venice is said to have been built on alder wood pilings. Since the 1980s, there has been a deadly disease that causes alders to wither, transmitted by the fungus Phytophthora alni, which enters through the roots, especially when they are planted near slow-moving, warm watercourses, on loamy-clayey substrates, with little shade, close to technical structures that slow down water flow. Cutting back all affected trees to ground level is recommended. To prevent this, monoculture plantings along watercourses should be avoided.