Shade is arguably one of the most ubiquitous abiotic stresses faced by golf course superintendents around the world, and selection of the best-adapted turf species and cultivars for low-light environments is key to long-term success at many courses. In the past, this has been challenging due to difficulty in quantifying light levels throughout a golf course as it relates to the light requirements of specific grass species or cultivars. The complexity and differences among shade environments can make it difficult to specify a minimum light requirement in terms of hours per day or percent shade that can extend across different situations. Biologically speaking, rather than responding to the number of hours of sunlight or percent shade level, plants ultimately respond to the cumulative daily number of photons – measured in moles per square meter per day (mol/m2/d) – they receive within the photosynthetically active wavelengths of 400-700 nanometers (nm), known as the daily light integral (DLI). For reference, in Houston, Texas, ambient DLI levels in full sun fluctuate from approximately 45 mol/m2/d during summer to less than 20 mol/m2/d during the winter months. In sunnier areas such as the desert southwestern United States, DLI may approach as high as 65 mol/m2/d during summer months (Faust et al. 2018). Furthermore, minimum DLI requirements needed to support acceptable quality (DLIm) for a particular cultivar may not remain constant throughout the year, sometimes varying by month and temperature.
With the recent development of relatively inexpensive and easy-to-operate DLI meters, superintendents can readily determine the approximate DLI levels within particular areas of their course. Furthermore, methods are available for using multiple instantaneous handheld photosynthetically active radiation (PAR) sensor readings taken over the course of a day to estimate DLI for a given site (Richardson et al., 2019). Numerous apps are also now available that allow turf managers to predict the sun’s path for a given month of the year, and thus, selectively remove only limbs or trees that may be of concern based on the DLIm of the turf. Use of DLI data along with these types of technologies can aid the superintendent in making data-driven decisions about tree pruning, tree removal or turf cultivar selection.
There has been growing interest by turfgrass researchers in quantifying DLIm (Baldwin et al., 2009; Bunnell et al. 2005a; Bunnell et al. 2005b; Chen et al., 2021; Glenn et al., 2014; Meeks et al., 2015; Russell et al., 2019), however, published field-study data of this type for warm-season fairway and rough cultivars have been limited. The goal of this research was to determine seasonal DLIm in five zoysiagrass and four bermudagrass cultivars commonly utilized on southern and transition zone golf courses. The research also sought to assess the effects of monthly growth regulator application on DLIm.
A two-year field study was conducted at Texas A&M University in College Station, Texas, during the 2016-2017 growing seasons. A 15,000-square-foot shade research facility was constructed to accommodate replicated shade treatments offering 0% to 90% reductions in PAR. Turfgrasses utilized in this project included cultivars of bermudagrass and zoysiagrass commonly used on golf courses in the southern United States. The study was arranged in a completely randomized design with four replicate plots per treatment and six density-neutral shade levels that provided approximately 0%, 30%, 50%, 70%, 80% and 90% photosynthetic photon flux reductions. Plots were established from washed sod in July of 2015 and given six weeks to establish under full sun conditions before shade structures were moved onto the plots. Shade structures remained on the plots through the duration of the project (August 2015 to November 2017), including winter months, and were only removed for short periods for routine maintenance and data collection.