It is estimated that 20% to 25% of turfgrasses are maintained in some degree of shade from buildings, trees, or shrubs (Beard, 1973). Tree shade represents, arguably, the most stressful of these environments for the establishment and persistence of turfgrass, because it simultaneously reduces light intensity and alters the red:far-red ratio of sunlight (Bell and Danneberger, 1999; Wherley et al., 2005). Changes in microclimate (Giesler et al., 2000) and root competition (Whitcomb, 1972; Whitcomb and Roberts, 1973) associated with tree shade further exacerbate the situation. Shade-avoidance responses by turfgrasses often culminate in undesirable morphological changes, including increased leaf elongation, internode length, and plant height as well as decreased tillering and root:shoot ratio (Shnyder and Nelson, 1989; Tan and Qian, 2003; Wherley et al., 2005; Winstead and Ward, 1974). Compared with plants grown in full sun, shaded plants exhibit lower rates of photosynthesis (Alexander and McCloud, 1962), which leads to a reduction in total nonstructural carbohydrates (Burton et al., 1959; Shnyder and Nelson, 1989). As a result, stand density often declines over time as individual tillers die and cannot be replaced as a result of a lack of available photosynthate. Turfgrass persistence in shade is therefore commonly limited as a result of development of a weakened, thinned turfgrass stand with an increased vulnerability to damage from a host of factors, including traffic (Cockerham et al., 1994; Jiang et al., 2003), winter injury (Steinke and Stier, 2004), and disease pressure (Giesler et al., 2000; Vargas and Beard, 1981; Zarlengo et al., 1994).
Zoysiagrass is a warm-season turfgrass that is increasing in popularity throughout the transition zone and southern United States. It has gained recognition as a low-maintenance turfgrass, primarily because of its minimal nutritional and mowing requirements relative to other turfgrass species (Christians and Engelke, 1994). Traditionally used for lawns, the species possesses broad genetic and morphological diversity (Anderson, 2000), which has in recent years led to development of varieties that are used on golf course tees, fairways, and greens (Engelke et al., 2002a, 2002b). Relative to other warm-season species, zoysiagrass possesses good shade tolerance (Beard, 1973; Stier and Gardner, 2008) and therefore has developed a niche for use in shady areas of lawns and golf courses where less shade-tolerant varieties of turfgrass are unable to persist.
Minimal research has been published regarding the comparative performance of zoysiagrass cultivars in natural shade. Riffel et al. (1995) evaluated zoysiagrass shade tolerance under 90% tree shade and identified ‘Diamond’ and ‘Zorro’ as two of the top performers in their evaluations. More recently, Sladek et al. (2009) evaluated the growth response of six potted zoysiagrass ecotypes under two levels of artificial shade under greenhouse culture. The authors reported that under 50% shade, all ecotypes exhibited positive changes in plug diameter, although quality was not always deemed acceptable. Levels of 90% shade proved more challenging, because acceptable turfgrass quality was difficult to achieve and plug diameters actually declined for the many of the ecotypes. The authors found that ‘Diamond’ and ‘Shadow Turf’ were two of the more shade-tolerant zoysiagrasses in their study. It is not known to what extent these findings would be relevant to the natural shade environment, although the authors did note that further shade evaluation of zoysiagrasses under more practical field research conditions was needed.
The need to reduce environmental impacts from managed landscapes emphasizes the importance of identifying turfgrass species and varieties that can be managed with reduced inputs. Although data were recently published for cool-season turfgrass systems (Watkins et al., 2011), information on low-input warm-season turfgrass options are lacking. Commonly, as landscapes age, grasses must be capable of adapting to and persisting as a functional turfgrass system when subjected to increasing levels of tree shade. Gardner and Taylor (2002) examined the change in cool-season turfgrass species and cultivars over time when maintained as a low-input, shaded environment in the Midwest. Data of this type for warm-season systems are unavailable.
Although St. Augustinegrass is the most shade-tolerant warm-season turfgrass (Beard, 1973; Stier and Gardner, 2008), it can often require a higher level of cultural intensity than zoysiagrass in terms of nitrogen fertilization, irrigation, and mowing frequency (Christians and Engelke, 1994; Turgeon, 2002). Therefore, identification of zoysiagrass cultivars adapted for low-input, shaded environments where inputs are conserved is of increasing importance. The objective of this study was to compare over a 3-year period the shade performance of 10 commercially available zoysiagrass cultivars maintained as a low-maintenance groundcover in heavy tree shade.
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