Oakleaf hydrangea (Hydrangea quercifolia Bartr.; Hydrangeaceae) is a woody shrub native to the southeastern United States, where it grows in calcareous soils in the understory of open woodlands (McClintock, 1957). There are ≈70 species in the genus Hydrangea with H. macrophylla (broadleaf or bigleaf hydrangea) accounting for the largest share of breeding and improvement work. Together, sales of Hydrangea species totaled over $73,000,000 in 2007 (USDA Census of Agriculture, 2007), making the genus one of the most economically important in the ornamental nursery industry.
Hydrangea quercifolia grows to 3 m or taller in height with a spread equal to or greater than the height as a result of vigorous spread by stolons (Dirr, 2009). Plants have an upright, small-branched, mounded habit. White to cream-colored conical inflorescences up to 30 cm long are produced in early summer. Inflorescences consist of large sterile sepals and inconspicuous fertile flowers. Sepals often turn an attractive pink or red color as they age. Mahogany red fall foliage and exfoliating bark provide fall and winter interest in the landscape.
At least 40 oakleaf hydrangea cultivars have been described (Dirr, 2009; Reed, 2010; van Gelderen and van Gelderen, 2004). Cultivars differ primarily in plant size and floral characteristics. Although most H. quercifolia cultivars reach at least 2 m in height, large and compact forms are available. The cultivars Alice and Alison were reported to reach 4 m and 3 m in height, respectively (Dirr, 2009). Compact forms are of special interest to the nursery industry for use as residential landscape plants. Two compact cultivars, Pee Wee and Sikes Dwarf, reach only to 1 m in height but lack some of the attractive floral traits of other cultivars. Two new compact selections were introduced in 2010 by the U.S. National Arboretum. ‘Munchkin’, a result of crossing two open-pollinated selections from ‘Sikes Dwarf’, grows to 0.9 m in height and possesses an attractive rounded habit with flowers that age to a deep pink (Reed, 2010). ‘Ruby Slippers’, an F2 hybrid between ‘Snow Queen’ and ‘Pee Wee’, is slightly larger and less rounded than ‘Munchkin’ with flowers held strongly above the foliage and aging to a deep rose red color (Reed, 2010).
Where most cultivars produce single flowers, ‘Snowflake’ produces hose-in-hose double flowers. Hydrangea quercifolia ‘Alison’ produces an equal mix of sterile and fertile flowers, ‘Snowflake’ is known for producing large and numerous sterile flowers, and cultivars such as ‘Harmony’, ‘Roanoke’, and ‘Vaughn’s Lillie’ produce only sterile flowers. The sepals of most varieties turn pink or brown with age. ‘Alice’ and ‘Amethyst’ develop some of the deepest pink colors with aging, whereas the sepals of ‘Ruby Slippers’ develop into a vibrant red color. Hydrangea quercifolia cultivars flower for ≈6 weeks in early summer, from approximately mid-May to the end of June in zone 7. Development of an attractive, landscape quality oakleaf hydrangea with a shifted bloom period would be an important addition to the appeal of this species in the landscape.
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