Spiraea betulifolia Pall.
A shrub to about 2 ft high, of rounded habit, with zig-zagged terete or slightly angled, reddish brown stems; buds ovoid, glabrous. Leaves pinnately veined, mostly elliptic or oblong-elliptic, rounded to cuneate at the base, or sometimes obovate and tapered to a narrow base, obtuse at the apex, crenately toothed or double-toothed in the upper part or throughout, glabrous on both sides or downy beneath; petiole to 1⁄4 in. long. Flowers white or rose, densely arranged in flat or slightly convex downy or glabrous corymbs up to 31⁄2 in. wide. Petals much shorter than the stamens. Follicles glabrous; sepals reflexed in the fruiting stage.
Native of N.E. Asia, south to Japan; introduced in 1812. The true species is rare in cultivation.
var. corymbosa (Raf.) Maxim. S. corymbosa Raf., not Muhl.; S. betulifolia sens. some authors, not Pall. – A shrub to about 3 ft high, with terete, sparsely branched stems. Leaves to about 3 in. long, coarsely and often doubly toothed in the upper part, glabrous, glaucous beneath. Flowers white, about 1⁄6 in. across in rounded corymbs 2 to 4 in. across. The chief difference from typical S. betulifolia is that the sepals are upright in fruit. Native of the eastern USA; introduced 1819. A handsome shrub, which, like typical S. betulifolia renews itself by stems pushed up from the base annually; these should be encouraged by pruning out the older wood. Both flower from about midsummer on the shoots of the year.
In western North America S. betulifolia is represented by var. lucida (Dougl.) C. L. Hitchc. (S. lucida Dougl. ex Greene; S. corymbosa var. lucida (Dougl.) Zab.), which is very near to the typical state of S. betulifolia but is more constantly glabrous.
S. densiflora Nutt. – A deciduous shrub up to 2 ft high, with glabrous, round, rich brown young shoots. Leaves oval or ovate, rounded at both ends, rather coarsely toothed towards the apex; 2⁄3 to 13⁄4 in. long, glabrous on both surfaces, deep green above, paler beneath; stalk 1⁄12 in. or less long. Flowers rose-coloured, densely packed in dome-shaped corymbs 1 to 11⁄2 in. wide, opening in June.
Native of western N. America, from British Columbia to Oregon. It was no doubt gathered by early collectors like Douglas and Lobb, and, as it was growing in Veitch’s nursery at Exeter in 1861, was probably introduced by the latter. It is useful as a very hardy, low-growing shrub that will keep dwarf without pruning, and its rose-coloured flowers are pretty and attractive.
S. densiflora is very near to S. betulifolia var. lucida, differing in its rosy flowers. The true species is still in commerce, but some plants in the trade as S. densiflora are a dwarf form of S. japonica (see its cv. ‘Nana’).
var. splendens (Baumann) Abrams S. splendens Baumann ex K. Koch – Stems and inflorescence-axes finely downy. This extends into California. Described from plants distributed by Baumann’s nursery and introduced to Britain shortly before 1883.
S. virginiana Britt. – Near to S. betulifolia var. corymbosa but with the leaves entire or almost so, oblong-lanceolate, glaucous beneath, 1⁄2 in. or slightly more wide. Native of the south-eastern USA.