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Owen Johnson (2021)
Johnson, O. (2021), 'Diabelia' from the website Trees and Shrubs Online (treesandshrubsonline.
A genus of about 4 species of deciduous shrub. Twigs hollow, not grooved; buds in opposite pairs, exposed, with several pairs of scales. Leaves short-stalked, lacking stipules. Flowers paired at the ends of short shoots and opening simultaneously, but sometimes grouped in 3s (or in clusters of up to 8) due to supernumerary flowers axillary to the bracteoles, especially on long shoots; paired flowers with 6 small bracts. Sepals 2–5, narrowly oblong to elliptic and persistent but seldom brightly coloured. Corolla 5-lobed, white, yellow, pink or red. Ovary 3-locular, 2 locules with sterile ovules, the third with a single fertile ovule. Stamens 4. Fruit an oblong, leathery, wind-dispersed achene, crowned with the persistent sepals which serve as wings. (Landrein & Farjon 2020).
Diabelia are in effect the Japanese equivalents of the Abelia species from mainland China, differing in their adaptations to a colder climate which include fully-deciduous foliage and flowers borne together in spring; one diagnostic peculiarity is their typically paired flowers (which should not be confused with paired flowers in some Abelia which open consecutively rather than simultaneously, or with the distinctive paired inflorescences, each reduced to a single flower, of the related Twin-flower, Linnaea borealis). The species were conventionally placed within the genus Abelia but genetic analysis has shown them to be slightly closer to the Chinese genera Dipelta and Kolkwitzia (Wang et al. 2015), and, to reflect this relationship, the genus Diabelia was proposed by Sven Landrein in 2010 (Landrein 2010); the new name is a slightly awkward combination of Clarke Abel’s (commemorated in Abelia) with the classical Greek prefix di-, indicating paired (the flowers). As none of these species are at all well known as garden plants in the west, this name-change should be a relatively painless one, though just for now such information as there is for these plants online is spread chaotically across names within Abelia, Diabelia and Linnaea. (See the introductions to Abelia and to Linnaea for a discussion of the ‘alternative’ nomenclature suggested by Maarten Christenhusz, and the reasons for preferring Landrein’s system here.)
Although the vernacular name ‘Twin Abelia’ has been suggested for the new genus by Landrein (Landrein 2010), ‘Japanese Abelia’ is proposed here, since all of the species recognised by Landrein and Farjon in their definitive monograph on this group (Landrein & Farjon 2020) are native to that country, although absent from the northern island of Hokkaido; Diabelia serrata (Sieb. & Zucc.) Landrein and D. stenophylla (Honda) Landrein both have small populations persisting from ice-age refugia in Zhenjiang in eastern China, while the range of D. spathulata (Sieb. & Zucc.) Landrein extends into South Korea. In southern and central Japan all four of the species recognised by Landrein and Farjon are abundant plants and may often grow together (Landrein & Farjon 2020), leading to hybrid populations which can be very hard to define within the constraints of Linnaean binomial taxa.
When understood to be members of Abelia, the Japanese Abelias were Cinderella plants, with pretty and colourful flowers and bright autumn colours in a continental climate (Dirr 2009; Garden.org 2021) but lacking the spectacular and long-lasting floral display of many of their Chinese cousins. It is possibly that their establishment within a genus of their own will focus more garden interest on the group. In particular, a variant with showy red flowers, accepted by Landrein and Farjon as Diabelia sanguinea (Makino) Landrein and found at higher altitudes in Japan’s more central prefectures, has apparently never been introduced to cultivation in the west.
|1a||Sepals 2, often dissected into 2–4 lobes; flowers often in clusters of 4–7:||D. serrata|
|1b||Sepals 5 (or 4 plus a reduced lobe); flowers always in pairs:||2|
|2a||Corolla tubular in the upper part, to 3.5 cm long:||D. stenophylla|
|2b||Corolla lobe bell-shaped in the upper part, less than 3 cm long:||3|
|3a||Corolla red, 1.1–2.2 cm long; sepals blunt-tipped:||D. sanguinea|
|3b||Corolla white or yellowish, 1.6–2.8 cm long; sepals pointed:||D. spathulata|