is not abundant.
IX. PROPAGATION
Hydrangea quercifolia is readily propagated from seed and vegetative cuttings. Seeds are extremely small (0.6 mm long; Hufford 1995) but require no stratification treatment in order to achieve germination. Surface sown seeds typically germinate in 7–14 days with or without a light covering, so long as seeds are not allowed to dry out or become buried under germination substrate (Halcomb and Reed 2012). Overall, germination rates are fairly low, on average 62% (Sherwood et al. 2019); however, this is within the range observed for H. paniculata (37–65%) and H. macrophylla (5–65%; Greer and Rinehart 2009). Additionally, seed germination significantly varies by mother plant and population in wild collected seed (Sherwood et al. 2019). Seed germination percentages can also be increased by germinating in a growth chamber rather than a greenhouse (Sherwood et al. 2021). Oakleaf hydrangea roots readily from softwood cuttings, although IBA may increase the rate of rooting (Halcomb and Reed 2012).
H. quercifolia can be propagated via tissue culture. When introducing explants into culture, microbial contamination is a widespread problem in Hydrangea (Kitamura et al. 2008; A. Sherwood, unpubl.). However, the use of dormant buds as explant material has been used successfully in H. quercifolia (Sebastian and Heuser 1987; Ledbetter and Preece 2004). Both type and concentration of cytokinin were shown to have significant effects on number and length of shoots induced in tissue culture, with 5 μM thiadiazuron (TDZ) inducing the highest number of shoots but also the smallest shoot size; therefore, intermediate concentrations (0.1–0.5 μM TDZ) are recommended to produce a moderate number of adequately elongated shoots (Ledbetter and Preece 2004). Additionally, inclusion of gibberellic acid in the medium can enhance shoot elongation when higher cytokinin concentrations are used (Sebastian and Heuser 1987). It was also determined that micropropagated shoots have a high ability to root; on average 93%. Sebastian and Heuser (1987) demonstrated that shoots derived from callus tissue had a greater ability to root than shoots derived directly from dormant buds; this difference was attributed to increased juvenility in the callus derived shoots. Conversely, Kästner et al. (2017) reported that tissue cultures of H. quercifolia declined in vigor after many repeated subcultures, and that it was necessary to reestablish explants periodically. Cochran et al. (2014) found significant (but inconsistently) improved branching and symmetry in container‐produced H. quercifolia ‘Alice’ which were propagated in tissue culture, compared to cutting propagated material.
X. FUTURE PROSPECTS
Hydrangea quercifolia is a particularly understudied and underutilized species with unique horticultural potential. Because it is unrivaled among hydrangeas for its year‐round aesthetic value and intriguing foliar characteristics, it can be described as “a shrub with a difference” (Lawson‐Hall and Rothera 1995). Hydrangea quercifolia has immense potential waiting to be developed. The amount of improvement seen in H. macrophylla, H. paniculata, and H. arborescens over the previous century serves as an example of the possibilities the genus has to offer. To date, progress has been made in selecting for compactness and floral traits, however, there are other traits which would benefit from improvement.
Resistance to biotic and abiotic stressors are still a priority and have not yet received enough attention. Cold tolerance and deacclimation timing are limiting factors in cold climates. This is currently being examined in the species across its latitudinal range which will reveal the extent of cold hardiness and variation for deacclimation timing that is available for selection. Resistance to wilt and foliar pathogens still needs to be studied in order for H. quercifolia to be robust enough to survive nursery production reliably and still be attractive in the landscape with minimal pesticide applications. Tolerance to heat and direct sunlight are important traits to identify for southern growers.
Another trait that has yet to receive any appreciable amount of attention is floral scent. The floral scent in wild oakleaf hydrangea plants is sweet and, although not strong, it is generally stronger than that of other Hydrangea species (A. Sherwood, pers. observ.). Because Hydrangea are typically not considered to possess fragrant flowers, a cultivar with a stronger fragrance would be quite significant. It is unknown what compounds make up the scent, nor what floral organs produce them. This trait would be interesting as a breeding target and likely plays a role in pollination in the wild.
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18 Deans,
