Fothergilla L. spp. (fothergilla or witch-alder; Hamamelidaceae R. Brown) are exceptional garden plants (Clark, 1987; Dirr, 1998; Flint, 1984; Weaver, 1971) that display showy, white, fragrant flowers in a terminal spike that resembles a bottlebrush. Summer foliage color can be dark green to blue-green with fall foliage ranging from and including multicolored combinations of yellow, orange, maroon, and scarlet. Fothergilla have few pest problems, and they tolerate a broad range of climates (USDA hardiness zones 4–9), soil types, and shade. As a result, Fothergilla have become valuable nursery and garden plants.
There are two species of Fothergilla: F. gardenii Murray and F. major Lodd. Both are native to the Southeastern United States. Fothergilla gardenii is found in wet savannas and pocosins in the coastal plains of North Carolina, South Carolina, Georgia, Florida, and Alabama (Flora of North America Editorial Committee, 1993+; Weakley, 2006; Weaver, Jr., 1969). This species generally is smaller in stature (3–10 dm) than F. major and is distinguished sometimes by smaller leaves ranging from 1.9 to 6 cm long and from 1.3 to 5.2 cm wide that are generally toothed only on the upper half and symmetric at the base. Stipules are 1.5–4 (6.1) mm long. Stamens generally number from 12 to 24. The hypanthium at anthesis ranges from 1.5 to 2.6 mm wide and from 0.9 to 1.5 mm deep. Cytology determined a chromosome number of 2n = 4x = 48 (Weaver, Jr., 1969). In contrast, F. major is found on upland sites in the piedmont and mountains of North Carolina, South Carolina, Georgia, Alabama, Tennessee, and Arkansas (Flora of North America Editorial Committee, 1993+; Weakley, 2006; Weaver, Jr., 1969). This species generally is larger in stature (7–65 dm) than F. gardenii and is distinguished by larger leaves ranging from 2.5 to 13 cm long and 4.2 to 12.5 cm wide that generally are toothed from below the middle and conspicuously asymmetric at the base. Stipules are 2.8–7 (10.2) mm long. Stamens generally number (18) 22–32. The hypanthium at anthesis ranges from 2.4 to 3.9 mm wide and from 1.5 to 3 mm deep. Cytology determined a chromosome number of 2n = 6x = 72 (Weaver, Jr., 1969). Although there is no known diploid species of fothergilla, Parrotiopsis (Niedenzu) C. Schneid. is a closely allied genus with 2n = 2x = 24 (Goldblatt and Endress, 1977; Li and Bogle, 2001; Weaver, Jr., 1969) and may represent a parallel lineage from an ancestral diploid.
Often, the two species of Fothergilla are confused, but they can be separated by comparing key characteristics (Clark, 1988). Also, there has been speculation that the two species of Fothergilla hybridize (Dirr, 1998). Hybrids between these species should have a chromosome number of 2n = 5x = 60. Microscopic determination of chromosome numbers is not a practical approach for separating species and hybrids among large numbers of cultivars. However, flow cytometry can provide a fast and accurate determination of nuclear DNA content that is related directly to ploidy level (among closely related taxa) and can be used as a taxonomic tool (de Laat et al., 1987; Doležel, 1991; Doležel et al., 1998; Galbraith et al., 1983).
The objectives of this research were to verify the existence of hybrids between F. gardenii and F. major and to clarify the proper taxa designations for clones of Fothergilla commonly grown in the nursery industry.
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