Horticultural Reviews, Volume 49

Oakleaf hydrangea does not have any particular insect pest issues, however generalist nursery and landscape pest insects will sometimes feed on H. quercifolia . Japanese beetle ( Popillia japonica Newman) has been noted to be the most abundant insect feeding on H. quercifolia in our studies at the University of Minnesota (A. Sherwood, unpubl.). Mmbaga and Oliver (2007) demonstrated that kaolin powder is as effective at controlling Japanese beetle feeding on H. quercifolia as a broad‐spectrum insecticide. This would be an option for use in nursery or greenhouse settings when additional pest defense is needed. However, kaolin powder may not be suitable for landscape use considering the unsightly appearance of the leaves after application.

Figure 1.7 Photograph showing range of bacterial leafspot ( Xanthomonas campestris ) severity in oakleaf hydrangea.

( Source : Photo credit: A. Sherwood.)

Genome size and ploidy have been well studied in Hydrangea . Although there is some speculation that the ancestral base chromosome number is n = 9 (Cerbah et al. 2001), the current base chromosome number in Hydrangea is n = 18, with most species (including H. quercifolia ) having 2n = 2x = 36 chromosomes (Van Laere et al. 2008). Exceptions to this include H. aspera and H. involucrata , which have 2n = 34 and 2n = 30, respectively (Mortreau et al. 2010). Another exception to this is H. platyarguta Y. De Smet & Granados, which has 2n = 2x = 24 chromosomes (Qiu et al. 2009). Cerbah et al. (2001) found that the North and South American hydrangeas had the smallest genome sizes with H. quercifolia being the smallest of all species investigated (1.95 pg 2C DNA; 1.9 Gb). Zonneveld (2004) reported similar findings, noting H. quercifolia with the smallest genome at 2.17 pg 2C DNA (2.1 Gb). However, H. seemannii has been reported to have a similar genome size at 2.09 pg (2.1 Gb) (Cerbah et al. 2001).

Although most Hydrangea are diploid, higher ploidy levels are not uncommon in the genus. Triploidy has been determined to arise from unreduced gamete production in H. macrophylla and triploid plants were noted to have an increased stem thickness and a decreased number of flowers (Jones et al. 2007; Alexander 2017). While, no naturally occurring tetraploid H. macrophylla have been documented, induced polyploidy has recently been reported in H. macrophylla (Deans et al., 2021) and H. febrifuga (syn. Dichroa febrifuga ), which is the one of the most closely related taxa to H. macrophylla , has been determined to exhibit diploid, tetraploid, and hexaploid cytotypes (Rinehart et al. 2010). H. paniculata has also been documented in the diploid, triploid, tetraploid, pentaploid, and hexaploid states with tetraploidy being the most common (Cerbah et al. 2001; Funamoto and Ogawa 2002; Zonneveld 2004; Beck and Ranney 2014). Oakleaf hydrangea has thus far only been documented in the diploid state; however, morphological traits could potentially be enhanced by induced polyploidy. Polyploidy can be induced using antimitotic chemicals or by the strategic use of unreduced gametes, although no H. quercifolia genotypes have been reported to produce 2n gametes.

Gametophytic self‐incompatibility seems to be widespread throughout Hydrangea (Reed 2000, 2003, 2004, 2005; Mortreau et al. 2003). Reed (2000, 2004) reported the capacity of self‐pollen to germinate on H. quercifolia stigmas, but not grow long enough to reach the ovaries, except in a very low percentage of cases. Additionally, it was shown that cross‐pollen typically outcompetes self‐pollen when present. The stigmas are receptive to pollen from the day after anthesis until five days post‐anthesis (Reed 2004). The pollen tube grows to the bottom of the style within 48 h after pollination and has fertilized the ovule within 72 h of pollination. It has been shown with H. arborescens and H. macrophylla that pollen is able to be stored at –20°C for at least three months with only a marginal decrease in viability (Kudo and Niimi 1999). However, viability of stored pollen has yet to be confirmed empirically in H. quercifolia .

Breeding for root rot resistance should be a priority in H. quercifolia , considering the disease is often lethal. No resistance genes have been reported to date, but by screening diverse, wild germplasm, the possibility exists to find tolerant material. Opportunities for high throughput screening for resistance to Phytophthora has been demonstrated in crops such as Nicotiana , Solanum , and Lycopersicon using culture filtrates incorporated into a tissue culture medium (Behnke 1979; van den Bulk 1991). Although promising, more information about the host–pathogen interaction is needed in order to implement such procedures in H. quercifolia . Because Phytophthora is a vascular disease and may not use a chemical toxin as a pathogenic 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.

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.

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).