Ceratostigma willmottianum Stapf

A garden classic in all but the coldest parts of our area, C. willmottianum is grown for its copious bright blue flowers in rather dense heads, opening successively for three or four months in summer and autumn. The leaves often develop attractive red tints towards the end of the flowering season. Subshrubby in nature, it dies back more decisively in cold areas, behaving in effect as a herbaceous perennial, and even when aerial shoots survive many gardeners encourage vigorous new growth by cutting it back to near ground level in spring. Annual growth approaching 1 m in height is possible with a warm growing season and adequate moisture, but in gardens it rarely approaches the 2 m recorded in the wild. A sunny position is ideal; poor, dry or chalky soils are tolerated (Bean 1976). Beyond our area, it makes an excellent garden plant even in East Africa, where it grows best in light shade (Cameron 2017).

C. willmottianum belongs to a trio of rather similar western Chinese species, with C. griffithii and C. minus. C. griffithii has a more westerly range in Bhutan and southern Xizang, usually smaller, obovate leaves (closer to diamond-shaped in willmottianum), with denser hairs on stems and both leaf surfaces. It is also more shrubby than C. willmottianum, although it too will grow back freely from the base and flower when cut back annually (Havlis 2021). At least in the usual garden form, C. willmottianum is less branched, with most flowers in terminal inflorescences. C. minus is less clearly differentiated; while the older literature distinguishes it from the other two by the combination of shrubby habit and a more or less glabrous upper leaf surface, Flora of China (Peng & Kamelin 1996) allows hairier forms in its circumscription and there is ongoing uncertainty about the identification of at least one collection (see discussion under C. minus).

C. willmottianum was well established in Western cultivation by the time it was described. It was first collected as seed for the Arnold Arboretum by Ernest Wilson, in the semiarid Min River valley, W Sichuan (W. 1373 & 2314 of 1908; Wilson in Sargent 1916). Two seedlings were raised by Ellen Willmott at Warley Place, Essex, UK from seed sent by the Arnold (presumably a share of the original gathering), and from these the species was described (Stapf 1914). The specific epithet commemorates Miss Willmott, whose once fabulous garden is now a delightful ruin.

Fast-growing and easily propagated by semi-ripe cuttings in summer, it seems likely that Wilson’s introductions dominate cultivated stock, perhaps even as a single clone in Europe. Later introductions from Sichuan attributed to C. willmottianum include H&M 2103 (grown at Quarryhill Botanical Garden, California). SICH 59 of 1988, grown at RBG Edinburgh, Howick Hall, and Quarryhill has been variously interpreted as C. willmottianum and C. minus (Royal Botanic Garden Edinburgh 2021; Quarryhill Botanical Garden 2021) but as seen on the Edinburgh rock garden is lower and more dense than typical C. willmottianum of British gardens (J. Grimshaw pers. comm. 2021).

This is a heterostylous species, with long- and short-styled morphs, coupled with a degree of self-incompatibility (Gao et al. 2021). Long-styled plants seem to be the norm in British cultivation (pers. obs.), perhaps reinforcing the suggestion of a single dominant clone, but further investigation is required. In a Min River valley population, Gao et al. (2021) found that the corolla limb (and hence the ‘face’ of the flower) was on average larger in short-styled plants, something to be considered by plant breeders. Single clones seem to set little seed: at the start of his breeding programme – see cultivars below, Peter Catt (pers. comm. 2021) sowed ‘5 or 6 boxes’ of uncleaned seed heads, raising just 5 seedlings, but intercrossing these yielded far more seed.