The alders are deciduous trees and shrubs closely allied to, and only likely to be confounded with, the birches (Betula). Leaves with stipules, alternate, more or less toothed in all the cultivated species. Winter buds nearly always stalked. Male and female flowers borne on the same tree but on separate catkins. Male catkins long and slender, usually in clusters of two to six; the flowers small, with a four-lobed calyx, no petals, and usually four (sometimes one to three) stamens. Female catkins shorter, clustered, or rarely solitary, developing into woody, cone-like “fruits”, correctly, ‘strobiles’, 1⁄3 – 11⁄4 in. long. The “seed” (the true fruit) is a minute, flattened nutlet, often with thin membranous wings at the sides. Apart from three species – A. maritima, A. nepalensis, and A. nitida – which flower in autumn, the cultivated alders form their catkins in the late summer and autumn; these expand the following spring, either very early before the leaf-buds begin to grow, or along with the leaves; the fruits develop during the summer and persist until the succeeding spring. From the alders the birches are distinguished by the fruits being longer, not woody, and falling to pieces (those of the alders falling whole), and the flowers of birches have never more than two stamens.
In gardens and parks the alders are chiefly valuable for growing in wet situations unsuited to the majority of trees. Some, however, such as A. japonica, nitida, cordata, and firma, succeed quite well in ordinary good soil. All are best propagated by seed except the garden varieties, which may be grafted on their respective types, or, better still, rooted from cuttings made as soon as the leaves fall, and put in sandy soil, as willow or poplar cuttings are – compared with which, however, they do not strike root so readily. The following is a selection of the best worth growing, irrespective of their use in damp places: cordata, firma, nitida, rubra; glutinosa ‘Imperialis’; incana ‘Laciniata’ and ‘Aurea’.
From the Supplement (Vol. V)
The American species are revised by John F. Furlow in Rhodora, Vol. 81, no. 825, pp. 1-121 and no. 826, pp. 151-248 (1979). See also the article by the same author in International Dendrology Society Year Book 1981, pp. 115-19.