Alnus incana

The Grey Alder or Alnus incana is a deciduous tree of medium size that owes its name to its grey-white and smooth bark, which is very decorative. Its dark green, ovate and pointed leaves, with a grey-white underside, are characteristic. The pendulous male catkins appear well before the foliage, illuminating the garden in late winter. This deciduous tree has a conical to ovoid habit, with an irregular crown, and has rapid growth and easily regenerates from the stump, often having multiple trunks. As a very hardy pioneer species, the mountain alder is a natural coloniser of alpine valley troughs. It is not very demanding and highly versatile, thriving in wet soil but tolerating dry soil, unlike the common alder. However, it prefers a sunny location and fairly calcareous soil. It is often planted for landscaping purposes, to populate barren land, reforest unstable soil such as spoil heaps or riverbanks, or as a windbreak in flat areas.

Alnus incana is a deciduous tree or large bush from the Betulaceae family, native to temperate regions of the northern hemisphere. It can withstand temperatures as low as -36°C. It is found in the wild along rivers, wetlands, and riparian forests in the Jura, the Alps, and the Rhine valley, up to an altitude of 1,800 m. Thanks to its flexibility and ability to produce new shoots, it is one of the few trees capable of withstanding avalanches. It is a fast-growing pioneer species, capable of forming dense stands in poor, calcareous soils. It has an annual growth of 35 cm (13.8 in) in height and 20 cm (7.9 in) in width. Its roots are rather shallow (less than 60 cm (23.6 in) deep) and less buried than those of the common alder, allowing it to withstand very windy regions and establish firmly even in very unstable, sloping soils. Also known as mountain alder, it can reach a maximum height of 20 m with a 12-m spread, and generally has ascending branches and an irregular conical crown. Its 10 cm (3.9 in) leaves are distinguished from those of the common alder by their sharp ovate shape, their light underside that captures light, and their doubly indented edges. The lower surface of the youngest leaves is covered in hairs. The pendulous 7-9 cm (2.8-3.5 in) long male catkins appear in a yellow-brown colour ahead of the foliage from February to April. The female flowers, in the form of dark red spikes, are much more discreet and located higher up on the branches. The fruits are woody ovoid cones called strobiles, with almost absent stalks, which remain on the tree year after year. They release narrow-winged nuts that are dispersed by the wind and provide an important food source for birdlife.

The Grey Alder is a tree that is not demanding in terms of soil but prefers a sunny location and moist, calcareous soil. It can tolerate temporary flooding and drier soil, but the common alder is the preferred variety for constantly waterlogged soil. When cut back to the ground, it quickly forms low shoots. It is particularly interesting as a solitary tree, in a line, in groves, and on slopes or riverbanks to retain soil, serve as a windbreak, or to reforest difficult areas in larger gardens. It can be combined with limestone-loving species such as holm oak or hornbeam, or in wetter soils with ash trees and basket willows. Functional and ecological hedges can be created by combining it with fruit species such as elderberry.

The mountain alder is widely planted to provide vegetation cover over poor and unstable mountain soil as well as more artificial landscapes such as spoil heaps. Indeed, its root system forms nodules containing nitrogen-fixing bacteria, thus enriching the soil with this nutrient. It is also often found in windbreak hedges in flat areas.

Alnus incana comes from the Latin word "incanus" which means "covered with grey-white hairs", referring to the hairy underside of the young leaves. The leaves are used as a diuretic and vermifuge. The bark is astringent and tonic.