A tree of small or medium size, occasionally 70 ft or more high; bark of trunk white, peeling off in papery layers, eventually dark and rugged at the base; young shoots downy, not warted. Leaves broadly ovate, 11⁄2 to 21⁄2 in. long, 1 to 2 in. wide; usually tapered, sometimes rounded or slightly heart-shaped at the base; pointed, coarsely toothed; upper surface thinly downy at first; lower one downy on the midrib and veins, sometimes only in the vein-axils, sometimes over the whole surface; veins in five to seven pairs; stalk more or less downy, 1⁄3 to 3⁄4 in. long. Fruiting catkins about 1 in. long; lobes of scale minutely downy, side ones rounded, terminal one ovate.
Native of Europe (including Britain) and N. Asia, and one of the two birches (the other is B. pendula) which make up the B. alba of Linnaeus. This is not so attractive a tree for the garden as B. pendula; its bark is darker, and its branching being more erect, it lacks the graceful, pendulous habit of that species. It affects moister places than B. pendula, and is especially abundant in Highland glens. Easily distinguished from typical B. pendula by its downy, not warted twigs.
It is a more variable species than B. pendula and in Flora Europaea is divided into three subspecies, of which two for certain occur in the British Isles:
subsp. pubescens. – This is the typical downy birch, which is usually treelike in habit, with downy young growths; leaves more than 11⁄4 in. long; wings of fruit as half as wide again as the nutlet.
subsp. carpatica (Willd.) Aschers. & Graebn. – Of more shrubby habit; young growths soon glabrous; leaves usually less than 11⁄4 in. long; wings of fruit as wide as the nutlet. Birches found in Scotland (referred to subsp. odorata in Flora of the British Isles) apparently belong here. Their young twigs and leaves are covered with resinous warts when young and are aromatic at that time; buds viscid. To this subspecies the Flora Europaea also refers var. murithii (Gaudin) Gremli, which is a dwarf tree up to 10 or 15 ft high, or a shrub, found in a few localities in Switzerland. It was discovered near Mauvoisin, Val de Bagne (Valais), and Dr Christ described it as a very pretty little tree. Var. pontica (Watson) Bean appears also to fall under this subspecies.
B. pubescens differs from B. pendula in having twice the number of chromosomes of that species (2n = 56). This difference restricts inter-breeding between the two species, and the first-generation crosses, when they occur, are sterile. The intermediates usually have the same chromosome number as B. pubescens, and must be hybrid swarms of complex origin (Flora Europaea),
The downy birch produces its seeds very freely, and is, as a rule, one of the first trees to find its way back to deforested areas. Like the grey birch in N. America, it is sometimes useful in affording shelter for young timber trees of better class. The wood is of very little value in this country except for turnery and pulping. In Scandinavia, where it is used for veneers, the occasional tree is found in which the wood is beautifully grained and commands a high price. A fragrant oil is obtained from it which is used in the manufacture of russia leather. The bark is water-tight, and is used in the construction of roofs in Sweden, etc. Under certain conditions it is curiously indestructible. I have seen pieces unearthed during peat-cutting in the Highlands, which must have been buried some centuries, but were still quite silvery.
The following are the most important garden varieties:
cv. ‘Aurea’. – Leaves yellow when young; shoots very downy,
cv. ‘Crenata Nana’. – A dwarf, round bush growing at the rate of 2 or 3 in. annually.
cv. ‘Urticifolia’. – A small tree whose leaves have a drawn-out apex, and are sharply double-toothed, very dull green, densely downy above when young. Fruiting catkins up to 11⁄2 in. long, and more slender than in B. pubescens itself. Loddiges’ nursery 1836. Hylander considers that this clone may be B. pendula × pubescens and that it was probably of garden origin. Similar nettle-leaved birches have, however, been reported from the wild. The Loddiges clone was later described under the name B. virgultosa Fries.
B. turkestanica Litvin. – This is one of the numerous allies of B. pubescens found in the mountains of Central Asia. The young branchlets bear resinous glands as well as down; in the fruiting catkins the lobes of the scales are only about half as long as the central one (about the same length in B. pubescens).