Hydrangea xanthoneura Diels

This large shrub or small tree closely resembles Hydrangea heteromalla, with its toothed, sharply pointed leaves, matt dark green above, and white or cream flowers in flat corymbs ringed by well developed sterile flowers. At present there is no botanical consensus on whether it should be treated as a species in its own right. We include it tentatively at species level to reflect the real uncertainties and incompletely understood variation patterns in this group, but to create an illusion of settled knowledge would serve nobody. The issue is discussed fully under H. heteromalla.

H. xanthoneura extends eastwards in China beyond Yunnan, Sichuan and Xizang where typical H. heteromalla is found. While Flora of China lists it for Guizhou and Hubei (Wei & Bartholomew 2001), Chinese botanists seem happy to identify it from as far north as Gansu, Shaanxi and Ningxia (Chinese Academy of Sciences 2024). The leaf undersides are pale green rather than grey or white, and lack hairs except on the veins: indumentum has always been a major identification feature for those who believe in splitting the heteromalla complex (Sargent 1913). Unlike H. bretschneideri, H. xanthoneura has a tree-forming reputation, and has conspicuous lenticels even on first year branchlets (Wei & Bartholomew 2001). Herbarium specimens and plant images identified as H. xanthoneura on the PlantPlus website tend to have narrower leaves than most identified as H. heteromalla, strongly acuminate apices and sharp, regular teeth (Chinese Academy of Sciences 2024). While the specific epithet refers to yellow veins on both leaf surfaces in the type specimen (Diels 1900), this does not seem to be diagnostic or even usual. In truth, while it is quite easy to point to ‘typical’ examples of each species, the dividing lines seem hazy. Several varieties in the complex, most described by Alfred Rehder (1912 and in Sargent 1913) and now largely ignored, shifted confusingly between species and have not always been sunk into the species in which they were first described (Royal Botanic Gardens, Kew 2021).

This species has probably been in Western gardens – never commonly – since the early 20th century. It is hard to trace details of introductions and their survival, partly due to conflicting taxonomic treatments from the beginning, and a period when it was normal to label the whole complex simply as H. heteromalla. It seems to have been introduced by the Veitch nursery around 1904 from an Ernest Wilson collection (Haworth-Booth 1984), number and location uncertain. Wilson’s sometimes rather chaotic records include for 1904 W 4899 & 4900 from Emei Shan, Sichuan, of herbarium specimens only, plus several subsequent collections (Sargent 1913).

Another surviving garden introduction probably linked to him is the plant first called H. heteromalla var. wilsonii Rehder, treated here as a cultivar ‘Wilsonii’ and listed under Hydrangea heteromalla Species Complex Cultivars.

Several late 20th–21st century introductions have been placed here. These include SICH 179 (N Sichuan, 1988) as grown at Benmore Botanic Garden, Scotland (Royal Botanic Garden Edinburgh 2021) and SICH 1653 (C Sichuan, 1995). NACPEC expeditions between 1994 and 2010 made seed collections under NACPEC, BJG and QLG numbers in the Beijing municipality, Shanxi and Shaanxi, which have sometimes been labelled H. xanthoneura, sometimes (perhaps more likely given the provenance) H. bretschneideri: without studying each it is impossible to express an opinion here. Some of these are grown at the Arnold Arboretum, US National Arboretum and other collections in Eastern North America (US National Arboretum 2024; Arnold Arboretum 2024). Two wild collections studied at the Yorkshire Arboretum and associated Ray Wood, UK, also conform well to H. xanthoneura: both are distinctive and very attractive plants (J. Grimshaw, J. Sutton pers. obs. 2024). One (previously unidentified) was collected under GUIZ 53 (Fanjingshn, Guizhou, China, 1985): it completely lacks pink or purple in the leaves, is glaucescent between the veins beneath, and self-seeds true to type in Ray Wood where no other member of the complex flowers at the same time. The second, from HNE 171 (collected N Vietnam as H. heteromalla) has striking red petioles and major veins, and is almost shiny between the veins beneath. A collection from a 2.5 m shrub on Leigongshan, Guizhou in 2011 (NJM 11.009) has been commercially available in the UK from PanGlobal Plants, who comment “Big creamy lacecaps are seen by the dozen in summer over narrowly ovate foliage, pale beneath and with red petioles. Hardy and tolerant of exposure and drought” (Pan-Global Plants 2023). However, a plant without provenance distributed commercially as this species in the 2010s by Cherry Tree Nursery, UK seems intermediate between typical H. xanthoneura and H. heteromalla with rather thick elliptic leaves, sharp, spreading teeth, and thick white hairs across the underside, of similar type to classic H. heteromalla but less dense and not woolly to the touch (pers. obs.). Hybridization in cultivation between what are anyway weakly separated species seems highly likely.