Horticultural Reviews, Volume 49. Группа авторов

Figure 1.3 Map of historic herbarium specimens and recent samplings. Small open circles indicate approximate location of herbarium specimens collected between 1882 and 2017. Larger solid points indicate location of oakleaf hydrangea confirmed by the author in 2017 and 2018. Note: many of the herbarium specimen collection sites were also searched but Hydrangea could not be located; this is discussed in the Conservation section.

Figure 1.4 Photographs of Hydrangea quercifolia in its natural habitat. Left photograph shows representative seasonal streambank population. Center photograph shows flowering H. quercifolia growing out of bedrock. Right photograph shows small population growing out of near‐vertical bluff.

      (Source: Photo credits: A. Sherwood.)

V. MORPHOLOGY OF OAKLEAF HYDRANGEA

      The phenotypic diversity in H. quercifolia is less than that of other Hydrangea species. In her 1957 monograph of the genus, McClintock notes that oakleaf hydrangea is “remarkably uniform throughout its range” and a review of the available cultivars confirms this phenomenon (discussed in further detail below) (McClintock 1957). There is little variation in the growth habit other than overall size; dwarf forms are available, but all display a broadly mounded habit. Variation exists in floral characteristics to an extent, mostly in size of panicle and sepals, but also in coloration and flowering time.

Hydrangea quercifolia has opposite leaves with 5 to 7 lobes and pinnate venation, resulting in a superficial resemblance to the leaves of lobed oak (Quercus) species. The margins are coarsely serrate, and both leaf surfaces are pubescent with the abaxial surface more so than the adaxial surface. Apical buds and young leaves often have very high trichome densities and are therefore highly pubescent. Interestingly, H. quercifolia is the only Hydrangea species to have branched trichomes with tannin deposits (Stern 1978). Fall leaf color is variable according to genotype and environmental conditions, such as shade, and ranges from pale yellow to burgundy red.

      The stems are densely pubescent and coarse, developing exfoliating bark with age. The growth rings of H. quercifolia, like that of several other Hydrangea species, are semi‐porous; a distinguishing feature of H. quercifolia wood is the presence of axial xylem parenchyma (Stern 1978). Year‐old and older stems have abundant root initials or latent buds, which likely aid the plant in self‐propagation by layering.

In the wild, mature plants occasionally grow to greater than 5 m in height. Oakleaf hydrangea is often at least as wide as it is tall, and older plants tend to develop a spreading habit as the lower branches naturally propagate by layering. Wild plants tend to produce few to several clumps in close proximity to one another, spreading both by stem layering and by rhizomatic suckering (Figure 1.5). In the landscape, many cultivars can be 2 to 3 m in height, with dwarf cultivars reaching only 0.5 to 1 m at maturity.

Oakleaf hydrangea blooms from May through June in its native range, flowering on the previous season’s wood (Figure 1.6a). Inflorescences are panicles with fully fertile, perfect florets in the center that are partly covered by semi‐sterile florets with conspicuous petaloid sepals around the exterior (Figure 1.6b). Each fertile floret contains two stigmas and up to 10 stamens. Conspicuous florets indeed do possess all reproductive parts, albeit in reduced size. There are four orbicular conspicuous sepals which are typically white in color, fading to various shades of pink or brown as the inflorescence senesces. In a study of floral morphology in the tribe Hydrangeeae, Jacobs (2010) found H. quercifolia to have the highest number of conspicuous florets per inflorescence out of 36 taxa examined, with up to 50 per inflorescence. This is twice as many as the species with the next highest number, H. paniculata, having up to 25 conspicuous florets per inflorescence. In that study, H. quercifolia was found to be the only species to consistently produce more than 20 conspicuous florets per inflorescence. These data show the promising potential for H. quercifolia to serve as a striking landscape plant with large conspicuous inflorescences.

Figure 1.5 Photographs of Hydrangea quercifolia clonal habit. Each photograph displays the clumping nature of oakleaf hydrangea in the wild.

Figure 1.6 Stages of floral development: (a) young flower bud; (b) full flower;(c) senescing flower with developing fruit; (d) senesced flower with dehiscent fruit.

      (Source: Photo credits: A. Sherwood.)

      Hydrangea quercifolia fruit are dry dehiscent capsules, which fade from green during the growing season to brown at maturity when the seeds dehisce. Once pollinated, the conspicuous florets invert and the fruit begin to ripen (in Figure 1.6 compare the orientation of florets in 1.6b and 1.6c). The stigmas dry and form horn‐shaped appendages on the top of the capsule with an aperture between them to allow seed dispersal. Each fruit contains several seeds, which are relatively small, approximately 0.6 mm in length, with longitudinal striations (Figure 1.6d) (Hufford 1995). Seed color ranges from light tan to dark brown, being ellipsoidal to ovate in shape.

VI. BIOTIC AND ABIOTIC CONSIDERATIONS

Oakleaf hydrangea has the unique ability to flower in dense shade as well as tolerate full sun. The capacity to flower even in deep shade sets it apart from most other flowering shrubs (Dirr 2004). Nevertheless, blooming improves with sunlight, as does flower color on genotypes which become pink with age. Similarly, fall foliage color is also enhanced on sun grown plants. However, if planted in prolonged direct sunlight in hot climates, the leaves have a tendency to wilt and desiccate. Oakleaf hydrangea are more sun tolerant in landscapes located in cooler climates, although flowering may be less impressive under such conditions (Lawson‐Hall and Rothera 1995). In the wild, H. quercifolia tends to occur in shady forest understories and is rarely found in daylong full sun situations.

      Like most hydrangeas, H. quercifolia requires moist but well drained soil and will wilt in hot periods of the day when moisture is limiting; even when adequately watered, plants may still wilt when under extremely high temperatures unless protected from direct sun. Oakleaf hydrangea is susceptible to several root rot diseases, particularly in wet soil (discussed below), so care must be taken not to irrigate too frequently. Indeed, wild H. quercifolia most frequently occurs in well‐drained substrates such as cliff faces, steep ravines, and rocky outcrops (Figure 1.4).

      Limited information is available regarding cold hardiness in Hydrangea quercifolia. Dirr et al. (1993) used controlled freezing tests