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.
D. Breeding for Compactness
Wild plants of the species are on average about 2 m tall and 1.8 m wide with mature plants often being much larger (A. Sherwood, pers. obs.), making breeding for smaller forms desirable in order to allow H. quercifolia to be grown in smaller garden settings. Mature plant size is often cited as a quantitative trait in other crops (Miller and Hammond 1991), although in certain cases there may be a single major effect gene which induces compactness (Ishimaru et al. 2003). Considerable progress has been made in breeding more compact H. quercifolia cultivars. By using ‘Sikes Dwarf’ and ‘Pee Wee’ as parents in breeding populations, The USDA‐ARS Floral and Nursery Crops Laboratory (McMinnville, TN) has introduced three cultivars that are considerably smaller than the species (Reed 2010; Reed and Alexander 2015). Using these cultivars as parents in crosses should yield progeny with smaller stature. Additionally, in first and second year wild collected oakleaf hydrangea seedlings, variation in plant architecture traits has been detected with populations from the northeastern extent of the species range being the most compact (Sherwood et al. 2021). Therefore, wild germplasm can serve as a novel source of compactness for breeding.
E. Breeding for Cold Hardiness
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).
Because winters are variable and unpredictable, controlled freezing experiments offer the most reliable information on cold tolerance at a given point during the winter (Pagter and Williams 2011; Hokanson and McNamara 2013; Pagter et al. 2014). These controlled freezing experiments can be used to identify populations or individuals that are more cold hardy or that deacclimate later than average (McNamara and Hokanson 2010). Identifying populations with increased cold tolerance or deacclimation resistance can provide an advantageous starting point for improvement. Although midwinter hardiness was not significantly different, the variation in deacclimation between ‘Alice’ and ‘Alison’ in controlled freezing tests (Dirr et al. 1993) indicates that genetic variation for winter survival traits may exist in H. quercifolia. A similar pattern was detected between H. macrophylla and H. paniculata where the latter, in addition to being far hardier in midwinter, retained its ability to withstand cold temperatures longer into an experimental warming period (Pagter et al. 2008a,b, 2011a,b). However, H. paniculata deacclimated faster than H. macrophylla, suggesting that midwinter hardiness and rate of deacclimation may need to be different breeding objectives.
F. Breeding for Floral Characteristics
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).
G. Germplasm Resources
At least 48 cultivars of oakleaf hydrangea have been named and introduced (Table 1.1, Figure 1.8), many of which were selected directly out of the wild or from chance seedlings. Several have unique characteristics that would be useful as breeding material.
Table 1.1 Cultivars of oakleaf hydrangea including cultivar name (trade name or synonym in parentheses), plant and flower size, notes, and origin.
| 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 |