Mountain laurel is an evergreen flowering shrub in the heath family (Ericaceae) that occurs throughout temperate areas of the world (Jaynes, 1988). Mountain laurel is native to the eastern United States, specifically from southern Maine west through southern New York to central Ohio, south to eastern Louisiana, southern Mississippi, Alabama, Georgia, and northwestern Florida (Kurmes, 1967). The plant has been considered by many horticulturists, nursery growers, and gardeners to be an outstanding flowering native species (Dirr, 2009; Jaynes, 1988). Its attractive, lustrous green foliage; showy inflorescences; and variations in morphological traits have made mountain laurel a valuable ornamental shrub in the nursery and landscape industries (Jaynes, 1988).
Many cultivars of mountain laurel have been introduced into the marketplace, beginning in the early 1960s (European Kalmia Society, 2018). Yet there is still considerable uncertainty about these plants and their performance in areas south of U.S. Department of Agriculture (USDA) Zone 7, particularly in the southeastern United States. In the southeastern United States, few cultivars are available in retail settings and those cultivars that are produced are principally done so by niche, heath-family nurseries. Ironically, although produced in several southeastern U.S. nurseries, mountain laurel is rarely used in southeastern U.S. landscapes (Jaynes, 1988). The majority of mountain laurel cultivars were evaluated (before release) and selected for release in the northeastern United States. As a result, the performance of cultivars outside the northeastern United States is relatively unknown, which has historically limited cultivar acceptance in other regions of the United States (Jaynes, 1982, 1988). An example of this regionality in cultivar development and use is ‘Sarah’, selected by R.A. Jaynes at the Connecticut Agricultural Experiment Station. Although well adapted to northeastern U.S. landscapes, no documentation exists regarding its broader adaptability due to a lack of evaluation outside the northeastern United States.
Southeastern U.S. climate varies significantly from that of the Northeast. Milder winter temperatures lead to less cold damage in nurseries and landscapes of the southeastern United States. Yet hotter and more humid summer conditions, as well as a lack of winter chilling hours, could limit adaptability of cultivars selected in the Northeast and transitioned into southeastern U.S. nurseries and landscapes. Limited information on physiological and adaptive responses, including heat tolerance, photosynthetically active radiation tolerance, growth rates, disease and insect tolerance, required chilling hours to flower, soil moisture tolerance, for example, for most of mountain laurel cultivars is available (Jaynes, 1982, 1988). Additionally, although phenotypic traits of many mountain laurel cultivars have been well described at the time of release, these traits may differ across drastically different physiographic regions (Jaynes, 1988) due to genotype by environment interactions (Schlichting, 1986). This phenotypic variability was documented to occur in inkberry (Ilex glabra), whereby leaf traits varied when grown in varying environments (Givnish, 1984). This study evaluated 21 popular mountain laurel cultivars for their potential as both nursery and landscape plants. Our objectives were to provide comprehensive regional cultivar information for growers, landscape contractors, and breeders that could enhance mountain laurel production and landscape performance by identifying those cultivars with greatest adaptability to the southeastern United States.
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