Hydrangea ampla Megalocarpa Form — Giant-bract hydrangea vine

Introduction

The Megalocarpa Form represents the central-Sichuan climbing hydrangea historically described as Schizophragma megalocarpum Chun and currently accepted by some authorities as the species Hydrangea megalocarpa. Tree TSAR instead treats it as a distinctive Form within H. ampla. This preserves a recognizable regional morphology and a long taxonomic history while acknowledging that current evidence has not demonstrated the evolutionary independence required for formal species rank.

The historical concept is geographically concentrated in central Sichuan. Flora of China records S. megalocarpum from Baoxing and Tianquan, where it was reported from valley forests at about 600 m. Its principal diagnostic characters include broad ovate leaves about 11–15 cm long, relatively conspicuous pubescence along the veins beneath—especially on either side of the midvein—and unusually large obconical capsules approximately 8–12 mm long with a narrowed base. Typical H. ampla was distinguished in the same treatment by generally less pubescent foliage and smaller, more campanulate or turbinate capsules approximately 6–8 mm long with a broader base (Wei & Bartholomew 2001).

The distinction is therefore real at the level of traditional morphology. The unresolved question is whether it marks an independently evolving lineage. Liu & Zhu's later revision of Schizophragma, which incorporated both conventional morphology and leaf-epidermal characters, did not recognize S. megalocarpum separately from their broad S. integrifolium concept (Liu & Zhu 2011). Sutton (2025) emphasizes that this was a focused revision rather than a general checklist and notes that it otherwise retained meaningful variation where the authors judged it taxonomically defensible.

The name nevertheless returned to formal species usage when the combination Hydrangea megalocarpa (Chun) J.M.H.Shaw was published in 2021, and Plants of the World Online currently accepts it as a central-Sichuan species (Samain 2021; Royal Botanic Gardens, Kew 2026). Tree TSAR's different treatment reflects an evidentiary judgment rather than a nomenclatural problem: the name is valid and usable, but the biological case for treating its bearer as a species has not yet been tested by the kind of geographically explicit nuclear or population-genomic evidence that could distinguish localized variation from lineage independence.

Accordingly, the Megalocarpa Form should be understood as part of the variation currently included within Hydrangea ampla. Its separate page exists because the combination of central-Sichuan geography, consistently cited large-fruited morphology, historical species recognition, continuing acceptance by major databases, and persistence of the name in horticulture makes the entity too important to reduce to an unexplained synonym. If future genomic work demonstrates a discrete central-Sichuan lineage corresponding to these morphological characters, formal rank should be reconsidered. If no such structure exists, the Form can continue to describe a useful regional phenotype without taxonomic overstatement.

The working common name giant-bract hydrangea vine is retained for continuity with the current Tree TSAR roster. It is worth noting, however, that the historical diagnosis of megalocarpum emphasizes large fruit and leaf indument more strongly than uniquely enlarged sterile sepals; this common name may therefore merit separate review during final nomenclatural editing.

Horticultural and Agricultural Uses

The megalocarpa concept has persisted in horticulture and among wild-origin collections even during periods when taxonomists have treated it within H. ampla. Sutton (2025) notes that the name continues to appear on more recent Sichuan material. Some cultivated accessions have also been labelled aff. megalocarpa, reflecting uncertainty about whether their morphology falls within the historical central-Sichuan concept.

This continued horticultural use is one reason a Tree TSAR Form page is valuable. A plant received as Hydrangea megalocarpa need not be relabelled simply as undifferentiated H. ampla with its history erased. Instead, the botanical names Schizophragma megalocarpum and Hydrangea megalocarpa can remain associated with the Megalocarpa Form while the page explains why Tree TSAR currently considers that entity subordinate to H. ampla.

Cultivated accessions are also potentially important research material, provided their wild provenance is secure. Collections from Baoxing, Tianquan, and adjacent central Sichuan should be prioritized for comparison with typical H. ampla. Horticultural collections can preserve populations no longer easily accessible in the field, but only well-documented accessions should be used to test the original taxonomic concept.

Conservation Concerns

The Megalocarpa Form has no separate conservation status under the Tree TSAR treatment because it is not regarded as a formally independent taxon. Current Kew taxonomy accepts H. megalocarpa as a species confined to central Sichuan, illustrating how strongly conservation interpretation depends upon the unresolved taxonomic boundary.

Even if species rank is ultimately unsupported, the historical restriction of the morphotype to Baoxing and Tianquan makes these populations conservation-relevant. A broad assessment of H. ampla cannot determine whether the genetic or ecological variation represented in central Sichuan is secure. Field surveys should establish the current abundance, habitat condition, elevational distribution, and continuity of populations corresponding to the megalocarpa morphology.

The decisive taxonomic and conservation study would combine replicated sampling from the historical central-Sichuan localities with surrounding H. ampla populations, nuclear genomic data, quantitative capsule and leaf measurements, habitat comparisons, and reproductive or phenological observations. Concordant differentiation would justify reconsideration at formal rank; extensive intergradation and genetic continuity would support continued treatment as a Form or ordinary geographic variation.

Additional Information

Plants of the World Online — Hydrangea megalocarpa (opens in a new tab): current treatment as an accepted central-Sichuan species and nomenclatural details.

Flora of China — Schizophragma megalocarpum (opens in a new tab): historical morphology, habitat, and distribution.

Trees and Shrubs Online — Hydrangea ampla (opens in a new tab): detailed discussion of the megalocarpa problem and cultivated material.

References and Further Reading

Chun WY (1954) A census and preliminary study of the Chinese Hydrangeoideae. Acta Phytotaxonomica Sinica 3(2): 101–205. https://cpni.ibiodiversity.net/26764 (opens in a new tab)

Liu W, Zhu X-Y (2011) Leaf epidermal characters and taxonomic revision of Schizophragma and Pileostegia (Hydrangeaceae). Botanical Journal of the Linnean Society 165(3): 285–314. https://doi.org/10.1111/j.1095-8339.2010.01101.x (opens in a new tab)

Royal Botanic Gardens, Kew (2026) Hydrangea megalocarpa (Chun) J.M.H.Shaw. Plants of the World Online. https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:77296097-1 (opens in a new tab)

Samain MS (2021) Hydrangea grows bigger. The Plant Review 3(4): 52–56.

Sutton J (2025) Hydrangea ampla. Trees and Shrubs Online. International Dendrology Society. https://www.treesandshrubsonline.org/articles/hydrangea/hydrangea-ampla/ (opens in a new tab)

Wei Z, Bartholomew B (2001) Schizophragma Siebold & Zuccarini. In: Wu ZY, Raven PH (eds.) Flora of China, vol. 8. Science Press, Beijing & Missouri Botanical Garden Press, St. Louis, pp. 408–410. https://www.iplant.cn/foc/pdf/Saxifragaceae.pdf (opens in a new tab)