SportsTurf provides current, practical and technical content on issues relevant to sports turf managers, including facilities managers. Most readers are athletic field managers from the professional level through parks and recreation, universities.
Issue link: http://read.dmtmag.com/i/318924
www.stma.org June 2014 | SportsTurf 19 associate with lower light, they begin to react and try to "grow out" of shaded conditions. This is usually seen as elongated, thin leaves, and can lead to unsightly scalping. If light levels are not increased, turfgrass quality will eventually begin to decline. Daily light integral, or DLI, is a method of measuring light that quantifies total light intensity accumulated during the course of a day. It is measured in moles of light per meter squared per day (mol/m2/ day). In the past, light has been reported in hours of full sun or percent shade. These are often vague as incoming solar radiation changes peri- odically due to sun movement, cloud cover, and changing shadows caused by objects such as buildings and trees. DLI is a more precise method to evaluate available light in a given location on the field, as it takes into account the dynamic nature of shade. To put DLI into perspective, the average summer ranges are 40-45 moles in the eastern U.S., and can get as high as 60 moles in parts of the southwestern US. These ranges can fall significantly during the winter months. In certain areas where warm-season turfgrass is grown year- round, ranges can drop to as low as 15 moles. If these levels are already marginal for growing a specific turfgrass in your area, reductions in light caused by shade can further impact turf quality and growth. By using some light-monitoring equipment, turfgrass managers can easily determine exactly how much light is falling on a particular site. The question becomes, how can this information be used to make more informed decisions about turfgrass management from a species standpoint? We set out to determine threshold light levels using DLI to maintain quality turfgrass. We also wanted to see how much tem- perature impacted these DLI requirements, so that managers could determine if the amount of light measured was adequate for their turf, no matter the time of the year. Greenhouse trials were conducted at the Turfgrass Envirotron at the University of Florida over 2 years to evaluate minimum DLI requirements to maintain acceptable turfgrass quality for twelve warm-season turfgrasses (Figure 2). Four treatments (0%, 30%, 60% and 90% shade) were used to develop a light gradient to determine the point at which turfgrass quality becomes unacceptable (Figure 3). These grasses were shaded for a period of two months. All treatments simulated either summer or winter average temperatures in south Florida (87 F and 74 F, respectively). When DLI requirements were calculated after the trials were com- pleted, there was a substantial difference between the summer and winter ranges (Table 1). The highest requirement from the grasses that were included was 22 moles, where that number dropped down to around 11 moles during lower temperatures. Turfgrasses in both temperatures were actively growing, but the samples in the cooler envi- ronment seemed to tolerate shade better. The answer can most likely be attributed to lower energy demands on the turf with lower temperatures, allowing the plant to maintain quality without as much light. M a n y o f t h e results when com- paring grasses were e x p e c t e d b a s e d on past research and observations. Bermudagrass had the highest light requirements, while the zoysiagrasses had the lowest. Some of the species that were selected for the studies are marketed for their "shade tolerance," including Celebration and TifGrand bermudagrasses. Now that we have an idea of the relative light requirements for differ- ent grasses, how can they be used? With the right tools, this information can help turfgrass managers establish a starting point when dealing with shade on their fields. One instrument that can be used is a small light sensor that measures DLI over a 24 hour period (Figure 4). After a few days of monitoring, the average DLI can be determined for the site. Multiple units can be used across a field if various microclimates exist. If the DLI is below the requirement for the given season and declines in turf quality have been observed, a different turfgrass species with a lower DLI requirement may be recommended. These values are an approximation for each of the species tested, but different factors can potentially alter DLI requirements for a spe- cific grass. Low mowing heights could lead to unacceptable turfgrass quality, even with an acceptable amount of light. Using a plant growth regulator (PGR) could lead to higher quality under lower light levels. Minimum acceptable quality may also not be acceptable on high profile sports turf, so these requirements may need to be adjusted according to expectations. When used for comparison purposes, these values can help managers determine if quality issues are a product of shade or if another possibility should be considered. Research using DLI is ongoing, including determining the effect of different mowing heights on DLI requirements within the same species. New information using DLI could potentially help manag- ers account for the effects of low light on turfgrass growth. Raising mowing heights, applying PGRs, and other cultural practices could be proactively altered to maximize turfgrass health and minimize negative effects due to shade and other reductions in light. ■ Figure 4. DLI100 Light Meter from Spectrum Technologies. Figure 1. Shade on bermudagrass Figure 2. Twelve warm-season turfgrass species under 30% shade Figure 3. Twelve warm-season turfgrass species