Shade is a mood killer for warm-season turfgrasses. Sure, we try. We select for cultivars that will tolerate a little more shade, we prune the shrubbery, and southern homeowners unfailingly plant cool-season tall fescue under their trees because they read on the internet that tall fescue “likes shade.”
Still, when trying to put a quantitative number on the shade tolerance of various warm-season turfgrasses, we find we really do not have a lot of research for reference. So, a recent paper that studied the daily light needs of warm-season turfgrasses is certainly helping to cast some light on the topic.
Conducted by the folks at the University of Florida, this study was carried out in greenhouses located in Gainesville, Fla., with three separate experiments, all repeated. Trials took place in winter (January-March), summer (July-September) and spring (April-June and March-May), simulating different temperature and day length regimes for each season.
Twelve different turfgrasses were evaluated, including Tifway and Celebration bermudagrass, Sea Dwarf seashore paspalum, and JaMur and Diamond zoysiagrass. Centipedegrass, bahiagrass, and some other zoysiagrass and bermudagrass cultivars were also studied, but those listed here may be of most interest to the golf course management community.
Different levels of shade were imposed on all of these grasses, using black shade cloth to produce reductions in light of 33%, 61% and 92%.
How does one measure the amount of light a turfgrass plant needs? It’s measured as a daily light integral (DLI), which measures the light within photosynthetic active radiation (PAR) wavelengths. Light sensors are used to do this, and daily totals are recorded using units of moles/square meter/day. The researchers measured turfgrass quality and clipping yield, and used these results to determine the DLI at which turfgrass quality was reduced below acceptable standards.
Editor’s note: Find a rundown of the light requirements of various turfgrasses and get a look at some handy tools available for measuring light in Measuring light for healthier turf.
Overall, most of the warm-season turfgrasses maintained acceptable quality under 33% shade, except for Tifway bermudagrass, which had lower quality. The zoysiagrasses had good quality up to 61% shade, and there were no differences in quality based on the cultivar. Every turfgrass had poor quality at 92% shade, and there were no differences based on turf species or cultivar.
How did this shading translate to the DLI for these grasses? Well, grasses had a lower DLI in the winter (5.9 to 9.9 moles/square meter/day) than in the spring (9.4 to 17.4 moles/square meter/day) or summer (9.9 to 21.4 moles/square meter/day). For acceptable quality, Celebration and Tifway bermudagrass require about 20 moles/square meter/day DLI in the summer (Tifway does require more than Celebration), and this was the highest DLI of all grasses in the study. Zoysiagrass and seashore paspalum had lower requirements: 5.9 to 10.9 moles/square meter/day. Interestingly, St. Augustinegrass, which is often marketed to homeowners as a very shade-tolerant turfgrass, had the same DLI as zoysiagrass.
As a final note, the ability of golf course superintendents to measure DLI in real time and real life is pretty easy. A quick browse of the internet will reveal several companies that make affordable and reliable hand-held sensors that measure DLI and PAR. You can place these on putting greens or in tree lines to get quantitative information about the DLI at your course.
Source: Zhang, J., B. Glenn, J.B. Unruh, J. Kruse, K. Kenworthy, J. Erickson, D. Rowland and L. Trenholm. 2017. Comparative performance and daily light integral requirements of warm-season turfgrasses in different seasons. Crop Science 57:2273-2282 (https://doi.org/10.2135/cropsci2017.01.0052).
Editor’s note: Read all of Beth Guertal’s recent Verdure columns.
Beth Guertal is the Rowe Professor of Soil Fertility in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and past president of the Crop Science Society of America. She is a 22-year member of GCSAA.