A deciduous shrub, very variable in habit and flower, the largest forms 10 to 15 ft high and wide-spreading bushes of open growth; branchlets four-angled, downy. Leaves 4 to 12 in. long, 1 to 3 in. wide; lanceolate or linear-lanceolate, finely toothed, with very long, tapered points; dark green and soon becoming glabrous above, white-felted beneath; stalks very short. Flowers fragrant, arranged densely in short, rounded clusters on slender panicles 6 to 30 in. long, according to the vigour of the plant and the variety, and appearing from July to October, or later. Corolla varying in colour from lilac to purple, orange-yellow at the mouth, the tube 1⁄3 to 1⁄2 in. long, the limb 1⁄/6 to 1⁄3 in. wide, flower-stalks and calyx more or less felted. Seed-vessel smooth 1⁄4 to 1⁄3 in. long, cylindrical, pointed. Bot. Mag., t. 7609.
Native of Central and W. China up to 9,000 ft; discovered by the French missionary David in 1869; specimens collected later by Henry were described by Hemsley under the name B. variabilis, under which it was long known in gardens. It was first introduced from Russia, no doubt from seed collected by one of the Russian explorers, but this was an inferior form, being of comparatively weak, low, semi-prostrate habit, poor in colour of flower. A second and much superior form was introduced to France in 1893 and was raised in the Jardin des Plantes, Paris, and by the nursery firm of Vilmorin. This introduction had a more erect habit and flowers in denser and longer panicles; it resembled var. veitchiana, later introduced by Wilson. It is from Wilson's seed, collected in Hupeh and Szechwan during the years 1900-8, that the garden varieties of the present day are descended. The following botanical varieties have been distinguished:
var. alba Rehd. & Wils. – Flowers white, leaves narrow. It occurs wild with the type.
var. magnifica (Wils.) Rehd. & Wils. – Flowers violet-purple, the divisions of the corolla reflexed at the margins. Similar in habit to var. veitchiana, but later to flower. Introduced by Wilson from W. Hupeh, where it occurs by stream-sides up to 6,500 ft.
var. nanhoensis (Chittenden) Rehd. B. variabilis var. nanhoensis Chittenden – A dwarf variety usually not more than 3 to 5 ft high, more compact in habit than the type; leaves smaller, panicles shorter. Introduced by Farrer from Kansu in 1914.
var. veitchiana (Veitch) Rehd. B. variabilis var. veitchiana Veitch – A variety with arching branches; flowers in denser panicles than in the type and deeper-coloured, with a conspicuous orange eye. The earliest to bloom of Wilson's introductions from Hupeh.
var. wilsonii (Wils.) Rehd. & Wils. – Leaves long and tapering; flowers rosy lilac in lax panicles; corolla lobes more or less erect, with crinkled, reflexed margins.
Since 1945 many named clones have come into the trade in Britain, nearly all of them raised in the U.S.A. or on the continent, of which the following is a short selection:
'Black Knight'. – Very dark purple.
'Empire Blue', – Rich violet-blue with orange eye, upright growth.
'Fascinating'. – Broad panicles of a vivid lilac-pink. Usually known as 'Fascination'.
'Royal Red'. – Rich purple-red.
'White Profusion'. – White flowers followed by attractive russet-coloured seed heads.
There are two variegated forms in commerce – 'Variegated Royal Red' and ‘Harlequin’.
In its best forms B. davidii is one of the most effective of late summer-flowering shrubs, and a very popular one. Its flowers have a singular attraction for butterflies and this has increased its popularity. Possibly the way its seeds are carried off by the wind explains its existence in unexpected places. It is now thoroughly naturalised in waste places in the south of England and soon established itself on bombed sites in London after the last war.
The species in all its forms needs a good garden soil; the more robust its growth, the finer the flower-panicles. The previous year's growth should be pruned hard back every spring. Vigorous young plants treated in this way will make shoots 8 ft long in a season, terminated by panicles 2 to 21⁄2 ft long. Growth should not, however, be forced by heavy feeding.
From the Supplement (Vol. V)
Only one white-flowered selection was mentioned – 'White Profusion'. This is perhaps the best of those available, but 'Peace' runs it very close.
An interesting new introduction from Holland is B. davidii 'Nanho Purple', the result of a cross between B.d. var. nanhoensis and B.d. 'Royal Red', back-crossed with the latter. It is of lax, spreading habit, with willow-like, sea-green leaves mostly under 1 in. wide, and narrow trusses of violet-purple flowers with orange eyes (Dendroflora No. 17, p. 46 (1980)). It is very vigorous and free-flowering, but needs plenty of room.