Key Takeaways:

  • Shade is a problem for superintendents everywhere, but quantifying light levels at a given location and the light requirements for various grasses has been a challenge.
  • New technology makes it easier to measure the photosynthetically active radiation (PAR) reaching the turf and the daily light integral (DLI) in a given location.
  • Knowing the minimal daily light integral required to maintain acceptable quality (DLIm) for various grasses can indicate whether shade is an issue in a given area and guide tree removal or turf selection decisions.
  • This study identified marked differences in DLIm, which were affected by season, cultivar and growth regulation in the fairway-height bermudagrasses and zoysiagrasses studied.

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.

Approach

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.

During the study, plots were irrigated twice weekly to prevent wilt based on reference evapotranspiration from an onsite weather station. Plots were mowed one or two times weekly, depending on the time of year. Mowing was performed at a 0.75-inch fairway height using a walk-behind reel mower. Plots were further subdivided to receive either no trinexapac-ethyl (TE) or a monthly rate of 0.2 pounds of active ingredient per acre from May through September using a handheld boom sprayer. Preventative fungicides were applied during spring and fall months using granular fungicides. A 21-7-14 sulfur-coated urea fertilizer was supplied to all plots at a rate of 0.75 pounds of nitrogen (N) per 1,000 square feet every six weeks from May through September for a total annual application of 3 pounds N per 1,000 square feet.

PAR measurements were continually recorded using PAR sensors and data loggers mounted underneath shade structures. These data were used to calculate mean monthly DLI levels within each respective shade treatment. Turf quality data were collected twice monthly throughout the study. At the end of the study, regression analysis was used to determine DLIm, or minimal DLI thresholds needed to support acceptable turf quality in each entry. Mean DLI values recorded for full-sun treatments during summer months at the study site averaged 47 mol/m2/d. The shade treatments resulted in reduced summer DLI values that ranged from 28 mol/m2/d to as little as 5 mol/m2/d with increasing shade density.

Findings

For the purposes of this article, we will highlight DLIm of fairway-height turf for the final spring, summer and fall of the project, at which time grasses had been exposed to shade stress for nearly two full years. An effect of season on DLIm was observed across all cultivars, with higher DLIm generally required during summer compared to spring and fall seasons. The DLIm were generally higher for bermudagrass than for zoysiagrass cultivars, regardless of TE treatment. (Figures 1-3)

Spring DLIm

During spring, bermudagrass cultivars required between 16-25 mol/m2/d to achieve acceptable quality, with TE application resulting in lower DLIm for all cultivars. In the absence of TE, ‘Tifgrand’ showed the lowest DLIm (19 mol/m2/d) of any bermudagrass. TE-treated ‘Tifgrand’ and ‘Latitude 36’ each showed the lowest DLIm during spring (16 mol/m2/d). Overall DLIm reductions due to TE ranged from 16% to 36% in bermudagrass cultivars during spring.

Zoysiagrass culitivar DLIm ranged between 9-16 mol/m2/d during spring, and again, TE reduced DLIm in all cultivars. In the absence of TE, ‘Zorro’ showed the lowest DLIm (10 mol/m2/d) of any zoysiagrass. TE-treated ‘Zorro’ showed the lowest DLIm during spring (9 mol/m2/d). Overall DLIm reductions due to TE ranged from 8% to 20% in zoysiagrass cultivars during spring.

Summer DLIm

During summer months, bermudagrass cultivars required slightly higher DLIm compared to spring, ranging from 24-26 mol/m2/d. Also, while TE application slightly decreased (4% decrease) DLIm in ‘Tifgrand’, it had no effect on DLIm of any other bermudagrass cultivars. Regardless of TE application, ‘Celebration’ and ‘Latitude 36’ showed the lowest DLIm (24 mol/m2/d) of any bermudagrass cultivars during summer.

Zoysiagrass culitivar DLIm notably increased from spring to summer, with summertime DLIm ranging between 13-26 mol/m2/d. Unlike bermudagrasses, TE was effective at reducing summer DLIm in all cultivars, with the greatest DLIm reduction noted in ‘JaMur’ (35%). In the absence of TE, ‘Zorro’ and ‘Zeon’ showed the lowest summer DLIm (19 mol/m2/d) of any zoysiagrass. TE-treated ‘Zorro’ again showed the lowest DLIm during summer (13 mol/m2/d).

Fall DLIm

Similar to spring, DLIm of most bermudagrasses declined during fall compared to summer, ranging from 19-25 mol/m2/d. With the exception of ‘Tifway’, which showed a 16% decrease in DLIm due to TE, application of TE again had limited effect on reducing DLIm in bermudagrass cultivars. ‘Latitude 36’ and ‘Celebration’ (19 mol/m2/d) showed the lowest DLIm during fall, regardless of TE application.

Zoysiagrass culitivar DLIm were also reduced in fall compared to summer, ranging from 11-18 mol/m2/d. ‘Zorro’ showed the lowest fall DLIm, regardless of TE application (11 and 13 mol/m2/d, respectively for + and - TE). In the absence of TE, the medium-textured ‘JaMur’ and ‘Palisades’ zoysiagrasses showed slightly higher DLIm (17-18 mol/m2/d) than the finer-textured cultivars ‘Zorro’, ‘Geo’, and ‘Zeon’ (13-15 mol/m2/d). ‘Palisades’ showed the greatest benefit from TE application in fall, with 22% reduction in DLIm observed due to TE.

Summary

Shade stress is a widespread problem faced by superintendents throughout the world. However, characterization of light levels and the corresponding DLI for problem areas combined with the use of adapted cultivars can lead to greater long-term success in shaded environments. This shade study sought to determine DLIm of commonly used zoysiagrass and bermudagrass cultivars for golf course fairway and tee situations. It should be noted that although shade may be more of a seasonal occurrence in many situations, our shade treatments remained on the plots throughout the duration of the study – i.e., shade structures were not removed during dormancy months – and this may have produced a greater amount of plant stress than may be typical for some shade situations. Furthermore, the 0.75-inch mowing height used in our study may have been somewhat higher than that commonly used for these cultivars under fairway conditions. Lower mowing heights would be expected to lead to higher DLIm.

Our data showed that DLIm varied by season, with greater DLIm needed during summer than fall and spring months. TE application was beneficial in reducing DLIm, primarily in zoysiagrass cultivars. Minimal summer DLIm for fairway bermudagrasses, regardless of TE application, were generally highest in ‘Tifway’ (26 mol/m2/d), but relatively similar among ‘Tifgrand’, ‘Celebration’ and ‘Latitude 36’ which all had DLI requirements in the range of 24-25 mol/m2/d. In the zoysiagrasses studied, DLIm were lowest for ‘Zorro’ compared to other cultivars, and higher DLIm were generally found for the medium-textured ‘Palisades’ and ‘JaMur’ cultivars.

Knowledge of minimal DLI requirements for zoysiagrass and bermudagrass cultivars should allow for improved species and cultivar selection for shaded golf course fairway and rough environments and improved sustainability of golf course management. While shade reductions are not often uniform across the season, the data provided through this study should give superintendents seasonal DLIm thresholds to consider when assessing suitability of cultivars for low-light environments. Considering that the highest light requirements for these grasses are needed during summer, it would make sense from a practical standpoint to conduct DLI assessments during summer months. While it was not a focus of our study, future research should begin to address the effects of traffic on DLI requirements in shade, which may lead to increased DLI needs relative to those reported here.

Literature Cited

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Bunnell, B.T., L.B. McCarty, J.E. Faust, W.C. Bridges, and N.C. Rajapakse. 2005a. Quantifying a daily light integral requirement of a ‘TifEagle’ bermudgrass golf green. Crop Science 45(2):569-574.

Bunnell, B.T., L.B. McCarty, and W.C. Bridges. 2005b. ‘TifEagle’ bermudagrass response to growth factors and mowing height when grown at various hours of sunlight. Crop Science 45(2):575-581.

Chen, Z., B. Wherley, C. Reynolds, R. Hejl, and B. Chang. 2021. Daily light integral requirements for bermudagrass and zoysiagrass cultivars: Effects of season and trinexapac-ethyl. In press Crop Science.

Faust, J.E. and J. Logan. 2018. Daily light integral: A research review and high-resolution maps of the United States. HortScience 53:1250-1257.

Glenn, B., J. Kruse, and J.B. Unruh. 2014. Effect of mowing height on DLI requirements of warm-season turfgrasses. ASA-CSSA-SSA Annual Meetings Abstract 88070.

Meeks, M., A. Chandra, and B. Wherley. 2015. Growth responses of hybrid bluegrasses treated with and without trinexapac-ethyl under shade. HortScience 50(8):1241-1247.

Richardson, M., D. Karcher, and D. O’brien. 2019. What the tech? Measuring light for healthier turf. GCM Online. October.

Russell, T.R., D.E. Karcher, and M.D. Richardson. 2019. Daily light integral requirement of a creeping bentgrass putting green as affected by shade, trinexapac-ethyl, and a plant colorant. Crop Science 59:1768-1778.