Hydrangeaceae
Hydrangea Family
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Family Overview
Hydrangeaceae, the Hydrangea Family, is a family of flowering plants in the order Cornales best known for Hydrangea, but its diversity also includes Deutzia, Philadelphus, Jamesia, Carpenteria, and several smaller lineages. Tree TSAR recognizes nine genera and approximately 235 species, although the species total remains somewhat dependent on taxonomic treatment within the largest genera. The difference is especially apparent in Hydrangea: Tree TSAR currently recognizes 111 non-hybrid species, compared with 98 accepted species in the current Plants of the World Online treatment. These differences concern species delimitation and synonymy rather than the outer boundary of Hydrangeaceae, for which modern classifications show considerable agreement (Royal Botanic Gardens, Kew 2026; De Smet et al. 2015; Yang et al. 2024).
Most Hydrangeaceae are woody trees, shrubs, or vines; however, there are some herbaceous taxa that depart from the usual woody architecture. Broadly circumscribed Hydrangea includes enormous woody root-climbers as well as perennial subshrubs from woody bases and genuinely herbaceous rhizomatous geophytes. Kirengeshoma provides another conspicuous herbaceous expression within Hydrangeoideae. These growth forms make Hydrangeaceae more architecturally diverse than the familiar garden hydrangeas may suggest (Freeman 2016; Yang et al. 2024).
The leaves are usually simple and opposite, lack stipules, and may have entire, crenate, toothed, or occasionally lobed margins. Flowers are generally bisexual and radially symmetrical, although some Hydrangea inflorescences combine small fertile flowers with conspicuous marginal flowers whose enlarged petaloid sepals function primarily in display. Floral organization varies considerably across the family: flowers may bear four to twelve sepals and petals, stamens can range from eight to very numerous, and the ovary is partly to completely inferior. The fruit is most commonly a capsule, with fleshy fruits having evolved in some hydrangea lineages, most notable the bluebead hydrangea species (Freeman 2016; Royal Botanic Gardens, Kew 2026).
From Saxifragaceae to Cornales
The modern position of Hydrangeaceae differs substantially from many older classifications. Hydrangeas and their relatives were long associated with a broad and heterogeneous Saxifragaceae, in large part because of similarities in floral construction and ovary position. Molecular studies instead established Hydrangeaceae as a lineage of Cornales, far removed from Saxifragaceae sensu stricto. This placement is now supported by large nuclear genome datasets as well as earlier plastid and morphological analyses (Hufford et al. 2001; Thomas et al. 2021).
Within Cornales, Hydrangeaceae is closely allied with Loasaceae. Dense sampling in a recent comparative phylogenetic analysis recovered both as strongly supported, reciprocally monophyletic families, while broader nuclear phylogenomics likewise supports their close relationship within the capsular-fruited portion of Cornales (Thomas et al. 2021; Schenk et al. 2025). Hydrostachyaceae is also closely associated with this radiation, although its exact position has varied among phylogenomic and total-evidence analyses. That uncertainty affects the branching order among families rather than the monophyly of Hydrangeaceae itself (Thomas et al. 2021; Nguyen & Atkinson 2024).
The Hydrangeaceae–Loasaceae divergence is ancient. Schenk et al. (2025) estimated their separation within a 95% credible interval of approximately 79.5–92.6 million years ago, placing the split firmly in the Late Cretaceous. The two families nevertheless retain characters that make their relationship biologically intelligible, including predominantly capsular fruits and similarities in floral, seed, placentation, and indumentum structure. Their subsequent radiations followed markedly different trajectories. Hydrangeaceae remained predominantly woody and diversified extensively in forest and woodland environments, whereas Loasaceae underwent much greater expansion into herbaceous, xeric, montane, and seasonally dry life forms. Their morphological and ecological differentiation, ancient separation, reciprocal monophyly, and continued taxonomic usefulness strongly favor recognition as separate families (Schenk et al. 2025).
Biogeographic reconstruction places the ancestral area of Hydrangeaceae in the western Nearctic, followed by repeated dispersal into the Palearctic and subsequent movement into eastern and southeastern Asia and other regions. Several of the family's major radiations, particularly Deutzia, Hydrangea, and Philadelphus, now have important centers of diversity in eastern Asia, while western and eastern North America retain both old endemic branches and later representatives of more widely distributed lineages. These repeated movements between continents help explain the conspicuous eastern Asian–North American disjunctions found throughout the family (Schenk et al. 2025).
Deep Structure Within the Family
The deepest living division separates subfamily Jamesioideae from subfamily Hydrangeoideae. Molecular analyses consistently recover the western North American Fendlera–Jamesia lineage as sister to the remainder of Hydrangeaceae, and recent molecular dating places that split at approximately 57.8–78.4 million years ago (Hufford et al. 2001; Schenk et al. 2025). Its age approaches the temporal scale of some family-level divergences, but Tree TSAR retains Jamesioideae within Hydrangeaceae because age is only one component of rank assessment. The broad family remains strongly monophyletic and morphologically coherent, and elevating the comparatively small Jamesioideae lineage would add rank fragmentation without providing a corresponding improvement in biological interpretation.
Hydrangeoideae contains the remainder of the family and is divided into tribes Philadelpheae and Hydrangeeae. Philadelpheae includes the familiar ornamental genera Deutzia and Philadelphus together with several smaller woody or herbaceous lineages. Early molecular work recovered recognizable relationships among these genera, and recent plastome analyses continue to support the monophyly and substantial geographic structuring of Deutzia and Philadelphus (Hufford et al. 2001; Huang et al. 2025).
Hydrangeeae presents the family's most important generic-circumscription problem. Plants now assigned to broad Hydrangea were historically divided among Hydrangea and eight satellite genera: Broussaisia, Cardiandra, Decumaria, Deinanthe, Dichroa, Pileostegia, Platycrater, and Schizophragma. Molecular analyses demonstrated that these segregates are embedded among traditional hydrangeas, making narrow Hydrangea phylogenetically untenable unless the radiation is fragmented into numerous genera. Tree TSAR instead follows the broad solution developed by De Smet et al. (2015), retaining the biological information represented by the former genera principally at infrageneric rank. Phylogenomic work has since resolved this broad genus into five subgenera and 19 sections, providing a detailed internal classification without sacrificing generic monophyly (De Smet et al. 2015; Yang et al. 2024).
Hydrangeaceae also has a substantial paleobotanical context. Fossils referred to the family have been reported from the Upper Cretaceous, with a much richer record through the Cenozoic, consistent with the great age inferred from molecular dating (Freeman 2016). Fossil Cornales nevertheless include combinations of characters and entire higher lineages that do not fit neatly within surviving families. Total-evidence studies therefore provide an important caution against automatically assigning ancient cornalean fossils to Hydrangeaceae merely because they resemble parts of the modern family (Nguyen & Atkinson 2024).
Horticultural and Agricultural Uses
Hydrangeaceae is overwhelmingly important as an ornamental family, and Hydrangea accounts for much of that prominence. Garden culture is concentrated especially around H. macrophylla and its relatives, H. paniculata, H. arborescens, and H. quercifolia, but the broader genus offers far greater horticultural diversity than those familiar shrubs alone suggest. Woody climbing hydrangeas can cover substantial walls or ascend mature trees, while less familiar Asian species contribute distinctive foliage, bark, indumentum, and flowering characters. The herbaceous and suffrutescent lineages further expand the genus into woodland-perennial horticulture (De Smet et al. 2015; Yang et al. 2024).
Flower color in cultivated Hydrangea macrophylla is one of the family's best-known horticultural phenomena, but the familiar advice that acidic soil simply “turns hydrangeas blue” compresses a much more interesting chemical system. Red, purple, and blue sepals use the same principal anthocyanin framework; blue coloration develops through interactions among delphinidin-derived anthocyanin, copigments, aluminum ions, and cellular conditions including vacuolar pH. Soil acidity matters largely because it affects the availability and uptake of aluminum rather than acting as a direct color switch (Yoshida et al. 2021). This chemistry explains both the responsiveness of some cultivars to soil management and the limits of manipulating color in plants whose genetics or pigment composition differ.
Other Hydrangeaceae have long-standing ornamental roles of their own. Deutzia contributes predominantly deciduous flowering shrubs, while Philadelphus, the mock-orange lineage, is valued especially for masses of white flowers and, in many selections, strong fragrance. Carpenteria is a distinctive evergreen ornamental from California, and the herbaceous Kirengeshoma extends the family's cultivated range into shaded perennial plantings. Recent plastome work on Philadelpheae also emphasizes the strong geographic structure of Deutzia and Philadelphus, a reminder that horticultural germplasm represents only part of the evolutionary variation present in their wild populations (Freeman 2016; Huang et al. 2025).
Wild Hydrangeaceae remain important reservoirs for breeding and collecting. Within Hydrangea, species diversity provides potential sources of cold and heat tolerance, disease resistance, reblooming behavior, unusual plant architecture, evergreen foliage, climbing habit, and novel floral characters. The family's horticultural future therefore depends not only on continued cultivar development but also on maintaining well-documented wild germplasm and conserving the natural populations from which those traits arise.
Conservation Issues
The ubiquity of cultivated hydrangeas can obscure how localized some wild Hydrangeaceae are. Conservation concern is concentrated especially in narrow endemics, montane and cloud-forest species, island populations, and lineages whose apparent distributions have changed as taxonomic work has resolved previously overlooked species. Habitat loss, forest fragmentation, changing moisture regimes, and climate-driven alteration of montane refugia can therefore affect portions of the family that are scarcely represented by the common plants of gardens.
The climbing hydrangeas of Section Cornidia provide one of the clearest examples. These woody root-climbers are concentrated in humid tropical and subtropical forests from Mexico through Central and South America, with a remarkable Asian outlier. Intensive field and taxonomic work in Mexico demonstrated that material formerly placed within a few broad species concepts actually represented substantially greater diversity, including several highly restricted species. Deforestation, agricultural conversion, fire, and degradation of humid montane forest threaten many of these lineages; the specialist Mexican revision assessed several newly recognized species as Endangered or Critically Endangered under IUCN criteria (Samain et al. 2019). Subsequent phylogenetic work confirmed considerable evolutionary structure within Cornidia and showed that some traditional morphological species concepts remain in need of further genomic testing (Granados Mendoza et al. 2021).
This interaction between taxonomy and conservation is important across the family. Treating a broad, apparently widespread species as a single unit may conceal several geographically restricted evolutionary lineages with very different conservation needs. Conversely, genomic evidence can reveal strong population structure even where species boundaries themselves remain stable. In the herbaceous hydrangea lineage historically known as Deinanthe, population-genomic work recovered pronounced signatures of refugial history, range contraction, and population-specific responses to Quaternary climate change. The narrow Chinese lineage showed particularly low genetic diversity after demographic contraction, while Japanese populations retained geographically structured histories associated with changing glacial refugia (Sakaguchi et al. 2021).
For Hydrangeaceae, effective conservation therefore depends heavily on preserving geographic and genetic representation rather than merely maintaining a few horticultural accessions. Protection of humid forest, montane refugia, and narrowly endemic populations is especially important, while provenance-documented living collections can preserve material for future taxonomic, genomic, breeding, and restoration work. Continued field study is equally consequential: in parts of Hydrangea, finding and correctly identifying populations can change the conservation picture as much as subsequent management.
Major Clades
Jamesioideae. This early-diverging subfamily contains Fendlera and Jamesia, both centered in western North America. Its separation from Hydrangeoideae is one of the oldest surviving divergences within the family, estimated at approximately 57.8–78.4 million years ago. The lineage is retained at subfamily rank because its considerable age is accompanied by continued family-level cohesion rather than the suite of evidence that would make an additional family more informative (Hufford et al. 2001; Schenk et al. 2025).
Hydrangeoideae. This larger subfamily contains the remainder of the family and is divided into tribes Philadelpheae and Hydrangeeae. Philadelpheae contains Carpenteria, Deutzia, Fendlerella, Kirengeshoma, Philadelphus, and Whipplea. It encompasses much of the family's diversity outside Hydrangea, ranging from western North American shrubs to species-rich eastern Asian radiations and the herbaceous Kirengeshoma. Molecular evidence supports important affinities including Philadelphus with Carpenteria and Deutzia with Kirengeshoma, while continued genomic sampling is refining the branching pattern among the smaller lineages (Hufford et al. 2001; Huang et al. 2025). Hydrangeeae is treated by Tree TSAR as the single broad genus Hydrangea. The eight former satellite genera are nested within this radiation and are represented principally through infrageneric classification rather than separate generic recognition. The current phylogenomic framework divides Hydrangea into five subgenera and 19 sections, preserving its extraordinary morphological, ecological, and geographic structure within a monophyletic genus (De Smet et al. 2015; Yang et al. 2024).
Accepted Genera
| Carpenteria (page not yet published) | Subfamily Hydrangeoideae | Tribe Philadelpheae | Torr. (1851) | Bush-anemone |
| Deutzia (page not yet published) | Subfamily Hydrangeoideae | Tribe Philadelpheae | Thunb. (1781) | Deutzia |
| Fendlera (page not yet published) | Subfamily Jamesioideae | Engelm. & A.Gray (1852) | Fendlerbush | |
| Fendlerella (page not yet published) | Subfamily Hydrangeoideae | Tribe Philadelpheae | A.Heller (1898) | False-fendlerbush |
| Hydrangea | Subfamily Hydrangeoideae | Tribe Hydrangeeae | Gronov. ex L. (1753) | Hydrangea |
| Jamesia (page not yet published) | Subfamily Jamesioideae | Torr. & A.Gray (1840) | Cliffbush | |
| Kirengeshoma (page not yet published) | Subfamily Hydrangeoideae | Tribe Philadelpheae | Yatabe (1890) | Waxbells |
| Philadelphus (page not yet published) | Subfamily Hydrangeoideae | Tribe Philadelpheae | L. (1753) | Mock-orange |
| Whipplea (page not yet published) | Subfamily Hydrangeoideae | Tribe Philadelpheae | Torr. (1857) | Whipplevine |
Additional Information
- Plants of the World Online — Hydrangeaceae (opens in a new tab): Current Kew nomenclatural and taxonomic backbone for the family, including the nine accepted genera.
- Flora of North America — Hydrangeaceae (opens in a new tab): Detailed family morphology, North American genera, systematic history, and paleobotanical context. Its 17-genus classification preserves narrower historical generic concepts that predate the broad Tree TSAR treatment of Hydrangea.
- World Flora Online — Hydrangeaceae (opens in a new tab): Global taxonomic record and family-level morphological information.
- International Plant Names Index — Hydrangeaceae (opens in a new tab): Nomenclatural record for Hydrangeaceae Dumort., including publication details, conserved status, and typification.
- iNaturalist — Hydrangeaceae (opens in a new tab): Observation-based resource for photographs and occurrence records; cultivated observations should be distinguished from native records when interpreting distributions.
References and Further Reading
De Smet Y, Granados Mendoza C, Wanke S, Goetghebeur P, Samain M-S (2015) Molecular phylogenetics and new (infra)generic classification to alleviate polyphyly in tribe Hydrangeeae (Cornales: Hydrangeaceae). Taxon 64(4): 741–753. https://doi.org/10.12705/644.6 (opens in a new tab)
Freeman CC (2016) Hydrangeaceae Dumortier. In: Flora of North America Editorial Committee (eds.) Flora of North America North of Mexico, vol. 12. Oxford University Press, New York & Oxford, pp. 462–491. https://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=10425 (opens in a new tab)
Granados Mendoza C, Martínez Salas EM, Goetghebeur P, Wanke S, Samain M-S (2021) Molecular phylogeny, character evolution, and biogeography of Hydrangea section Cornidia, Hydrangeaceae. Frontiers in Plant Science 12: 661522. https://doi.org/10.3389/fpls.2021.661522 (opens in a new tab)
Huang Z, Li M, Liang C, Fan Q, Yu W, Wang S, Lin M, Li P, Zhang R, Gong W (2025) Evolutionary genetics analysis for complete plastomes and diversification rate estimation of Deutzia and Philadelphus in tribe Philadelpheae (Hydrangeaceae). BMC Plant Biology 25(1): 1104. https://doi.org/10.1186/s12870-025-07073-w (opens in a new tab)
Hufford L, Moody ML, Soltis DE (2001) A phylogenetic analysis of Hydrangeaceae based on sequences of the plastid gene matK and their combination with rbcL and morphological data. International Journal of Plant Sciences 162(4): 835–846. https://doi.org/10.1086/320789 (opens in a new tab)
Nguyen AT, Atkinson BA (2024) Cretaceous and Paleocene fossils reveal an extinct higher clade within Cornales, the dogwood order. American Journal of Botany 111(7): e16372. https://doi.org/10.1002/ajb2.16372 (opens in a new tab)
Royal Botanic Gardens, Kew (2026) Hydrangeaceae Dumort.. Plants of the World Online. https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:77126637-1 (opens in a new tab)
Sakaguchi S, Asaoka Y, Takahashi D, Isagi Y, Imai R, Nagano AJ, Qiu Y-X, Li P, Lu R, Setoguchi H (2021) Inferring historical survivals of climate relicts: The effects of climate changes, geography, and population-specific factors on herbaceous hydrangeas. Heredity 126(4): 615–629. https://doi.org/10.1038/s41437-020-00396-4 (opens in a new tab)
Samain M-S, Hernández Najarro F, Martínez Salas EM (2019) The climbing hydrangeas (Hydrangeaceae) of Mexico, including description of six (critically) endangered new species. Acta Botanica Mexicana 126: e1463. https://doi.org/10.21829/abm126.2019.1463 (opens in a new tab)
Schenk JJ, Jacobs S, Hufford L (2025) Comparative diversification analyses of Hydrangeaceae and Loasaceae reveal complex evolutionary history as species disperse out of Mesoamerica. American Journal of Botany 112(1): e16455. https://doi.org/10.1002/ajb2.16455 (opens in a new tab)
Thomas SK, Liu X, Du Z-Y, Dong Y, Cummings A, Pokorny L, Xiang Q-YJ, Leebens-Mack JH (2021) Comprehending Cornales: Phylogenetic reconstruction of the order using the Angiosperms353 probe set. American Journal of Botany 108(7): 1112–1121. https://doi.org/10.1002/ajb2.1696 (opens in a new tab)
Yang X, Xue T, Gao T, Liang Y, Smets EF, Gadagkar SR, Yu S (2024) An updated infrageneric classification based on phylogenomics and character evolution in Hydrangea (Hydrangeaceae). Taxon 73(2): 503–518. https://doi.org/10.1002/tax.13158 (opens in a new tab)
Yoshida K, Oyama K, Kondo T (2021) Insight into chemical mechanisms of sepal color development and variation in hydrangea. Proceedings of the Japan Academy, Series B 97(2): 51–68. https://doi.org/10.2183/pjab.97.003 (opens in a new tab)