Hydrangea viburnoides (Hook.f. & Thomson) Y.De Smet & Granados

Hydrangea viburnoides is an evergreen climbing species, attractive and fairly vigorous when well suited and properly established. Walls, tree trunks or rock outcrops in shade or sun are colonized by fast-growing extension shoots which cling by aerial roots. In subsequent years shorter side shoots grow outwards from this framework; their glossy green leaves and flowers are what is most obvious in a mature plant. The inflorescences of small, creamy white fertile flowers in late summer (peaking August or September in Britain) can make a good show even on shaded walls. Quite well known in Western Europe (where it is still usually labelled Pileostegia viburnoides), it is uncommon in North America.

One of just two species in Section Pileostegia, H. viburnoides can be distinguished from almost all other climbing hydrangeas in Western gardens by a set of characters: it is fully evergreen, the petals fall away as the flowers open, and the inflorescences lack the large, deciduous involucral bracts and hence ‘drumstick’ buds of H. seemannii and its relatives in Section Cornidia. Hydrangea obtusifolia (Section Decumaria) shares most of these characters, but is a less vigorous climber, with smaller, proportionately broader leaves and smaller inflorescences, whose petals fall away free rather than united. One other species in Section Pileostegia, H. tomentella (q.v.) has a much narrower distribution in China and is probably not in cultivation, although the name is sometimes attached to the unidentified Vietnamese collection FMWJ 13444. H. tomentella differs in its branchlets, leaves and inflorescences being densely covered in rusty-brown stellate hairs, and in its oblong to obovate-oblong leaves with shallowly cordate to rounded bases (Huang & Bartholomew 2001; Liu & Zhu 2011).

Entirely lacking sterile flowers, H. viburnoides looks at first glance less ‘hydrangea-like’ than most other climbing species. Moreover, because the petals fall early, the open flowers are dominated by their creamy stamens. Overall, the inflorescence looks superficially rather Sambucus-like. Pollination studies are lacking, but a generalist strategy seems likely. Flowers in British gardens seem very attractive to bees, flies, and especially wasps (pers. obs.), as well as hornets (Beth Chatto’s Plants and Gardens 2023). Along with H. petiolaris and H. anomala (Section Calyptranthe), H. viburnoides is very unusual in the way the deciduous petals fuse after they have been formed to make a cap, the calyptra, which pops off as a unit when the stamens expand (Hufford 2001). This feature (most uncommon among flowering plants in general) seems to have evolved independently in the two groups. Its evolution has not been adequately explained, beyond the observation that selection for petal characters might be relaxed in species with tight inflorescences, where the inflorescence as a whole takes on the role of attracting pollinators. In the garden, the lack of sterile flowers makes for rather unattractive inflorescences after flowering, with little more than bunches of blackened, wiry peduncles and pedicels persisting into late autumn.

In well grown plants the leaves are strikingly glossy and dark green above. In stressed or weakly growing plants they may look paler and chlorotic; good nitrogen availability seems important for this species to look its best (Architectural Plants 2023; Cardunculus 2015). Most plants in Western cultivation have quite narrow oblanceolate leaves, although the Plantplus website includes some images of rather broader leaved forms in the wild (Chinese Academy of Sciences 2023). Leaves of young extension shoots tend to be smaller than those of flowering side shoots (pers. obs.). Plants from Hainan with sparsely stellate-hairy branchlets, leaves, and inflorescences have sometimes been called Pileostegia viburnoides var. glabrescens (C.C.Yang) S.M.Hwang (Huang & Bartholomew 2001); a combination in Hydrangea has not been published.

H. viburnoides was first described scientifically from the northwestern end of its range, in the Khasi Hills of northern India (Hooker & Thomson 1857). It was first introduced to Western gardens from China in 1908, by Ernest Wilson collecting for the Arnold Arboretum, from W Sichuan (W 1385); most garden material probably derives from this (Sargent 1913; Bean 1981). Exactly how it reached Europe is unclear, although the Arnold frequently exchanged material with botanic gardens, serious amateurs and commercial growers during this period, and sometimes ensured the future of less hardy introductions by sending them to Europe. It probably first flowered in Britain in 1914 when the Hertfordshire politician and plantsman Vicary Gibbs exhibited a flowering shoot at a Royal Horticultural Society show (Bean 1981). It was described at the time as ‘a bushy shrub somewhat resembling Skimmia japonica [vegetatively]’ (Anon. 1915), suggesting that it had not yet attached itself to a surface. Slow establishment is a common complaint with this species. By 1922 French nurseryman Léon Chenault (a regular recipient of material from the Arnold) was able to show off a specimen covering the front of his Orléans house (Edwards & Marshall 2019). Later collections include C.N. Page 10167A (Taiwan 1976, as grown under glass at RBG Edinburgh – Royal Botanic Garden Edinburgh 2023) and several commercial collections from N Taiwan by Crûg Farm Plants, BSWJ 3565 & 3570 of 1996 and BSWJ 7132 of 1999 (Shaw 2012). Other Crûg collections of Section Pileostegia from N Vietnam are less surely attributable to species.

As a species which can climb from the forest floor to the tree canopy, H. viburnoides suits both shaded and sunny sites. Every edition of the Hillier Manual (most recently Edwards & Marshall 2019) has gone so far as to call it ‘one of the best climbers for any aspect’. Unlike some climbing hydrangeas, it seems to flower as well on a north wall as it does on sunnier sides of a building (pers. obs.). Cold, strong winds may scorch the leaves on exposed sites, although the shoots survive (Bluebell Arboretum and Nursery 2023; Burncoose Nurseries 2023). Fertile soil is generally recommended. Weakly growing plants may be further hampered by competition from other plants (Cardunculus 2015), but the glossy leaves of strongly growing specimens make a lovely foil for other wall plants which are colourful outside its own flowering season; Thomas (1992) found it ‘a good contrast to red-berried pyracanthas’. The garden literature is full of comments on its slowness to establish. This is probably due in part to planting erect, pot-grown specimens whose branchlets are behaving like the short, flowering side-shoots of an established plant, rather than the rooting extension shoots. Bleddyn Wynn-Jones’ advice on establishing climbing hydrangeas seems particularly apposite here. He recommends pegging down the shoots to encourage buds to break and produce horizontal extension shoots which grow along the soil; they or their branches soon find their way onto the wall (pers. comm. 2023). Architects and landscape architects are currently paying attention to the engineering benefits of evergreen climbers, with phrases like ‘green facades’ and ‘vertical greening solutions’ appearing in the literature. In a study using small, brick-built model buildings, Thomsit-Ireland et al. (2020) found that H. viburnoides provided some thermal buffering, but far less than Ivy (Hedera helix).

This is close to being a mainstream garden plant in Western Europe, its garden range limited by the cold winters of Northern and Central Europe. It is most uncommon in North America, even in areas where it should prove hardy. On the Pacific seaboard it has grown well at University of British Columbia Botanical Garden, Vancouver, since 1981, where it was at least initially one of very few evergreen climbers in their extensive collection of East Asian woodland plants (Macdonald 1990; University of British Columbia 2023). It is also recorded from the more Mediterranean climate of San Francisco Botanical Garden (Quarryhill Botanical Garden 2003). There are a few records from serious Southeastern collections (JC Raulston Arboretum 2021); Dirr (2021) mentions a plant in Atlanta, GA, which flowered only once in 20 years, questioning its value in that climate.