Hydrangea × ytiensis
Putative / unconfirmed parentage
(Hydrangea davidii × Hydrangea aff. hwangii)
Y Tý hydrangea
Synonyms
- × Didrangea ytiensis
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Introduction
Y Tý hydrangea, Hydrangea × ytiensis (J.M.H.Shaw) J.M.H.Shaw, is a putative natural intersectional hybrid discovered near Y Tý in northern Vietnam. It is known principally from the wild-origin Crûg Farm collection BSWJ 11790, made in 2006 from a solitary purple-leaved shrub about 1.5 m tall at approximately 1900 m. The plant was initially identified as Hydrangea lingii, but subsequent cultivation revealed a combination of characters inconsistent with that species.
The plant was named in 2014 as × Didrangea ytiensis, reflecting the then-common interpretation of its parents as belonging to different genera. When the bluebead hydrangeas were incorporated into broad Hydrangea, Shaw recombined it as Hydrangea × ytiensis in 2021.
Its parentage is putative and requires unusually careful wording.
Working hybrid formula: Hydrangea davidii × Hydrangea aff. hwangii
The first parent, H. davidii, belongs to Section Hirtae. The proposed second parent is a bluebead hydrangea from Section Dichroa that resembles H. hwangii but has not been securely identified as that species. The abbreviation aff. therefore matters biologically: it signifies a plant showing affinity with H. hwangii, not a confirmed H. hwangii individual.
This wording also resolves a nomenclatural complication in the older literature. Shaw originally described the putative bluebead parent as Dichroa aff. yunnanensis S.M.Hwang. Hwang's Dichroa yunnanensis could not retain the epithet yunnanensis when transferred into Hydrangea, because that name was already occupied; it is now called Hydrangea hwangii. Trees and Shrubs Online consequently renders the modern hypothesis as H. davidii × H. aff. hwangii.
POWO currently simplifies the formula to H. davidii × H. yunnanensis, but that presentation obscures both the replacement-name issue and, more importantly, Shaw's original aff. qualification. Until the bluebead parent is demonstrated genetically or tied securely to a species concept, the more cautious H. davidii × Hydrangea aff. hwangii formulation better reflects the evidence.
The evidence for hybrid origin is morphological and reproductive rather than genomic. Most flowers resemble ordinary fertile flowers, but some develop slightly enlarged calyx lobes in the positions where a conventional lacecap hydrangea might produce showy marginal flowers. Even more unusual are stamens with widened, partly petaloid filaments and connectives. Pollen grains appear to abort, suggesting impaired fertility.
The vegetative morphology is also intermediate or anomalous: the leaves are thick, glossy, elliptic, finely toothed, and bear sparse stiff hairs above. The original wild plant was reported as purple-leaved, although cultivated material at Crûg Farm does not retain that coloration strongly under lower light.
The hybrid hypothesis is therefore persuasive, but H. × ytiensis remains explicitly provisional as a biological interpretation. No genomic admixture test has yet demonstrated ancestry from either proposed parent. It is possible that future data will confirm the formula, modify the identity of the bluebead parent, or reveal a still more complex origin.
Assuming the present hypothesis is correct, H. × ytiensis represents natural gene flow between Section Hirtae and Section Dichroa—two of the most morphologically divergent shrub lineages within Subgenus Dichroa.
Horticultural and Agricultural Uses
Y Tý hydrangea is primarily a specialist collector's plant. The original BSWJ 11790 accession is cultivated at Crûg Farm, where its unusual flowers, thick foliage, and presumed hybrid origin make it scientifically and horticulturally interesting. It is described as rather tender and experimental in cultivation.
Its floral abnormalities could be particularly informative developmentally. Petaloid stamens and partially enlarged calyx lobes suggest disrupted or intermediate expression of floral developmental programs inherited from two substantially different parental lineages.
The plant is therefore potentially more valuable as research material than as a conventional ornamental. Maintaining clonal material of BSWJ 11790 also ensures that the original named hybrid remains available should genomic methods later be applied to test the proposed parentage.
Conservation Concerns
Hydrangea × ytiensis should not presently be treated as a separately evolving threatened species. It is known principally from one wild-origin collection, and available evidence suggests a hybrid individual rather than a stabilized population-level lineage.
The natural site is nevertheless scientifically important. Conservation of the surrounding Y Tý forests preserves the parental populations and ecological circumstances capable of producing unusual intersectional hybrids. The area also lies within a botanically rich montane region where several poorly resolved continental Asian hydrangeas approach one another.
The living BSWJ 11790 accession has additional value as a documented reference specimen. Because the type itself was prepared from the cultivated plant at Crûg Farm in 2009, continued preservation of the clone provides unusually direct continuity between the original field discovery, nomenclatural type, and material available for future genomic study.
Additional Information
References and Further Reading
Samain MS (2021) Hydrangea grows bigger. The Plant Review 3(4): 52–56.
Shaw JMH (2014) Hybrids between Dichroa and Hydrangea. The Plantsman, New Series 13(1): 16–21. https://www.rhs.org.uk/about-us/pdfs/publications/the-plant-review/general-pdfs/index-for-2014 (opens in a new tab)
Sutton J (2025) Hydrangea × ytiensis. Trees and Shrubs Online. International Dendrology Society. https://www.treesandshrubsonline.org/articles/hydrangea/hydrangea-x-ytiensis/ (opens in a new tab)