Hydrangea hullaballoo

For over 65 years, the majority of horticulturists, botanists, nursery operators and gardeners accepted the Hydrangea taxonomy and nomenclature of Elizabeth McClintock [Proc. Calif. Acad. Sci. 29(5): 147-156, 1957]. She recognized 23 species on a worldwide basis. Most (all) of her taxonomic work was based on herbarium specimens without accounting for the tremendous variation that occurs in nature. Her work, in my professional opinion, before the advent of molecular studies, proved extremely successful and useful. Certainly, taxonomists do not agree on the number of species within Hydrangea. The “Flora of China” lists 73 species worldwide with 33 species (25 endemic) in China. The 2020 edition of “Hillier Manual of Trees and Shrubs” legitimizes 29 species. In the United States, H. arborescens, H.cinerea, H. radiata, and H. quercifolia were the only members of the genus. With the new revision of the Hydrangeaceae, Decumaria barbara becomes Hydrangea barbara and the Hawaiian Islands native, Broussaisia arguta, is now Hydrangea arguta, for a total of six.

Taxonomy is the classification of plants and their relationships to each other. Scientific nomenclature is the naming of plants based on relationships, now dictated by DNA-based hypotheses of the most recent common ancestry. Morphological characteristics (leaves, flowers, fruits, seeds, etc.) are generally no longer considered absolute, but utilized to support molecular classifications. In 2007, the taxonomic winds of change in Hydrangea were softly blowing. At the International Hydrangea Conference in Ghent, Belgium, I listened to Dr. Larry Hufford of Washington State University discuss “Relationships and evolution of Hydrangeaceae based on rbcL sequence data.” [American J. Bot. 82: 504-514. 1995] and “A phylogenetic analysis of Hydrangeaceae using morphological data” [International J. Plant Sciences 158: 652-672. 1997]. Some of the discussion was beyond my ability to understand and appreciate, yet I grasped that DNA would undergird/support all taxonomic revisions going forward. The conclusion from the second listed reference by the author was ...

“The classifications of Hydrangeaceae require extensive revision to provide greater utility and reasonable insights on evolutionary relationships.” Thus, his conclusions have come to pass.

In my 2004 book, “Hydrangeas for American Gardens,” I stayed true to McClintock’s classification. I noted more molecular Hydrangea taxonomic research published in 2010 and 2015. These publications emanated from the University of Ghent and Dr. Paul Goetghebeur and associates with the seminal/transformational publication in 2015, “Molecular phylogenetics and new (infra) generic classification to alleviate polyphyly in tribe Hydrangeeae (Hydrangeaceae, Cornales)”. [Taxon 65: 741-753.] The title sounds frightening but the research supporting the taxonomic changes is sound and is being adopted.

Hydrangea ‘Barbara Ann’ fka Decumaria

In “Hydrangea grows bigger,” [The Plant Review 3(4): 52-56], Marie-Stephanie Samain discussed the new taxonomic and nomenclatural changes. In addition to the genus Hydrangea, the Hydrangeaceae includes eight genera… Broussaisia, Cardiandra, Decumaria, Deinanthe, Dichroa, Pileostegia, Platycrater and Schizophragma. The new combinations are presented herein.

The major nursery/garden species, H. arborescens, H. macrophylla, H. paniculata, H. quercifolia, and H. serrata remain unchanged. Drs. Reed and Rinehart believed that H. serrata should be considered a subsp. of H. macrophylla and presented as H. macrophylla subsp. serrata [Proc. SNA Res Conf. 51: 573-576 (2006)]. This was not accepted as a valid name.

Hydrangea species

The following species (lefthand column) are legitimate according to GRIN and “Taxon” 2015. In parentheses, the most recent names in common use in “Hillier Manual” (2019) and the “Royal Horticultural Society Plant Finder” (2020).

H. angustipetala      (H. scandens subsp. chinensis f. angustipetala)

H. anomala               (H. anomala subsp. anomala)

H. aspera

 H. bretschneideri    (H. heteromalla Bretschneideri Group)

 H. chinensis              (H. scandens subsp. chinensis)

 H. chungii                 (H. macrophylla subsp. chungii)

 H. davidii                  (H. yunnanensis)

 H. dumicola

 H. febrifuga

 H. heteromalla

 H. hirta

 H. hypoglauca

 H. involucrata

 H. kwangsiensis

 H. liukiuensis           (H. scandens subsp. liukiuensis)

 H. longipes

 H. longipes var. fulvescens

 H. luteovenosa

 H. petiolaris             (H. anomala subsp. petiolaris)

 H. robusta                (H. aspera subsp. robusta)

 H. sargentiana        (H. aspera subsp. sargentiana)

 H. scandens

 H. sikokiana

 H. strigosa               (H. aspera subsp. strigosa)

 H. stylosa                 (H. macrophylla subsp. stylosa)

 H. xanthoneura

Generic name changes

The eight generic name changes initiated by the “Taxon” 2015 research are presented below. Those names indented are not well represented in U.S. cultivation.

According to Samain in “The Plant Review” article (2021), several largely unknown varieties and subspecies of Schizophragma and Dichroa have been recombined. These are based on the work of J. M. H. Shaw who previously published “Classification of climbing hydrangeas and their relatives” [The Plantsman 10: 226-231, 2012] and “Hybrids between Dichroa and Hydrangea” [The Plantsman 23: 16-21, 2014]. In 2021, Samain stated, “to complete the classification of the Hydrangea clade, several recombinations for plants in horticulture are required.” They are listed above with Shaw as the authority.

The 12 evergreen vining species native to Mexico, Central and South America with an outlier in southeast Asia are not discussed in this article. To my knowledge their classification and nomenclature has not changed. I recommend Samain, MS, and E. Martinez Salas “Hydrangea hunting in the Neotropics” [The Plantsman n.s. 14(1):30-35].

H. integrifolia (Taiwan, Philippines), H. seemanii (Mexico), and H. serratifolia (Chile) are occasionally listed in U.S. commerce.

The information is based on sound science and the new molecular approach to plant taxonomy. When writing an article, I always check GRIN. There are other sources for plant names.

Perhaps, the best is Plants of The World Online (www.powo.science.kew.org), managed by Kew Science, England. Type in Hydrangea and be prepared to be overwhelmed with names. I did note Kew uses the taxonomic changes reported in “Taxon” 2015.