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Horticultural Reviews, Volume 49


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A set of three photos depict Hydrangea quercifolia in its natural habitat. Left photograph shows representative riverbank 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.)

      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.

      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.

A set of three photos depict Hydrangea quercifolia clonal habit. Photos depict the 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.

      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