Because of recurring drought and increased demands for potable water throughout the United States, municipalities are enacting landscape irrigation restrictions to conserve water. Homeowners and turf managers are often the first to be affected by restrictions on landscape irrigation. Commonly, these restrictions include a variance period of ≈35 d postplanting, during which the homeowner or turf manager may water as needed to facilitate establishment (San Antonio Water System, 2015; Southwest Florida Water Management District, 2015). During this time, establishment practices promoting rapid development of a deep and expansive root system may help ensure long-term success of the turf as irrigation is scaled back.
Within Texas and across the southeastern United States, st. augustinegrass is a predominant warm-season turfgrass used for lawns because of its ability to withstand heat and tolerance to shade (Duble, 2013). St. augustinegrass cultivar Floratam is widely used throughout the southeastern United States for home lawns because of its good drought resistance (Duble, 2013; Siefers and Beard, 1999). Steinke et al. (2010) reported that ‘Floratam’ exhibited significantly delayed leaf firing as well as superior turfgrass quality and green groundcover relative to other st. augustinegrass cultivars during a 60-d drought in San Antonio, TX. In the same study, ‘Floratam’ also demonstrated superior recovery attributes as compared with other cultivars following resumption of irrigation.
Genetic variation in st. augustinegrass manifests in morphology, adaptability, and stress tolerance (Busey 2003; Chandra et al., 2015). Such genetic variation is observed to be partitioned between ploidy levels. Diploids tend to exhibit superior turfgrass quality because of their finer leaf texture, bright green color, and compact growing habits, whereas polyploids such as ‘Floratam’ have been shown to exhibit increased resistance to insects, diseases, and drought stress (Busey, 1986, 2003). Genovesi et al. (2009) reported the use of embryo-rescue technique to develop interploid hybrids between polyploid and diploid germplasm of st. augustinegrass to facilitate gene exchange between different ploidy levels. ‘TamStar’ (formerly tested under the experimental name DALSA0605), is the first and only reported embryo-rescue-derived interploid cultivar of st. augustinegrass and was recently released by Texas A&M AgriLife Research (Chandra et al., 2015).
Much of the research involving rooting and drought resistance in established turfgrasses highlights the importance of rooting depth and density as factors contributing to drought avoidance (Carrow, 1996; Marcum et al., 1995; Miller and McCarty, 1998; Steinke et al., 2011). Busey (2003) reported that drought resistance in st. augustinegrass was likely not because of reduced evapotranspiration but rather because of wilt avoidance resulting from deeper and more extensive root systems. Huang and Gao (2000) examined developmental characteristics of roots of tall fescue (Lolium arundinaceum) and reported that a general root decline occurred in the upper 20 cm of soil as the drought stress ensued. However, drought-resistant cultivars in the study developed increased root length within the 40- to 60 cm depth, which reportedly aided plants by increasing water uptake from greater depths and compensated for decreased water uptake near the soil surface. Similar observations have been reported by Huang et al. (1997) as well as Sharp and Davies (1985) for warm-season turfgrasses and maize (Zea mays). To promote deeper and more extensive root systems, it has generally been recommended to increase the height of cut in managed turfgrass situations. As mowing height is reduced, the development of the root system is reduced through a root–shoot compensatory effect (Liu and Huang, 2002; Madison, 1962; Salaiz et al., 1995; Trenholm et al., 2000). Although increasing the mowing height during a 35-d establishment period would likely encourage increased root development, withholding mowing altogether could conceivably produce even greater and more rapid benefits, so long as turf quality was not significantly sacrificed once mowing commenced at the end of the establishment period.
Less research has been focused on agronomic factors related to root development during early sod establishment. Amthor and Beard (2014) evaluated effects of transplant timing, receiving soil texture and moisture content, and nitrogen–phosphorus–potassium (N–P–K) fertilizer amounts and placement (surface applied vs. soil incorporated) on root growth and anchoring strength of ‘Tifgreen’ hybrid bermudagrass (Cynodon dactylon ×C. transvaalensis) measured after 30 d of sod establishment. The authors reported that delaying sod harvest/transplantation by 1 month after the end of shoot dormancy increased root anchoring strength 200% and root dry weight 640% 30 d after sodding. Furthermore, there was no effect of receiving soil moisture or N–P–K fertilizer amount or placement on sod root anchorage. Wherley et al. (2011) studied effects of cutting height and nitrogen application rate on root development of four warm-season turfgrasses during a 70-d establishment period in Florida. The authors reported that increasing N rates during establishment resulted in greater rates of root extension into deep soil; however, cutting height had no effect on root extension in three of the four species, including ‘Floratam’. However, shorter mowing heights reduced total weight in all species during the establishment period.
PGRs are widely used in turf management for reducing shoot growth and frequency of mowing and/or seedhead suppression. TE is a commonly used PGR that reduces turfgrass shoot growth by inhibiting biologically active forms of gibberellins (King et al., 1997; Turgeon, 2002), but has shown little to no effect on seedhead suppression in st. augustinegrass (McCarty et al., 2004). TE has been shown to have little to no effect on photosynthesis (Qian et al., 1998) and to lead to reduced rates of maintenance respiration (Heckman et al., 2001). As such, it is believed that greater net photosynthesis may occur in TE-treated plants, with excess photosynthate not used for leaf elongation allocated to other organs (Ervin and Zhang, 2008). In fact, Fagerness et al. (2004) found that hybrid bermudagrass allocated 50% more N with an atomic weight of 15 (15N)-labeled ammonium nitrate to roots and rhizomes when treated with TE. This suggests that the altered allocation of nonstructural carbohydrates may promote greater rates of tillering, stem growth, or rooting during the period of TE suppression (Ervin and Zhang, 2008). Although it is currently labeled for use on a range of warm-season turfgrasses including st. augustinegrass, limited published information is available regarding the effects of TE on st. augustinegrass growth and development. Weinbrecht et al. (1998) reported the growth responses of st. augustinegrass to five commercial PGRs and noted that root length density was unaffected by PGR application. Information on the effects of TE application and mowing practices during early establishment of st. augustinegrass would be of interest to sod producers and turf managers as tools for potentially improving success of newly planted sod in the context of tight establishment timelines.
The objectives of this greenhouse study were to 1) evaluate the effects of mowing and TE application on shoot growth, final turf quality, and root development characteristics (weight, total length, and extension rate) of st. augustinegrass sod during a 35-d establishment period, and 2) compare the quality and rooting potential of ‘TamStar’ and ‘Floratam’.
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