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Owen Johnson (2021)
Johnson, O. (2021), 'Dipelta' from the website Trees and Shrubs Online (treesandshrubsonline.
A genus of 3 or 4 species of vigorous, deciduous shrubs to 6 m tall, rarely arborescent. Twigs longitudinally ridged; older bark greyish to rich brown, flaking in strips. Buds exposed, in opposite pairs, with several pairs of more or less glabrous scales. Leaves ovate to lanceolate, 2–13 × 1–7 cm, margin entire or with a few distant teeth near the tip; stipules absent; petiole short. Inflorescence of single or paired fragrant flowers in panicles on short shoots and year-old growths in late spring; peduncle 10–25 mm long with 2 lanceolate bracts near the middle. Ovaries with 4 bracts (episepals) at the base, 2 of these large and persisting to form dry winglike appendages to the fruit, 15–25 mm wide; ovary 4-locular, 2 locules each with a single fertile ovule. Sepals 5, linear to lanceolate, often fused in the lower part. Corolla 2–4 cm long, bell-shaped with two lips, distinctly narrowed at the base; upper lip 2-lobed, lower lip 3-lobed; white to purple with bold yellow or orange markings. Stamens 4, included. Style often slighly exserted. Fruit in autumn a 2-seeded achene, enclosed in the 2 papery episepals; seeds globose (Flora of China 2021; Landrein & Farjon 2020).
With their winged fruit, which resemble the samarae of elms, and their brightly-marked and scented late spring flowers which recall individual foxglove bells or the blooms of some kind of Catalpa, Dipelta form a distinct group which, ever since its orginal description by Karl Maximowicz in 1877, was universally treated as a genus; the classical Greek name means ‘two shields’ and refers to the shape of the seeds’ two major bracts. Phylogenetic research since 1998 has revealed these distinctive features to be of relatively recent evolution and has shown Dipelta to be nested among a group of closely related plants traditionally placed within the genus Abelia; to make sense of this position, the nomenclature adopted for Trees and Shrubs Online conserves the familiar genus Dipelta while the remainder of Abelia has been split into separate genera following Landrein et al. (2012). We have elected to follow this approach over an alternative system, proposed by Maarten Christenhusz (Christenhusz 2013), to unite all of these plants (plus Kolkwitzia) within a single large and diverse genus, for which Linnaea is the first available name: see the generic introductions to Abelia and to Linnaea for further discussion of this issue. The advantages of the former scheme are perhaps most obvious in the case of Dipelta, in which the three species familiar to western gardeners, and whose shared lineage is obvious, need no change of name. Very few growers in the UK have begun to use names in Linnaea – for example Linnaea floribunda instead of Dipelta floribunda – though in the United States, influenced perhaps by the adoption of Christenhusz’s nomenclature in Wikipedia, references to these plants are now split between the rival treatments.
In the wild, in western China, Dipelta tend to be more abundant and hardier plants than the corresponding Abelia species; they grow larger, are fully deciduous, and prefer more wooded habitats at higher elevations (Landrein & Farjon 2020); they are often grown around temples, and this may have influenced their apparent natural distribution (Landrein & Farjon 2020). Like so many plants from this part of China Dipelta were first introduced to cultivation in the west by Ernest Wilson, and three taxa, D. floribunda Maxim., D. ventricosa Hemsl. and D. yunnanensis Franch., are about equally well – or poorly – known as garden shrubs, at least in the UK and Ireland; the plants of D. yunnanensis now in cultivation here probably derive from later collections by George Forrest and others (Landrein & Farjon 2020). D. ventricosa is considered synonymous with D. yunnanensis by many authorities, include the authors of Flora and China and by Z.L. Liu et al. in their phylogenetic study of the genus (Liu et al. 2013), but it is maintained as a distinct species by Landrein and Farjon in their monograph on the Linnaeeae (Landrein & Farjon 2020).
The fourth species, Dipelta elegans, is endemic to a small area in north Sichuan and is assessed as Endangered by Landrein and Farjon; seed was probably collected by Reginald Farrer in 1917 but this dainty and graceful species has never become established in cultivation (Landrein & Farjon 2020). (The better documented seed collection by Farrer and William Purdom in the same year was in fact from D. floribunda (Landrein & Farjon 2020).) D. elegans differs from D. floribunda in its fused, cupular sepals, its very hairy style and its fruit with wings more than 2 cm wide. D. wenxianensis, described by Y.F. Wang & Y.S. Lian in 1994, is sometimes held to be another species, while Liu’s analyses have suggested that it originated as a hybrid between D. floribunda and D. yunnanensis (Liu et al. 2013); it is treated as D. floribunda var. wenxianensis (Y.F. Wang & Y.S. Lian) Landrein by Landrein and Farjon, and does not seem to be in cultivation.
As large and vigorous shrubs with a single season of interest, Dipelta have never become really popular garden plants. Their flowers are perhaps most interesting when viewed in detail, and for the rest of the year, without fruit at least, they are slightly dull, anonymous, Philadelphus-like bushes, though the bark peeling in long strips can provide winter interest (Dirr 2009). They can be trained into the shape of a small tree (Stewart 2010), though when pruning it should be remembered that the genus flowers on the last season’s wood. All the cultivated forms are quite hardy and easy to grow in any good garden soil; most are easily propagated from half-ripe cuttings (Bean 1976).
|1a||Episepals (the fruit's 2 wings) making a circle, attached centrally; calyx divided to the base:||D. floribunda|
|1b||Episepals kidney-shaped, attached at the margin; calyx divided no more than 2 thirds of the way to the base:||2|
|2a||Corolla bulging slightly at the base:||D. yunnanensis|
|2b||Corolla inflated and bulging markedly at the base:||D. ventricosa|