Horticultural characterization of wild Hydrangea quercifolia seedlings collected throughout the species native range

Authors: SHERWOOD, ANDREW - University Of Minnesota; Alexander, Lisa; MCNAMARA, STEVE - University Of Minnesota; CLARK, MATTHEW - University Of Minnesota; HOKANSON, STAN - University Of Minnesota

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2021
Publication Date: 9/1/2021
Citation: Sherwood, A., Alexander, L.W., Mcnamara, S., Clark, M., Hokanson, S. 2021. Horticultural characterization of wild Hydrangea quercifolia seedlings collected throughout the species native range. Journal of the American Society for Horticultural Science. 56(9):1-11.
DOI: https://doi.org/10.21273/HORTSCI15889-21

Interpretive Summary: Oakleaf hydrangea is a flowering shrub native to woodlands of the southeast United States. Oakleaf hydrangea has immense ornamental potential with four-season interest, including traits like showy panicles, striking foliage textures, intense fall color, and exfoliating bark. Cultivars are often derived from wild selections either directly or only a few generations removed. Full genetic and phenotypic variation has not been evaluated for the species, and little is known about the diversity in horticulturally important traits for oakleaf hydrangea. USDA scientists in McMinnville, TN, along with partners at the University of Minnesota, collected seed form 55 oakleaf hydrangea populations across the southeast for horticultural characterization. Significant differences between populations were found for plant size, tolerance to leaf spot, and tolerance to cold. Plant height was correlated with collection site latitude, with populations from the northeastern extent of range being the most compact and populations from Florida being the largest. Leaf spot severity varied significantly among populations and cultivars and was also correlated with latitude in the wild seedlings. Two populations in Florida were identified as potential sources of resistance to leaf spot, while cultivars Flemygea and Alice were identified as having moderate tolerance to leaf spot. Midwinter cold hardiness also varied by latitude, and several populations displayed high levels of cold tolerance. These results indicate that certain wild oakleaf hydrangea populations will be useful for introgressing novel variation into breeding programs.

Technical Abstract: Oakleaf hydrangea (Hydrangea quercifolia) is an understory shrub native to the southeastern United States. Hydrangeas are popular ornamental landscape plants; however, little is known about the diversity in horticulturally important traits for oakleaf hydrangea. Information regarding the variation in important traits could guide future breeding efforts for the species. Seed was collected from 55 populations throughout the species range for the horticultural characterization. Seed germination percentage was characterized in a greenhouse and growth chamber. Plant architecture was characterized as plant height, number of nodes, internode length, number of branches and plant width. Plant architecture was measured in potted and field grown plants in two locations. Tolerance to leaf spot (Xanthomonas campestris L.) was characterized in wild collected seedlings and cultivars by measuring disease severity under natural exposure to ambient inoculum. Cold hardiness was characterized in two winters with a controlled freezing experiment. The first winter, seedlings were tested in January only while in the second winter, seedlings and cultivars were tested monthly throughout winter. Significant variation among wild populations and cultivars was found for all traits measured in all environments. Mean population seed germination percent ranged from 11% to 93% (mean=61% in greenhouse, 74% in growth chamber). Plant architecture varied by environment, with plants growing larger in Tennessee than in Minnesota. Plant height was correlated with collection site latitude (r=-0.66) with populations from the northeastern extent of the species range being the most compact and populations from Florida being the largest. Leaf spot severity varied significantly among populations and cultivars and was also correlated with latitude in the wild seedlings (r=0.70). Two populations in Florida were identified as sources of resistance to leaf spot while ‘Flemygea’ and ‘Alice’ were identified as having moderate tolerance to leaf spot. Cold hardiness varied among populations and cultivars and among months of the winter. Overall maximum cold hardiness was observed in February (mean LT50=-33.7°C), and several populations maintained an extreme level of cold hardiness into late winter. Midwinter cold hardiness also varied by latitude (r=-0.65). These results indicate that certain wild oakleaf hydrangea populations will be useful for introgressing novel variation into breeding programs.