Witchhazels are represented by about six species distributed across temperate regions of North America and Asia (Leonard, 2006; Wen and Shi, 1999). Although as many as 15 species have been reported (Wiersema, 2017), such as southern witchhazel (Hamamelis macrophylla) and mexican witchhazel (H. mexicana), morphological and phylogenetic analyses support a monophyletic clade of witchhazels with six species (Li et al., 2000). Witchhazels are large shrubs or small trees bearing characteristically narrow, strap-like flower petals and capsulate fruit that co-occurs with flower buds and flowers. North American species include vernal (or ozark) witchhazel, which is found in the Ozark Mountains of Oklahoma, Missouri, and Arkansas, and in Texas; common witchhazel, which is widely distributed in rich but dry woodlands from southern Canada into the eastern and central United States; and bigleaf witchhazel (H. ovalis), a new species represented by a few populations in Mississippi and Alabama (Leonard, 2006). Vernal witchhazel is smaller than common witchhazel and is grown as an ornamental in the U.S. Department of Agriculture (USDA) Hardiness Zones 3 to 8 (U.S. Department of Agriculture, 2012). It flowers December to March and has fragrant, orange–red flowers. Common witchhazel is a medium to large shrub producing lemon–yellow flowers from October to December. Bigleaf witchhazel is a large-leafed, creeping shrub producing orange–red flowers.
Asian witchhazel species include chinese witchhazel and japanese witchhazel. Chinese witchhazel is a small, rounded shrub native to central China, whereas japanese witchhazel is a low, spreading, or vase-shaped shrub distributed throughout Japan. A superior open-pollinated seedling from a chinese witchhazel observed by the Arnold Arboretum (Boston, MA) proved to be a hybrid between chinese and japanese witchhazel, and in 1963 the first hybrid witchhazel, ‘Arnold Promise’, was registered (Gapinski, 2014). This hybrid combined the dense, yellow blossoms of chinese witchhazel with the cold-hardiness, larger petals, and less winter leaf retention of japanese witchhazel. Due to the variety of form and color, and a longer flowering period, most named witchhazel cultivars are hybrid witchhazel. Of ≈186 named cultivars, 106 are hybrid witchhazel. ‘Arnold Promise’ hybrid witchhazel remains a garden standard (Dirr, 2009); other notable hybrid witchhazel cultivars include Barmstedt Gold, Jelena, Primavera, and Westerstede (Gapinski, 2014).
Production and adoption of witchhazel often is hampered by production difficulties and a displeasing, irregular, open form of many cultivars in the landscape. Seedling rootstocks are used for bud-grafting of desired cultivars, and rootstock selection is limited by the tendency of witchhazel to produce sprouts from the root collar. Other limitations of witchhazel, especially common witchhazel, include leggy, spreading forms, previous season’s foliage retention during flowering, and susceptibility to foliar diseases such as powdery mildew and phyllosticta leaf spot (Phyllosticta hamamelidis) in nursery production in the eastern and southeastern United States. Considerable improvement is warranted for quality and abundance of flower and the absence of foliage during the flowering period (Dirr, 2009). The objective of this study was to compare growth, flowering, and powdery mildew disease related responses of 23 witchhazel cultivars representing five species for nursery production and landscape value in Tennessee.
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