The name has preserved the Latin origin. The genus includes medium-sized trees, deciduous, which vegetate, except for a few species, in very humid soils. On the roots are nodules containing bacteria that have the ability to fix the nitrogen contained in the atmosphere, which is then absorbed by the host plant.
Cultivated species of Alnus:
four species live in the spontaneous state: AlnusAlnus cordata; A. viridis; Alnus glutinosa and A.incanta. Alnus cordata, Neapolitan alder, among the species of our country is the largest since it often reaches 25 m in height. It is a very beautiful and ornamental tree that finds optimal conditions of climate in the transition area between the range of oaks and that dominated by beech, where it is widespread. Among all the species of Italian alders, it is the least demanding in terms of soil moisture. It is a very effective plant to protect the soil from erosion and to improve its fertility, thanks to the powerful root system well developed, to the leaves that form, falling to the ground, a litter abundant and full of fertilizing elements, to the root tubercles, home of nitrogen-fixing bacteria. Today this species is much less widespread than in the past. This is due to the action of man, who has eliminated the Neapolitan alder from large areas, either directly, with the cut, to obtain wood or to make room for agriculture, either indirectly, with the practice of excessive pastures or fires. The residual natural forests must, therefore, be well protected, and in all areas of its area, ecologically suitable to meet their needs, the species must be reintroduced, also valuable for the rapid growth of which it gives proof. The wood is characteristic because, as soon as it is cut, it is tinged with red-orange and then it becomes darker. Alnus viridis, green alder: it is a bush and, sometimes, a tree that reaches a height generally not higher than 3 m. Unlike the previous species, it has a very vast range including a part of Europe, North Asia including Japan, Alaska, Canada, the United States of the North and Greenland. Here it colonizes the most ungrateful grounds and prepares suitable conditions for the establishment of more demanding species, such as, for example, spruce; it bears very well the low temperatures of the high mountain. Alnus glutinosa, black alder: it is a tree that can sometimes reach 20-25 meters in height, but generally does not exceed 8-10 m. It is the most widespread species of alder in our country, where it is present everywhere, including the islands, but limited to areas that are very humid; as a result, it is mostly found near watercourses, lakes, and swamps. Widespread in almost all of Europe, in some areas of North Africa and Asia Minor, it is the most used plant, at least in the past, for the manufacture of clogs, since it has a wood that is very suitable for these uses. Alnus incana, white alder, similar in size to those of the preceding species, also has a vast range. In fact, it lives in most of Europe, in Siberia and in North America. In Italy, it lives in the Alps and in the Pre-Alps up to 1700-1800 meters above sea level and further south it reaches just the Tuscan-Tuscan Apennines. It is a species suitable for strengthening landslides, thanks to the robust root system that goes very deep: Furthermore, it improves the soil, making it suitable to accommodate more demanding forest species, such as beech, white fir, and spruce. The wood of alders is of little duration in the air, but it is very resistant when it is used entirely submerged in water. This applies especially to black alders, and it should have been well known in the past because the piles used for the buildings of Venice and Ravenna were exclusively black alder.
as already mentioned, alders want a decent soil moisture; they can grow even in chalky soils, but benefit if they grow in conditions of good fertility. They can be propagated both by seed and cuttings, taken naturally, after the fall of the leaves. Artificial varieties will be diffused through grafts or cuttings. Both the sowing and the burial of cuttings must be performed in the spring.
Also read: Allamanda (Apocynaceae).