as: “How much full sunlight do various turfgrass species and varieties need to grow?” “Is afternoon sun better than morning sun?” “Is four hours of shade/partial sun better or worse than two hours of full sun?” “How do we measure it?” and so on.
Light intensity varies by location, diurnal cycle, time of year, atmospheric conditions, cloud cover, shade, etc. Sometimes this is measured as photosynthetic photon flux density (PPFD) in units of mol m -2
s -1 , which provides
an instantaneous value for how much PAR is reaching a leaf ’s surface. However, the item of most interest to turfgrass researchers is how much photosynthetically active radiation (PAR) strikes a surface throughout the course of a day, season or year. Tis integrates PPFD into a term called the Daily Light Integral (DLI) and is much more useful for quantifying shade tolerance in plants. DLI also is useful because there are simple, inexpensive devices capable of measuring DLI that are commercially available to turfgrass managers.
Publicly available data on DLIs, such as the information in Figure 2, illustrate the amount of PAR striking the Earth’s surface at various points in the United States at different times of year. One can see from this data the substantial differences that exist based on location and season. For example, during the spring months there can be 46 percent more PAR in the desert southwest than the northeast United States and from January to August in Houston, TX, the amount of PAR increases by 142 percent. Tese types of baseline data are important for calculating percent reductions using light meters under various shade levels.
Tips for growing turfgrass in shade
• Select the appropriate species and cultivar. Fine fescue and Tall fescue offer the highest shade tolerance for cool-season lawns while St. Augustinegrass and Zoysiagrass provide the highest shade-tolerance in warm-season turfgrasses.
• Shade source matters: Trees are more likely to alter light quality (red light, blue light, red/far red ratios) than buildings or other structures which primarily reduce light quantity.
• Tree species matters: Evergreen trees provide more shade than deciduous trees.
• Mowing height: Mow turfgrasses on the upper end of their recommended height range.
• Fertilization: Reduce nitrogen applications. Excessive leaf growth comes at the expense of roots and other stored carbon sources.
• Traffic: Limit traffic when possible; turfgrasses under shade grow less vigorously and therefore won’t recover as rapidly as in full sun.
• Plant growth regulators: PGRs have been shown to increase turfgrass quality in closely- mowed turf when grown under moderate shade.
TPI Turf News January/February 2018
43
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