By Brian Glenn, PhD Candidate, Environmental Horticulture Department, UF/IFAS
Living in the southeast, many areas are dominated by thick and large stands of trees. They thrive in the wet, warm climate that persists for most of the year. Under that majestic live oak in the front yard, it is quite common to see a large expanse of St. Augustinegrass turf that completes the scene. As much as one-quarter of all managed turfgrass is grown under shaded conditions, and that percentage is probably much higher in residential landscapes. In many southern landscapes, a yard is not complete without a magnolia or crepe myrtle being placed in the middle. But do these elements always grow in harmony, or is there an unseen competition being waged?
The fact is that the warm season turfgrasses that we use for lawns in Florida require full sun for optimal growth. If adequate light levels are not achieved, a few things start to happen. The leaf blades usually begin to elongate rapidly, and will take on a spindly appearance. These changes occur because the turf is attempting to “grow” its way to more light. If light quantity remains low, the plants essentially become starved and start to thin out. At this point, weeds and other undesirable species might begin to move in, or it will remain bare and not uniform with the rest of the yard. Many communities have specific regulations on the number of trees that must be on a property, as well as prohibition of existing tree removal. If a homeowner does have dense shade in their landscape, what options are there if they want a lawn?
Recently, there have been numerous cultivar releases that are branded as being “shade tolerant”. Common claims are that they have better quality “when grown under 60% shade”, but what does this number really mean? Measurements that quantify light intensity are often made at one instantaneous moment in time, but the full story may be missed. Due to the sun’s movement across the sky, shade and light conditions changes throughout the day. Day length throughout the year can also impact overall light interception by the plant.
Greenhouse growers have been using a measurement known as daily light integral (DLI), which uses moles/m2/day to quantify light needs of greenhouse grown crops. Bedding plants grown in greenhouse often depend on day length for optimum flowering. According to Dr. James Faust, the DLI is a compilation of “total quantity of light delivered over the course of the entire day.” By accounting for changes in shade due to sun angle, a better evaluation can be made of how much sunlight is actually reaching the turfgrass on a daily basis.
Little research has addressed quantifying the DLI for use in turfgrass. Recently, this preliminary data has been collected from greenhouse studies at the Turfgrass Envirotron in Gainesville, Florida. Twelve commonly grown warm-season turfgrasses were evaluated to determine the DLI required to maintain minimum acceptable quality. These included cultivars from the main species grown as lawn grasses such as centipedegrass, St. Augustinegrass, bahiagrass, and zoysiagrass, as well as others that are marketed for their shade tolerance.
Table 1 represents an experiment that was performed in the summer months of July and August. Of the grasses evaluated, the three bermudagrasses had the highest DLI requirements. The turfgrasses with the lower DLI requirements included the St. Augustinegrasses and zoysiagrasses (both the Zoysia japonica and Z. matrella species). Two bermudagrass species that are being marketed as shade tolerant, ‘TifGrand’ and ‘Celebration’ appear to be about equal in their light requirements. Preliminary requests indicate that these two grasses need about 20% less light than ‘Tifway’ bermudagrass. Additional research is being conducted to solidify the actual DLI requirements for these turfgrasses, as well as how the DLI values actually translate to turfgrass grown in the landscape.
With preliminary data in-hand, how can this information be used by homeowners, landscapers, and turf managers? New technology is in development to make determining the amount of light in a landscape as easy as a push of a button. Spectrum Technologies has developed the DLI100 – a device that can be placed in the desired location and then left for a period of 24 hours. During this time, the instrument collects light quantity based on one complete day, and then returns a DLI range value. Of course, this number can vary due to cloud cover and other factors, so a few days should be used to get an average. Sites with existing turfgrass can also be evaluated to determine if a decline in quality is due to low light or another culprit. We are evaluating this instrument in our research program.
Continued research in the area of the effects of low light 7 should help us have a better understanding of DLI need of our warm-season grasses and their responses. With these results, sod growers and marketers will be able to help clients choose the most appropriate turfgrass for their site, improving the satisfaction of all parties involved.