Hydrangea hydrangeoides (Siebold & Zucc.) Bernd Schulz

Primarily a Japanese plant, Hydrangea hydrangeoides is a deciduous, moderately vigorous, shade tolerant climbing species. Summer flowering, the sterile flowers ringing each inflorescence have just one conspicuous enlarged sepal each, usually white but variably pink-tinged in some cultivars. Still better known under its synonym Schizophragma hydrangeoides, it has been confused in gardens with the much commoner H. petiolaris which has a strikingly similar wild range and often grows alongside it, though rarely colonizing the same tree (Kato et al. 2014).

H. hydrangeoides belongs to Section Schizophragma: all species in this Section share its climbing habit, deciduous foliage, enlarged fertile flowers with free petals, and single (rarely 2–3) sepal per sterile flower (H. crassa, not described here, lacks sterile flowers). It can be distinguished from other schizophragmas by the roughly serrate leaves (versus entire, finely serrate or serrulate), as well as being the only species native to Japan (Liu & Zhu 2011). The only other widely grown species, H. ampla, usually has strikingly larger bracts. The common but much more distantly related H. petiolaris can look rather similar in leaf, but that species has about 4 sepals per sterile flower, a very different ‘look’.

When open, the fertile flowers are dominated by the spreading white stamens, much longer than the inconspicuous petals which may or may not fall before the stamens. It probably has a generalist pollination strategy; certainly it attracts a range of insect groups in cultivation (pers. obs.). In some atypical cultivated plants the petals are fused to form a cap which falls early, as in Sections Pileostegia and Calyptranthe (Hufford 2001). Peak flowering in our area is usually in July. While the plant grows well in part or even full shade, flowering is best in full sun, especially in areas with cooler summers (Edwards & Marshall 2019; Bluebell Arboretum and Nursery 2023). It often flowers best at the top of its support where light is brightest.

The growth form of H. hydrangeoides has been well studied in wild Japanese populations and serves as a model for understanding other climbing hydrangeas. It is a root climber, attached to trees (or rocks) by aerial roots. Seedlings on the forest floor develop branching horizontal stems rooting in the soil or litter layer as they go. Erect side branches grow usually from just one of the paired buds at nodes (Kato et al. 2014), and may go on to colonize vertical surfaces they meet. While the shortest shoots grow towards light, longer ones grow towards darker areas, presumably balancing the requirement for photosynthesis with the need to find the shadowy bases of tree trunks (Kato et al. 2012). Once on the tree, extension or ‘searcher’ shoots rapidly colonize branches, attaching by aerial roots; in subsequent years much shorter, non-rooting side or ‘ordinary’ shoots grow out from this framework, away from the branch. Leaf shape can be confusingly different on the two types of shoot (Shaw 2012), and different again in the leaves immediately beneath an inflorescence. The bulk of leaf area is contributed by the side shoots (Ichihashi, Nagashima & Tateno 2009), and only side shoots flower. Extension shoots are unable to colonize small branchlets, restricting the climber to relatively shady inner parts of the tree canopy (Ichihashi & Tateno 2011) and making spread to neighbouring trees almost impossible (Mori et al. 2021). All lianas have to make a trade off between the benefits of a large canopy for capturing light and the harm this can do to the host tree on whose survival its place in the forest depends. Limited to inner parts of the tree canopy and unable to escape to neighbours, H. hydrangeoides is forced into a ‘stay smaller, live longer’ strategy. Indeed, the canopy of established wild individuals seems to remain the same size over decades (Ichihashi & Tateno 2011). All this makes it an unthreatening climber for large, established trees in gardens and arboreta.

Like H. viburnoides, H. hydrangeoides has a reputation for being slow to establish (Edwards & Marshall 2019; Dirr 2021). This is probably due in part to planting erect, pot-grown specimens whose branchlets resemble the short side shoots of an established plant, rather than the rooting extension shoots. Bleddyn Wynn-Jones’ general advice on establishing climbing hydrangeas applies 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 colonize the wall (pers. comm. 2023).

This species was first described in 1838 from material collected by Philipp von Siebold in Japan; the tautological epithet hydrangeoides makes sense only because it was first described in Schizophragma. It is widely understood that Charles Maries, collecting in Hokkaido for the Veitch Nursery in 1877, introduced seed (Veitch 1906; Barnes 2001). However, many (perhaps all) plants under the name in late 19th century gardens on both sides of the Atlantic turned out to be H. petiolaris (Bean 1913; Anderson 1933). At this remove the confusion in the plants and hence in the written records of them seems intractable (Nevling 1964). Its first definite flowering in Britain seems to have been in 1905, with several plants flowering well in 1912, following a hot summer (Bean 1913). In today’s climate flowers can be expected every year in southern Britain. More recent Japanese collections in botanic gardens are too many to list, and show interesting leaf variation (Macdonald 1990), though none apart from ‘Moonlight’ (below) stand out especially. UBC Botanical Garden, Vancouver, and RBG Edinburgh have particularly large numbers, although many of the Edinburgh plants are not in public areas (University of British Columbia 2023; Royal Botanic Garden Edinburgh 2023). Commercial collections include Crûg Farm’s BSWJ 5489 and 5732 (Kinki district, Honshu), the latter with handsome red petioles and deep marginal teeth.

H. hydrangeoides is quite widely hardy in Europe, as far north as southern Scandinavia where it has been grown for many years in Gothenburg (Gothenburg Botanical Garden 2023). In southern Finland it is killed back to ground level by cold winters, and is most successful at Arboretum Mustila when covered by snow, making it best suited for ground cover (Arboretum Mustila 2023), something it will do well if vertical surfaces are absent. In North America it is well represented in serious collections and increasingly in general horticulture (especially the cultivar ‘Moonlight’) away from the very coldest central areas and arid climates.

This is a variable species, and attempts to fit variation into a taxonomic framework have not been entirely successful: indeed, Section Schizophragma in general remains poorly understood (Shaw 2012). One approach has been to pick out distinctive characteristics within Japanese material and describe formae (Ohba 2001, raised less satisfactorily to varieties by Liu & Zhu 2011). Apart from f. concolor (see below), these names have been published only in Schizophragma and include S. h. f. formosum (Nakai) Sugim. (flowers mostly sterile, apparently not in Western cultivation) and S. h. f. molle (Honda) H.Hara ex H.Ohba (leaves pubescent beneath). Another approach is geographical, keeping most Japanese populations in the type var. hydrangeoides and raising varities for distinctive island populations on Yakushima (var. yakushimensis), Jeju-do (var. taquetii) and Ulleung-do (var. ullungdoensis) – see below. More work is clearly needed on these island populations. Plants on the Kuril Islands have scarcely been investigated, although an example from Kurashir in the south of the archipelago is recorded at UBC Botanical Garden (University of British Columbia 2023).

Numbers of variegated forms have been picked out, mainly in Japan. Apart from ‘Shiro-fuka-fukurin-fu’ listed below, diverse other forms barely distributed in the West include yellow, white and yellow-green variegations, variously splashed, edged and sectored. Cédric Basset, France, has a good collection of these; for examples and names (sometimes perhaps tentative) see Pépinière AOBA (2023), Morand (2016) and (for a stimulating range of images) Basset (2023).