Crapemyrtles (Lagerstroemia indica, L. fauriei, and L. indica × L. fauriei hybrids) are popular flowering shrubs and small trees in US landscapes in regions with hot summers in USDA plant hardiness zones 6 to 10 and heat zones 7 to 10 (AHS Heat Zone Map, 2009; USDA Hardiness Zone Map, 2009). Because the commonly grown species have barriers to production of seed lines with uniform traits (Pounders et al., 2006), commercial crapemyrtle production is based on asexual propagation of named clones (Byers, 1997). To serve various landscape objectives, numerous clones that vary in growth rate, plant habit, pest tolerance, and flower color have been selected over the past century (Dix, 1999). Heirloom cultivars were selected as chance seedlings based on unique flower color or growth habit (Egolf and Andrick, 1978). Otto Spring of Okmulgee, OK, and Donald Egolf at the U.S. National Arboretum (USNA) pioneered controlled breeding of crapemyrtles in the United States and many cultivars released during the past 40 years originated from their efforts. Research at the USNA indicated that L. fauriei was cold-hardy and resistant to powdery mildew, coupled with the discovery of a chance interspecific hybrid between this species and L. indica in Texas (Egolf and Andrick, 1978), led to the introgression of L. fauriei into the USNA breeding program as a source of powdery mildew resistance. After selection of powdery mildew-resistant interspecific progeny, breeding focused on half-sib and backcrosses to introduce resistance into predominantly L. indica germplasm. More than 20 cultivars with high levels of powdery mildew resistance (Egolf, 1967, 1981a, 1981b, 1986a, 1986b, 1987a, 1987b, 1990; Pooler, 2006; Pooler and Dix, 1999) have been released by the USNA. Several popular cultivars with unique flower colors were derived by continued breeding with Otto Spring's germplasm (Whitcomb, 1985, 1999, 2000a, 2000b, 2004; Whitcomb et al., 1984).
Crapemyrtle cultivars have been evaluated for a number of traits, including growth (Bolques and Knox, 1997; Cabrera and Devereaux, 1999; Creech et al., 1999; Knox, 1996; Knox and Norcini, 1991; Laiche and Anderson, 1996), cold tolerance (Davis and Fare, 1995; Hayes and Lindstrom, 1992; Lindstrom et al., 2002), nutrient use (Cabrera, 2003; Cabrera and Devereaux, 1998; Devereaux, 1996; Martin and Ruter, 1996), flowering (Bolques and Knox, 1997; Fare, 1984; Goi and Tanaka, 1976; Guidry, 1977; Pair and Parsons, 1996; Stimart, 1986), resistance to disease (Hagan et al., 1998; Holcomb, 1994; Knox et al., 1993), and insect feeding preferences (Cabrera et al., 2008; Mizell and Knox, 1993; Pettis et al., 2004). Most of these evaluations were based on data from one site over multiple years.
Size categories based on plant height at maturity were first proposed by Egolf and Andrick (1978). Johnson and Dix (1993) pointed out that “at maturity” is a vague term that can lead to misclassification without designation of the age of specimens when the size would be realized. Under the improved system, cultivars are divided into four size categories: dwarf cultivars (less than 1.5 m at 5 years), semidwarf (1.5 to 3 m), intermediate (3 to 6 m), and tree forms (greater than 6 m) based on expected height after 10 years for the larger three classes. The revised classification system has been widely used in extension publications and other references comparing crapemyrtle landscape traits (Byers, 1997, 2000; Franklin et al., 2006; Gill and Owens, 2007; Knox, 2008; Wade and Williams-Woodward, 2001; Williams et al., 2000). Classification into the various size categories appears to be based on height data reported in the published cultivar releases (Egolf 1967, 1981a, 1981b, 1986a, 1986b, 1987a, 1987b, 1990; Pooler, 2006; Pooler and Dix, 1999; Whitcomb, 1985, 1999, 2000a, 2000b, 2004; Whitcomb et al., 1984) and extrapolation for other cultivars rather than replicated side-by-side comparisons of cultivars at multiple sites. This implies that popular cultivars can be expected to perform in a similar manner in all areas of the United States where crapemyrtle is adapted. None of the growth studies were replicated at multiple locations to determine how sensitive observed traits were to different environments.
Plant phenotypes vary according to the environments where they are evaluated (Hill, 1975). Evaluation of plant traits in diverse environments produces an understanding of genotype × environment (G × E) interactions that affect performance of a particular clone. Such information is important to horticulturists and plant breeders for proper selection of the most appropriate crapemyrtle clone for a particular application. Generally, genotypes with consistent performance in many environments are preferred over those that excel in restricted environments.
Observation of crapemyrtle clones in multiple random environments within the normal range of hardiness would give a much better understanding of the ultimate landscape performance of a clone. This study reports the variation for three traits (week of leaf-out, week of first flowering, plant height) in 34 common crapemyrtle clones (genotypes) observed in four diverse environments within the southeastern United States. Additionally, variation and phenotypic stability were identified for the three traits among the clones.
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