Kindly sponsored by a member of the International Dendrology Society.
John Grimshaw (2016)
Grimshaw, J. (2016), 'Stewartia' from the website Trees and Shrubs Online (treesandshrubsonline.
Stewartia is a genus of about twenty small to medium-sized deciduous or evergreen trees, sometimes with attractive bark, of which two occur in the eastern United States and the rest in Asia. Winter buds are laterally compressed. The leaves are alternate, sometimes with a winged petiole, with an entire, ovate to elliptic lamina with a finely toothed margin and a pointed apex. The flowers are axillary, either solitary or with a few in a short raceme, often nodding below the foliage, opening from rounded buds comprising two bracteoles and five sepals; the corolla is five-lobed, with the lobes joined at the base, often concave, pure- to creamy-white, silky-hairy externally and ciliate at the apex; stamens are numerous, joined at their base into a small tube that is fused to the corolla; anthers yellow or occasionally purple; ovary superior, with five locules, but with one or five styles. The fruit is a somewhat woody ovoid to conic capsule splitting into five sections, each containing a few winged or unwinged angular seeds. (Spongberg 1974; Min & Bartholomew 2007; Prince 2009).
The first species of this attractive genus to be recognised was the south-eastern American S. malacodendron, introduced to Britain by John Clayton, probably from Virginia, and flowered in 1742 in the garden of Mark Catesby, the famous author of The Natural History of Carolina. A year or so later it was portrayed by the great botanical artist Ehret from a plant growing in the garden of the young John Stuart, Earl of Bute, at Caenwood House, London (now Kenwood House). The Ehret portrait was sent to Linnaeus by Isaac Lawson, with a dedication to the Earl in which his family name was mis-spelt ‘Stewart’, and it was from this portrait, and from a Clayton specimen passed on to him by Gronovius that Linnaeus described the genus in 1746 (Bean 1981). Lawson is credited as the author of the generic name, though it was published within the binomial system by Linnaeus in Species Plantarum in 1753. This unintentional mis-spelling was emended to ‘Stuartia’ by L’Héritier in 1785, and the generic name was almost universally so spelt throughout the 19th century (Spongberg 1974, Bean 1981) – and indeed was used well into the twentieth century by horticultural authors such as Bean. The Botanical Code, however, requires adherence to the original spelling, so the ‘incorrect’ Stewartia is the form to be used. By the 1980s the editors of ‘Bean’ were well behind the curve in persisting to use Stuartia, and have undoubtedly caused much confusion over the spelling.
Closely related to Camellia, and sharing much of its distribution in Asia, Stewartia can be divided informally into evergreen and deciduous species. Formerly known as Hartia Dunn, the evergreen species cannot be maintained as a distinct genus (Prince 2002, Li & Del Tredici 2002, Xiang et al. 2004) and these studies are unable to support any system of subdivisions of the genus. The evergreen species, of which there are twelve in China, are mostly unknown in cultivation. They tend to be from lower altitudes and latitudes than the deciduous taxa and can be assumed to be less hardy. Their flowers are typically smaller and less showy than those of the deciduous species, and it seems that they do not have good bark, but the glossy foliage can be attractive.
With its many appealing characters – a good shape of tree, with often beautiful bark, the exquisite silky flowers borne through the summer, and often good autumn colour – Stewartia is justifiably popular in horticulture and has many aficionados, including members of a Facebook group. As a genus it can be grown in a wide range of climates, but the south-eastern American species prefer hot summers, while some of the Asian species are more tolerant of cooler maritime conditions in northern Europe. All require fertile acidic soil for best performance, with some shade in particularly hot places; the classic woodland garden with ‘high canopy’ is often ideal. In her notes on the genus, Polly Hill, founder of the eponymous arboretum on Martha’s Vineyard, says that to see stewartias at their best they should have plenty of moisture before flowering time. Cultivation and propagation techniques have recently been reviewed by Hsu, Boland & Camelbeke (2008), and Crock (2009) and there is a useful review of the deciduous species by Spongberg (1974). Hsu, Boland & Camelbeke (2008) note that there is a propensity to hybridise within the genus.
The following key, to the deciduous species, is from Spongberg (1974), adapted by Hsu, Boland & Camelbeke (2008).
|1a||Styles 5, distinct; petioles widely winged, enclosing the lateral and terminal buds; floral bract 1 or 2 (the latter soon falling off)||S. ovata|
|1b||Styles united, terminating in 5 or 6 stigmatic crests or arms; petioles narrowly winged, not enclosing the lateral and terminal buds; floral bracts 2||2|
|2a||Stamens with purplish filaments and bluish anthers; capsules dehiscent by the outward folding of the valve margins, the apices of the valves ± coherent; seeds angular||S. malacodendron|
|2b||Stamens with whitish filaments and yellow or orange anthers; apsules apically dehiscent, the valves spreading apart from the apex; seeds planoconvex||3|
|3a||Floral bracts conspicuously shorter than the calyx; small trees with smooth mottled bark; young branches usually compressed and zigzagged, rarely terete||S. pseudocamellia|
|3b||Floral bracts about equalling or longer than the calyx; small or large trees or shrubs with smooth or fissured bark; young branches usually terete, not zigzagged||4|
|4a||Ovaries/capsules subglobose, completely glabrous or pubescent only at the very base||5|
|4b||Ovaries/capsules conical, pilose or appressed-pubescent over the entire surface||6|
|5a||Ovaries/capsules pubescent only at the very base; sepals twisted, 4 ovules/seeds per locule; bark on older branches finely fissured||S. rostrata|
|5b||Ovaries/capsules completely glabrous; sepals flat, 2 ovule/seeds per locule; bark on older branches smooth and mottled||S. serrata|
|6a||Sepals oblong or ovate with acute apices||7|
|6b||Sepals ovoid with rounded, ciliate apices||S. × henryae|
|7a||Floral bracts ovate, subequal to the sepals; styles 68 mm long; seeds 7–9 mm long||S. sinensis|
|7b||Floral bracts oblong, conspicuously longer than the sepals; styles 3–4 mm; seeds 5–6 mm long||S. monadelpha|