2019 ASHS Annual Conference

Oakleaf hydrangea is a shrub with unique ornamental traits that are not available in other popular Hydrangea species such as exfoliating bark, intense fall color and deeply lobed leaves. Little is known about the variation in phenotypes in the wild germplasm of H. quercifolia. Moreover, woody ornamental breeding programs rarely begin with a comprehensive evaluation of the species but doing so allows the full range of phenotypic diversity to be assessed allowing for the optimal parental pool to be selected for breeding. In order to alleviate these two shortcomings, we sampled wild oakleaf hydrangea germplasm from across its range of occurrence in the SE U.S. and characterized the diversity in germination rate, compactness related traits (height and internode length), and midwinter cold hardiness (LT₅₀ and percent survival at -35°C) using a laboratory based controlled freezing assay. Seed from 17 natural populations representing a latitudinal cline (from Florida to Tennessee) was germinated in 2018 and variation was evaluated in the seedlings’ first year of growth. Significant variation in percent seed germination was found among populations, with the population means ranging from 14% to 97.3%. Height also varied significantly among populations and had a weak negative correlation with latitude (r=-0.41; p<0.001). Internode length was estimated by dividing plant height by node number and varied significantly among populations; shorter plants tended to have shorter internodes (r=0.92; p<0.001). Most seedlings had one main stem, however one family was identified with an increased number of branches. Significant variation for cold hardiness was found among populations with LT₅₀ ranging from -27.1°C to -33.1°C and percent survival at -35°C ranging from 4.3% to 36.4%. Both metrics of cold hardiness correlated significantly with latitude (r=0.72; p=0.003). These seedlings will continue to be evaluated in Minnesota and Tennessee for additional phenotypic variation along with a broader sample collected from the entire native range of H. quercifolia. Representative seed samples and clones will be made available to breeders via the USDA National Plant Germplasm System.