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Julian Sutton (2022)
Sutton, J. (2022), 'Chionanthus' from the website Trees and Shrubs Online (treesandshrubsonline.
A genus of about 140 species of shrubs and trees native primarily to tropical and subtropical Asia, Africa, the Americas and Australasia, with a few species extending to temperate China and North America. Most are evergreen; a few temperate-zone species (including all those described here) are deciduous. Leaves opposite or subopposite, simple. Flowers usually hermaphrodite (the functionally dioecious C. virginicus and androdioecious C. retusus are notable exceptions), in axillary or rarely terminal inflorescences. Calyx small,with 4 teeth or lobes. Corolla white or yellow, with 4 lobes, more or less free, or with a short tube. Stamens 2; style short; stigma entire or bifid. Fruit a drupe, with hard endocarp and usually a single seed. (Chang, Qiu & Green 1996; Royal Botanic Gardens, Kew 2022).
The genus Chionanthus as cultivated in our area comprises just three species bearing a clear resemblance to one another, and all described here. They are deciduous shrubs and small trees with showy inflorescences of narrow-petalled white flowers in spring or summer and (in female or hermaphrodite plants) attractive, rather olive-like dark blue drupes. All flower and fruit best in relatively continental climates. The type species, North American C. virginicus, is by far the longest- and best-known to gardeners, especially in Eastern North America, and other species are discussed in relation to it. The rare Florida endemic C. pygmaeus is a smaller shrub, potentially very showy in the garden, while the East Asian C. retusus differs in its inflorescences being terminal on new shoots rather than axillary on last year’s growth.
This simple picture belies real taxonomic difficulties around the scope of the genus, resulting not so much from confusion as from honest uncertainty. A core group of temperate zone species precisely matches the gardener’s concept of Chionanthus, and molecular studies confirm their close relationship (Hong-Wa & Besnard 2013; Dupin et al. 2020). C. virginicus is the type species, and it seems likely that however the scope of the genus may change, these species willl remain in Chionanthus. Linnaeus (1753) published the generic name suggested by his friend the Dutch botanist Adriaan van Royen, from the Greek chion (snow) and anthos (flower); C. virginicus was already known to Dutch gardeners as Sneeuwboom (Snow Tree) (Stearn 1976). Over time it became clear that Chionanthus could not easily be separated on morphological grounds from Linociera, a far larger, pantropical, mostly evergreen genus; Stearn (1976) formally united them. The much enlarged genus is currently widely accepted (e.g. Royal Botanic Gardens, Kew 2022), if only for the want of a more natural classification of this part of the family Oleaceae.
Chionanthus belongs to Subtribe Oleinae, along with horticulturally relevant genera such as Cartrema, Forestiera, Nestegis, Olea, Osmanthus, Phillyrea and Picconia. A swathe of molecular studies (notably Hong-Wa & Besnard 2013; Dupin et al. 2020, 2022) strongly suggest that many of these genera are far from natural; in particular Olea, Osmanthus, Nestegis and wide-sense Chionanthus are polyphyletic, Chionanthus badly so. However, attempts to set up a workable system of more natural genera are still in their infancy.
We know of no former Linociera species growing unprotected in our area. Most would be too tender, although a handful of southern Chinese species (Chang, Qiu & Green 1996) might be tried in areas such as the American Southeast, given availability and inclination. The deciduous C. pubescens Kunth (N Peru, S Ecuador) is sometimes planted as a street tree in tropical South America, for its copious, intensely pink flowers on naked branchlets. Breeders have their sights on it as a possible parent of hardier pink-flowered hybrids (Dirr & Warren 2019).
All species described here suit a reasonably moist, fertile soil in sun or light shade, with warm summers. Hardiness varies, with C. virginicus most cold-tolerant, and C. pygmaeus probably least. Significant pests and diseases are few, although they show some susceptibility to scale insects and leaf borers, in C. virginicus especially on dry sites (Missouri Botanical Garden 2022), and C. virginicus has been found with Ash Dieback (Hymenoscyphus fraxineus) in the UK (Forest Research 2022). Deer (and cattle, given the chance) will browse young shoots of C. virginicus, and plants may not survive if as little as one third of the annual growth is taken (Gill & Pogges 1974): Jones (2020) thinks it worth fencing a young tree where this may be a problem.
Propagation has always been a problem in Chionanthus. Seed shows double dormancy in the two more familiar species, while C. pygmaeus has more intractable issues. On the smaller scale, layering in autumn or spring can be effective (Huxley, Griffiths & Levy 1992; Bean 1976). For issues around cuttings and grafting see individual species.