Alnus incana (L.) Moench
Synonyms: Betula alnus var. incana L.
A tree 60 to 70 ft high, with a trunk occasionally 6 ft. in girth; young shoots covered with a short, grey down. Leaves ovate, oval, or occasionally obovate, rounded or wedge-shaped at the base, and with short, abrupt points; 2 to 4 in. long, 11⁄4 to 21⁄4 in. wide; the margins with six or more coarse teeth about the middle, these again being sharply toothed, the base entire; upper surface dull green covered with flattened down when young, lower surface grey with a close down, at least when young; veins in nine to twelve pairs; stalk 1⁄2 to 7⁄8 in. long, covered with minute down. Male catkins 2 to 4 in. long, usually three or four in a cluster, opening in February. Fruits ovoid, numerous, and rather densely clustered, 1⁄2 to 5⁄8 in. long.
Native of Europe and the Caucasus, not of Britain, but introduced in 1780. This alder is an exceptionally hardy tree, and useful for planting in cold, wet places. With the exception of A. glutinosa, it is the commonest of alders, but is more frequently represented in gardens by the various cut-leaved and coloured forms than by the type. From A. glutinosa in all its forms it is most obviously distinguished by the grey downy leaves and young shoots. The typical A. glutinosa is, of course, very distinct in the obovate, round-ended leaves, green and almost glabrous beneath.
The American form of A. incana, long known as A. incana var. americana, is now placed under A. rugosa (q.v.).
f. angustissima Holmberg – Leaves cut to midrib or near it; lobes very narrow, usually toothed at the base and prolonged at the apex into thread-like points. This remarkable form has been found wild in several places in Scandinavia and is in cultivation in Sweden. It is illustrated in Hylander’s monograph (for reference see A. glutinosa) in plates XXII to XXVI.
cv. ‘Aurea’. – Leaves yellowish, downy beneath; young wood reddish yellow and remaining so through the winter; young catkins orange. ‘Ramulis Coccineis’ (f. coccinea Callier) is similar, but has the leaves almost glabrous beneath; branchlets and buds a good red.
cv. ‘Laciniata’. – The handsomest of the cultivated cut-leaf alders, the blade being pinnately divided into six or eight pairs of narrow, lanceolate, toothed lobes, reaching two-thirds or more of the way to the midrib. Similar forms are found wild in Scandinavia and are referred by Hylander to f. laciniata (Loud.) Hylander. The nomenclature of the garden clone is very confused, but A. incana laciniata is the old horticultural name for it and the one under which it was given a first-class certificate in 1873; it should therefore be accepted as the clonal name. Botanical names under which it has been placed are: var. acuminata Reg.; var. incisa Bean (in previous editions); var. pinnatifida Dipp.; f. acutiloba (K. Koch) Hallier.
f. orbicularis Callier – Leaves round-oval, under 2 in. in length; veins in about five pairs. Described from a plant found wild in Silesia.
cv. ‘Pendula’. – Branches weeping. Van der Bom’s nursery, Holland, before 1903. Originally distributed as A. i. pendula nova.
cv. ‘Pinnata’. – Leaves small, with blunt, toothed lobes which sometimes reach to the midrib. According to Hylander, the original plant described by Lundmark in 1790 (as Betula pinnata) was found at Lesjöfors in Värmland; it was propagated in the Uppsala Botanic Garden around 1800 and is the parent of all the cultivated trees. Similar forms have been found elsewhere in Scandinavia and are referable botanically to f. pinnata (Lundmark) Willd. (syn. var. incisa Dipp.).
From the Supplement (Vol. V)
Regarded in a wide sense, this species is circumpolar in distribution. See further below under A. rugosa and A. tenuifolia.
The grey alder has attained 92 × 41⁄4 ft at Castle Milk, Dumfries (1984). It was planted in 1928.
cv. ‘Aurea’. – The most decorative feature of the golden grey alder is the brightly coloured young male catkins, which start to redden even before the leaves have fallen.