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Alasdair and Panny Laing
Tom Clark (2023)
Clark, T. (2023), 'Enkianthus' from the website Trees and Shrubs Online (treesandshrubsonline.
Shrubs or small trees, deciduous, rarely evergreen. Leaves clustered at ends of twigs, petiolate; leaf blade serrate or subentire. Inflorescence terminal, in umbels or corymbose racemes, flowers rarely solitary or in pairs, 5-merous. Corolla broadly campanulate to urceolate; lobes short. Stamens much shorter than corolla; filaments flattened, distinctly dilated towards base; anthers oblong, thecae each dehiscing by an elongate slit, awned at apex; pollen grains single. Ovary superior, with few ovules per locule; stigma truncate. Capsule loculicidal, ovoid. Seeds several or one; testa lamellate-winged. (Ruizheng & Stevens 2005).
The small genus Enkianthus, as currently understood, comprises 13 species (Plants of the World Online 2022) and is yet another exclusively Asian genus that brings much to gardens throughout many parts of the temperate world. The species, along with a modest number of cultivars, combine refined characteristics of flower, foliage and form with an accommodating nature in cultivation to provide the gardener with a fine assemblage of typically deciduous (rarely evergreen) shrubs and small trees. Hybrids seem to be quite frequent, but are poorly known.
The distribution of Enkianthus is primarily in Japan and China, but a few far flung species expand the range westward to the eastern Himalayas and southward into Vietnam and Laos (Clark, Hsu & Camelbeke 2011). Their habit tends to be upright in youth but broadening with age, especially when growing in shadier sites. Their branching is often distinctively whorled and generally neither particularly coarse nor overly congested, giving them, even in winter, a rather refined and tidy look. The growing season finds the branches clad in petiolate, finely serrate to entire, alternately arranged leaves that, like their branching, often occur in tufted whorls clustered at the ends of branches. On vigorous shoots, however, the leaves are typically well-spaced rather than in whorls, with readily apparent internodes, at least in their first year. Flowers occur in two- to many-flowered terminal umbels or racemes but singly in the appropriately named Enkianthus pauciflorus. Individual flowers are broadly campanulate or short tubular to urceolate ranging in colour from white to yellowish to reddish, often with darker striations and shading. Upon pollination and during the seed capsule’s development in several species, the section of pedicel closest to the developing fruit makes an abrupt turn, reorientating the capsule more or less 180° from a nodding position to an upright one. The resulting upright chalice-like dehiscent capsule ensures more effective wind-dispersal of the seeds. Interestingly, poorly formed capsules (presumably those resulting from flowers not adequately pollinated) do not undergo this change in orientation or only minimally so. This is a feature shared by other ericaceous genera such as Oxydendrum and Zenobia among others.
Derived from the Greek enkyos (pregnant) and anthos (flower), the Portuguese Jesuit missionary and naturalist João de Louriero established Enkianthus as a new genus along with its type specimen, Enkianthus quinqueflorus, in his seminal 1790 work Flora Cochinchinensis. His rather sparse Latin description of the type specimen includes reference to “a leafy flower, beautifully red” referencing the richly colored bracts that he fancifully thought appeared pregnant and “containing five flowers within” (Loureiro 1790). Extensive plant exploration during the mid and late 19th century brought to the attention of the scientific community an abundance of material from across eastern Asia resulting in a considerable number of new taxa now circumscribed within our current understanding of Enkianthus. At the time, however, many were described not as Enkianthus but either assigned to existing genera such as Andromeda, Rhododendron or Zenobia or, in other instances, entirely new genera such as Meisteria, Tritomodon and Melidora were established to accommodate the new taxa. It would be the better part of a century before taxonomic thinking would begin to coalesce around what would become our current understanding of Enkianthus.
Beyond various classifications at the generic level, several infrageneric classifications have been made, the first of which was Hooker’s informal classification of the genus into two sections followed by Makino’s informal recognition of two subgenera (Makino 1894). In 1899, Palibin’s revision divided the genus into four sections: Enkianthus, Andromedina, Meisteria, and Enkiantella. Ueno’s 1950 assessment assigned the species to three subgenera: Euenkianthus, Andromedina, and Tritomodon (Ueno 1950); and in 1982 Hsu revisited a sectional approach distributing the species into sections Enkianthus, Racemus, and Monanthus (Hsu 1982). The most recent comprehensive Flora of Japan takes a slightly different tack presenting four sections: Enkianthella, Meisteria, Andromedina, and a new section, Perulati (Iwatsuki 1993). Finally, a 1994 cladistic analysis of the genus (Anderberg 1994) brought more clarity to the taxonomic situation and reasserted Palibin’s four-section classification along with codifying the type specimens for each section which was an oversight of Palibin’s work. The sections noted in the species’ accounts are based on Anderberg’s 1994 revision.
Various cladistic and phylogenetic studies have supported placing Enkianthus in a basal position within the Ericaceae s.l. (Judd & Kron 1993). Furthermore, molecular cladistic analyses and morphological studies of several primitive characteristics including those related to pollen, stamens and seed have substantiated Enkianthus as being phylogenetically sister to all other ericaceous genera (Kron & Chase 1993, Kron et al. 2002). A study of Enkianthus pollen morphology and its taxonomic significance also includes a dichotomous key to most species based on pollen (Golam Sarwar & Takahashi 2006).
These taxonomic issues, of course, have little bearing on the substantial garden value of the species and their cultivars. Delightfully nodding flowers occur in spring before or after the leaves emerge depending on the species; the brilliant red, orange and yellow autumn foliage of many species can be spectacular and provides far greater visual impact in the garden, especially when contrasted with the variously hued and textured foliage of conifers and broad-leaved evergreens such as rhododendrons with which they grow so well. As with other members of the heath family, Enkianthus prefer an acidic, well-aerated, free-draining soil in a sunny to partly shaded position. Although they can tolerate a certain degree of drought once established, they strongly prefer a soil that remains evenly moist (Clark, Hsu & Camelbeke 2011). The fibrous root system typical of the genus allows them to be readily and successfully transplanted as container-grown or field-grown specimens. They tend to be shallow-rooted so cultivation around them should be kept to a minimum and a layer of organic mulch should be maintained, especially where drought is a concern.
In Europe, cultivation is most often met with success in northern and western areas, in gardens where moderate climates and acidic soils prevail. In North America, their adaptability in cultivation is broadly restricted to the eastern and northeastern United States, adjacent eastern provinces of Canada, and to a western strip of the Pacific states north of San Francisco extending north into western British Columbia. In other parts of Europe and North America successful cultivation is often precluded by higher soil pH and unfavourable soil chemistry, summer heat, seasonally dry conditions and low annual precipitation, extreme winter low temperatures, or a combination of these factors. No data was found nor any observations made by the author suggesting that any species of Enkianthus has invasive or aggressive tendencies outside their native habitat. Insect pests and diseases are few and occur only sporadically, though problems with scale have been reported.
The propagation of species is most effectively and efficiently carried out by seed, though beware of hybrids among the seedlings. Seed of E. campanulatus, E. cernuus, and E. deflexus germinate readily without pre-treatment, though seed of E. perulatus apparently presents some challenges possibly related to dormancy issues (Young & Young 1992). Although commonly encountered in gardens, the difficulty in germinating E. perulatus may acocunt, in part, for its scarcity in the nursery trade. Seeds of Enkianthus are small and, similar to many other ericaceous plants, are best surface sown on a well-aerated, free draining germinating mix; milled sphagnum moss is an excellent medium. Seed pots should be placed under mist, in a closed germination chamber or a similar enclosed environment. Provided with moderate warmth (18–25°C/65–75°F), ideally in the form of gentle bottom heat, seeds will germinate in two to three weeks (Young & Young 1992). Although very small, seedlings will grow quickly especially if provided with supplemental light and regular fertilising with a dilute liquid feed suited to acid-loving plants. Cultivars and forms with particularly desirtable characteristics should be propagated asexually. Enkianthus can be layered but when more than a small number of plants is desired softwood cuttings root quite easily. Leafy cuttings taken in mid-June are treated with talc or quick-dips of IBA at 5000 to 8000 ppm, and rooted under mist. Rooted cuttings should be allowed to harden-off and overwinter before they are disturbed. An extended photoperiod is helpful in ensuring overwintering success (Hartmann et al. 2018).
The author and editors are grateful to Roderick White for reviewing and commenting on the draft text for Enkianthus.