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Horticultural Reviews, Volume 49


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mechanism, screening with culture filtrates may not be a viable option. Instead, it may be necessary to screen seedlings by inoculating with the pathogen itself to identify variation in tolerance. Screening for Armillaria root rot is also a possibility, as high‐throughput procedures for field inoculations have been published (Beckman and Pusey 2001). Selecting for tolerance to foliar pathogens is also highly desirable, as foliar pathogens have an ability to induce an unsightly appearance to plants in production as well as in the landscape.

      Cold hardiness is the major limiting factor determining where oakleaf hydrangea can be cultivated, and therefore is a breeding priority in order to expand the cultivated range. Current hardiness estimates indicate USDA zone 5a may be the extent of cold hardiness for the species (Dirr et al. 1993; Halcomb and Reed 2012), although screening wild germplasm from the northern extent of the latitudinal cline may identify variation in cold tolerance (Hurme et al. 1997; Friedman et al. 2008; Pagter et al. 2010). Indeed, a latitudinal cline for midwinter cold hardiness is found in wild collected oakleaf hydrangea seedlings, with northern populations generally being more cold hardy than southern populations (Sherwood et al. 2019).

      Variation exists in H. quercifolia for floral characteristics such as flower color, flowering time, flower size, and double flowers (Dirr 2004). Because the white sepals become pink or brown as the inflorescence ages, the flower color variation consists of whether they turn pink, the timing of pinking, and the shade of pink. To date, there are no known genotypes which have flowers that open pink. Inheritance of flower color in Hydrangea is not well studied, but it appears to be a quantitative trait considering the variability based on environment (with light intensity likely being a factor) and the seemingly infinite number of intermediate phenotypes. Variation in flowering time is available in cultivars such as ‘Late Hand’, which blooms about one month later than typical (Dirr 2004), and ‘Queen of Hearts’, which blooms about 7–10 days later than other cultivars (Reed and Alexander 2015). In the wild, individual plants have been observed flowering later in the season than the surrounding plants, while others flower multiple times in one season (A. Sherwood, pers. observ.). However, research will be required to identify environmental effects from those that are genetically controlled. Considerable variation also exists for flower size, with panicles ranging from 7 cm long in some wild plants (A. Sherwood, unpubl.) up to approximately 30 cm long in many cultivars. In H. macrophylla, double flowers are a recessively inherited trait that may be controlled by a single major gene (Suyama et al. 2015; Waki et al. 2018). However, double flowered H. quercifolia genotypes produce little to no seed or pollen, and the fertility of any pollen or seed that is produced by double flowers has not been studied. Additionally, the double flowered H. quercifolia cultivars tend to produce a considerably higher number of sepals (up to 20 per floret) compared to most of the double flowered H. macrophylla cultivars which produce around eight sepals per floret (Dirr 2004).



Cultivar Plant height (m) Panicle length (cm) Notes Origin
Alice1 , 2 4 30 One of tallest cultivars; sepals turn pink Selected by Dirr on the University of Georgia, Athens campus
Alison1 , 2, 3 3 25 Selected by Dirr on the University of Georgia, Athens campus
Amethyst1 2 15 Sepals turn bright pink; nearly red Selected by Dirr
Applause1 , 2 Cultivated for a long time without a cultivar name
Back Porch1 Early flowers; sepals turn pink Introduced by Louisiana Nurseries