HYDRANGEA MACROPHYLLA AND SERRATA - SHOULD WE LUMP 'EM OR SPLIT 'EM?

Authors: Reed, Sandra; Rinehart, Timothy - Tim

Submitted to: Southern Nursery Association Research Conference
Publication Type: Proceedings
Publication Acceptance Date: 8/23/2006
Publication Date: 12/12/2006
Citation: Reed, S.M. and Rinehart, T.A. 2006. Hydrangea macrophylla and serrata - Should we lump 'em or split 'em? Proceedings of the Southern Nursery Association Research Conference. 51:573-576.

Interpretive Summary: There is confusion among the industry and scientists as to whether H. macrophylla subsp. serrata or H. serrata is the correct scientific name for the mountain hydrangea. This study used morphological, molecular and hybridization data to address this issue. While morphological data could support elevating the serrata form to species level, molecular and hybridization data do not support separating macrophylla and serrata into separate species. We recommend use of the H. macrophylla subsp. serrata designation as it is more appropriate from a breeding perspective.

Technical Abstract: The taxonomic treatment of Hydrangea serrata has long been disputed. While initially considered to be a separate species, it was later reclassified as a subspecies of H. macrophylla. Recently, many authorities on hydrangea have advocated elevating serrata to species level. The objective of this study was to use previously published and new morphological, molecular and hybridization data to evaluate whether this reclassification is merited. Serrata can be distinguished from macrophylla primarily by its smaller leaves and flower parts. While genome size of serrata is 5.8% smaller than macrophylla, intraspecific differences in genome size among populations of plant species collected from different geographical areas are not uncommon Hybrids between macrophylla and serrata forms have been produced and appear to be fertile. The decision as to whether it is valid to consider H. serrata a separate species from H. macrophylla depends on how one defines a species. Using morphological criteria, the separation of the species could be justified. While this might be convenient from a horticultural perspective, we suggest keeping serrata at the subspecies level. This is more appropriate from a breeding standpoint and might lessen confusion in the future if new cultivars are developed from hybridization of macrophylla and serrata taxa.