Hydrangea indochinensis Merr.

Nearly everything about this taxon is debatable. Those plants we have in gardens which best fit the concept are tender, tricksy evergreens with lanceolate leaves, sometimes purple-tinted especially on the leaf undersides, with white sterile and pigmented fertile flowers.

The following facts are not controversial: i) H. indochinensis was described by Merrill (1942) from a single specimen (Pételot 2687) collected two years previously in a humid ravine at around 2,200 m asl on Fan Si Pan, N Vietnam. ii) The type specimen, in fruit, is well preserved and viewable on the Arnold Arboretum’s website (Arnold Arboretum 2024). It shows a more or less inferior ovary, long styles and glabrous leaves with revolute margins which hide the small teeth, as per the description above. iii) The inferior ovary, long styles and glabrous leaves all link H. indochinensis to H. stylosa, although its leaves are narrower. McClintock (1957) sank the species entirely in H. stylosa; this is followed by Flora of China (Wei & Bartholomew 2001) and Plants of the Wrold Online (Royal Botanic Gardens Kew 2024), but many contemporary plant collectors use the name for a somewhat varied selection of (mainly Vietnamese) hydrangeas. iv) Two molecular studies place what we must cautiously call a plant identified as H. indochinensis as sister to H. stylosa (De Smet et al. 2015; Yang et al. 2024). v) Botanists seem never to have recognized H. indochinensis in China, and H. stylosa only in NW Yunnan and the Tibet Autonomous region (Wei & Bartholomew 2001; Chinese Academy of Sciences 2024).

The variation within and differences between these taxa, along with H. kwangsiensis and H. zhewanensis (see discussion under H. stylosa), is simply too poorly understood to be able to name unambiguously any of the numerous recent collections from N Vietnam made by Crûg Farm, Dan Hinkley, Ozzie Johnson, Scott McMahan and others. In some, there are even hints of hybridity with a member of Section Dichroa (B. Wynn-Jones, pers. comm. 2023; pers. obs. 2023). In theory, H. davidii (itself a dubious taxon) ought to be distinguishable by its half superior ovary, but in practice it can be very hard to draw the line between it and this group. For example, cuttings taken from two members of a single colony in N Vietnam (KWJ 12233 a&b) are currently listed under H. indochinensis (a) and H. davidii (b) (Crûg Farm Plants 2024). Clone b had rather long styles and more or less inferior ovaries when examined (pers. obs. at Crûg Farm 2023).

Plants which seem to fit the H. indochinensis concept best behave as evergreens and have glossy, narrow leaves; in Vietnam they tend to favour forest margins in half shade (T. Hudson pers. comm. 2024). They often (but not always) have purple-tinted leaves, especially beneath. Such collections have proved near-impossible to establish at mild, sheltered Tregrehan, Cornwall (T. Hudson, pers. comm. 2024) and in Athens, Georgia for Michael Dirr (2021). A plant from KWJ 12233b has beaten the odds in Alan Shamash’s mild, sheltered garden in SW Scotland to form a lovely small shrub (A. Shamash, T. Christian pers. comms 2024). Even in North Island, New Zealand they prove tender and troublesome, needing a warm, sheltered spot (Woodleigh Nursery 2023; Jury 2012). Those which prove hardier perhaps represent other taxa. But at this stage, who really knows?