Daily light integration — a new way to document shade issues

It has been estimated that more than 25 percent of all turfgrasses are exposed to some level of shade, with shade being produced by a variety of sources, including trees, hardscape structures and buildings. There are a number of factors that will contribute to whether a turfgrass can thrive in a shaded environment, including the species and cultivar planted, management practices that can be modified to enhance shade tolerance, and the presence or absence of other stress factors such as drought or traffic. Of the turfgrass species that are used worldwide, cool-season grasses such as tall fescue, Kentucky bluegrass and the fineleaf fescues are the most shade-tolerant grasses that we have available to us. For warm-season grasses, St. Augustinegrass and centipedegrass are adequately adapted to shaded environments, but are generally restricted to landscape turfgrass situations. The warm-season grasses, bermudagrass and zoysiagrass, are more widely adapted to a range of environments such as landscapes, athletic fields and golf courses. While zoysiagrass is relatively well-adapted to low light conditions, bermudagrass has very poor shade tolerance and will not survive in very low light conditions.

We often get questions from turfgrass managers related to shade, such as “is this site too shaded for a warm-season grass like bermudagrass” or “how much light does bermudagrass need?” Unfortunately, these are not always easy questions to answer because there are so many variables involved in shade tolerance and it can be difficult to quantify how much shade you actually have at your site. Fortunately, some of these answers are starting to become clear through research and there are also tools now available to turfgrass managers that will allow you to measure how much shade a site is actually receiving.

In relation to the first question regarding “is this site too shaded,” we need to be able to measure how much light the site is getting. Measuring light in an appropriate manner is one of the most important aspects of determining how much light is actually available to a turfgrass stand. When solar radiation passes through the atmosphere and strikes the earth, your skin, or a plant, it has a very broad range of radiation types (Figure 2). Some of the wavelengths are very short and have high energy like ultraviolet radiation, which can burn your skin if you get too much of it. Some of the light is made up of longer wavelengths that have lower energy and can include infrared waves. Infrared radiation can affect plants and is often used to signal changes in development such as flower production or seed germination. However, the most important radiation that comes from the sun in relation to plant growth is that radiation that is in the “visible” spectrum of light (400-700 nm) and will show up as one of the colors of the rainbow (red, orange, yellow, green, blue, indigo, and violet — i.e. ROY G BIV). When light passes through a tree or is blocked by a building, much of the light that ultimately reaches the turf will be in a longer wavelength and not available to plants for photosynthesis.

There are a number of types of meters available to measure light, but in order to get an accurate measurement of the light that is important for plant growth, you need to use a meter that only measures photosynthetically active radiation, or what is commonly referred to as PAR light (400-700 nm). In recent years, this technology has become more affordable and there are a number of different options that you can use to accurately measure PAR light (Table 1).

The first thing that you need to be aware of is that PAR light is measured in units of µmol/m2/sec. So, when you measure PAR light, you are only getting an instantaneous measure for that second. In order to really use the measurement that you get with a light meter, it is important to convert your measurements into a total light load per day or something that is commonly referred to as Daily Light Integral or DLI. Although there are some devices that will do this for you, such as the DLI meter listed in Table 1, with other devices you will need to make some simple calculations to get the total light load for a single day. For example, if you were using a hand-held meter and measuring the light once per hour during the day you could estimate the light load for each hour as following:

Meter Reading (µmol/m2/sec) x (60 sec/min) x (60 min/hour) = µmol/ m2/hour

If you take a reading once each hour during the day, you can do the same calculation with each reading and then sum all the readings for the day to get a total light load. The final number will be divided by 1,000,000 to convert µmol/m2/day into mol/m2/day, which is the total light load at the site or the DLI. An example of this type of measurement and calculation is given in Table 2.

Question — How much light does bermudagrass or other warm-season grasses need?

There have been a few published studies to date that have actually calculated the DLI requirements for warm-season grasses, and more studies are ongoing both at the University of Arkansas and the University of Florida and elsewhere. In a recent study at Florida (Glenn, 2012) the minimum light requirement for several grasses was calculated across two growing seasons (Table 3). In that study, more light was needed in the summer season than in spring and fall, but two of the bermudagrass varieties (Celebration and Tifgrand) needed less light to maintain acceptable quality than Tifway. You can also see from these results that grasses such as centipede, St. Augustine and zoysiagrass all had lower DLI requirements than bermudagrass. Remember, a lower DLI means better shade tolerance.

In a study conducted at Clemson University on a Tifeagle putting green (Bunnell, 2005), determined that this type of bermudagrass needed a daily light load of 32.6 mol/m2/day to sustain turfgrass quality. The differences between the Florida and Clemson studies are primarily due to intensity of management and clearly demonstrate that management practices such as mowing height will play an important role in how much light bermudagrass needs to be maintained in a shaded environment. It also points out that mowing heights should be raised in shaded environments to reduce the light requirement of the grass.

In an ongoing study at the University of Arkansas, we are looking at the performance of overseeded and non-overseeded bermudagrass in an athletic field environment in which we have four levels of shade including full sun, 30 percent shade, 60 percent shade and 90 percent shade. In Figure 3, you can see the average DLI for the 4 different light levels over 4 months in 2014. The performance of the bermudagrass in the 60 percent and especially the 90 percent shade treatments declined rapidly, as the DLI in those treatments was approximately 15 mol/day in the 60 percent treatment and 5 mol/day in the 90 percent treatment. The overseeded perennial ryegrass, which is a cool-season grass, continues to perform well in the 60 percent shade treatment and would have a lower DLI requirement than the bermudagrass. Based on these results and those from Florida, we would currently recommend a minimum DLI of 20-25 mol/m2/day to adequately maintain bermudagrass in a turf setting where the mowing height is 1 inch or higher.

Although research is beginning to quantify how much light is needed to sustain turfgrass species under varying management practices, more effort is needed to confidently apply these results to a range of turfgrass applications. However, even with the limited data that are currently available to turfgrass managers, understanding how to accurately measure the light received at a specific site is the first step in applying these results to management decisions.

References
Glenn, B., J. Kruse, and J. B. Unruh. 2012. Daily light integral requirements for twelve warm-season turfgrasses. Int. Ann. Meet. p. 72111.
Bunnell, B. T., L. B. McCarty, J. E. Faust, W. C. Bridges, N. C. Rajapakse, and W. C. Bridges. 2005. Quantifying a daily light integral requirement of a ‘TifEagle’ bermudagrass golf green. Crop Sci. 45(2):p. 569-574.