Editor's note: This post of the “Verdure” column by Beth Guertal, Ph.D., was published in the August issue of GCM.
Reptiles, spring-break undergraduates and hybrid bermudagrass ⎯ they all really like sunshine. In fact, all three of those groups would likely lie happily prostrate in sun for most of the day, given the chance. Hybrid bermudagrass, however, keeps getting shoved into shade, because we like trees on our golf course trees getting in the way of the shot, trees artfully arranged around the green, trees framing the hole in resplendent glory. Damn those trees.
Suffice it to say that hybrid bermudagrass and trees don’t get along all that well, and thus managing bermudagrass in the shade is a research topic of interest. The top research scientists that study bermudagrass and shade are in Dr. Bert McCarty’s research group at Clemson University. In 2005 they examined the combined effects of mowing height, growth promoters and various hours of sunlight on TifEagle bermudagrass.
Highest turfgrass quality was found in plots that received 12 hours of sun and added nitrogen, regardless of mowing height. As the amount of sunlight was reduced, turf quality decreased, although bermudagrass grown in eight hours of sun still had acceptable quality. If only four hours of sunlight was provided, turf quality was unacceptable, except when trinexapac-ethyl was applied and the turf was mowed at the higher height of 3/16 inch. In that case, the quality was equal to that found in some of the other treatments that received more sun.
Applying the additional ½ pound of nitrogen/1,000 square feet every two weeks also helped lateral regrowth of the bermudagrass, but only when the grass received 12 or eight hours of sunlight. Adding trinexapac-ethyl and mowing low produced the slowest lateral regrowth and, regardless of treatment, lateral growth was always poorest in the most shaded plots. The combination of only four hours of sun and extra nitrogen decreased total nonstructural carbohydrates, and the addition of trinexapac-ethyl or gibberellic acid did not improve total nonstructural carbohydrates.
Basically, differences in TifEagle bermudagrass quality, lateral growth, chlorophyll content and total nonstructural carbohydrates were fewer when sun was supplied at 12 or eight hours a day. So, a little bit of shade did not horribly affect the bermudagrass. Things got ugly and turf quality declined substantially when sunlight was reduced to only four hours a day. At that level of sunlight, applications of trinexapac-ethyl and a higher height of cut (3/16 inch) were needed to maintain any acceptable level of turf quality.
It’s important to remember that this study ran for only three months in each summer, and then the shade covers were removed. Thus, the results may not fully reflect what can happen to a putting green that is shaded for most of a growing season, nor would they account for additional stresses such as tree root competition. Regardless, this research was an excellent starting point to clearly show that extra nitrogen and/or trinexapac-ethyl, and increased mowing heights, can go a long way in helping bermudagrass handle shade.
Source: Bunnell, B.T., L.B. McCarty and W.C. Bridges Jr. 2005. ‘TifEagle’ bermudagrass response to growth factors and mowing height when grown at various hours of sunlight. Crop Science 45:575-581.
Beth Guertal is a professor in the department of agronomy and soils at Auburn University in Auburn, Ala., and the incoming editor-in-chief for the American Society of Agronomy.
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