The genus Hamamelis L. (Hamamelidaceae R. Br., the witchhazel family) is represented by about six species distributed across the temperate regions of North America and Asia (Wen and Shi, 1999; Leonard, 2006). Although as many as 15 species are reported (Wiesrma, 2017), such as Hamamelis macrophylla Pursh. and Hamamelis mexicana Standl., morphological and phylogenetic analysis support a monophyletic clade of Hamamelis with six species (Li et al., 2000). Hamamelis species are large shrubs or small trees bearing characteristically narrow, strap-like flower petals and capsulate fruit that co-occur with flower buds and flowers. North American species include Hamamelis vernalis Sarg. (vernal or Ozark witchhazel) which is found in the Ozark Mountains of Oklahoma, Missouri, and Arkansas and in Texas; Hamamelis virginiana L., widely distributed in rich but dry woodlands from southern Canada into the eastern and central United States; and Hamamelis ovalis S.W. Leonard (bigleaf witchhazel), a new species represented by a few populations in Mississippi and Alabama (Leonard, 2006). Hamamelis vernalis is smaller than H. virginiana and is grown as an ornamental plant in U.S. Department of Agriculture (USDA) hardiness Zones 3–8 (USDA, 2012). It flowers from December to March and has fragrant, orange-red flowers. Hamamelis virginiana is a medium to large shrub producing lemon-yellow flowers from October to December. Hamamelis ovalis is a large-leafed, creeping shrub producing orange-red flowers.
Asian Hamamelis species include Hamamelis mollis Oliv. (Chinese witchhazel), and Hamamelis japonica Sieb. and Zucc. (Japanese witchhazel). Hamamelis mollis is a small, rounded shrub native to Central China, whereas H. japonica is a low, spreading or vase-shaped shrub distributed throughout Japan. A superior H. mollis open-pollinated seedling observed by the Arnold Arboretum proved to be a hybrid between H. mollis and H. japonica, and in 1963 the clonal cultivar Hamamelis ×intermedia Rehder ‘Arnold Promise’ was registered (Gapinski, 2014). This hybrid combined the dense, yellow blossoms of H. mollis with the cold-hardiness, larger petals, and less winter leaf retention of H. japonica. Because of the variety of form and color, and a longer flowering period, most named Hamamelis cultivars are Hamamelis ×intermedia. Of ≈186 named cultivars, 106 are Hamamelis ×intermedia. Hamamelis ×intermedia ‘Arnold Promise’ remains a garden standard (Dirr, 2009); other notable Hamamelis ×intermedia cultivars include ‘Barmstedt Gold’, ‘Jelena’, ‘Primavera’, and ‘Westerstede’ (Gapinski, 2014).
Production and adoption of witchhazel is often 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 crown collar. Other limitations of witchhazel, especially H. virginiana, include leggy, spreading forms, previous season’s foliage retention during flowering, and susceptibility to foliar diseases such as powdery mildew (Podosphaera biuncinata Cooke and Peck) and leaf blight (Phyllostichta hamamelidis Cooke ex G. Martin) 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). Ease of propagation and tolerance to foliar diseases in production would facilitate the adoption of an improved North American witchhazel.
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