Shade is a problem when growing a warm season turf like Bermuda or Zoysia. It is well known that cool season grasses are generally much more shade-tolerant. The real problem, though, is cool season grasses have a hard time with our hot summers.
Cool season grasses have the ability to be grown in low light conditions and still produce an acceptable turf. Warm season grasses, while more heat tolerant, are much more susceptible to decline from shade. Zoysia grass has the ability to combat some lower light conditions and still produce acceptable turf. However, very shady conditions will drastically decline the health of zoysiagrass - making the turf more susceptible to insect, disease and weed infestations. Bermuda grass has an extremely poor shade tolerance and will not survive long in low light conditions. So, where's the middle ground? What must we do to have the happiest looking lawn possible?
Let's get more technical now... Photosynthesis is a chemical reaction that provides energy to the plant in the form of carbohydrates. Cool season turf is more shade tolerant than warm season turf because their compensation point is lower. A lower-light compensation point means that photosynthesis is producing more energy than is being consumed by the turfgrass. Warm season grasses in the same environment will not be able to store enough carbohydrates from photosynthesis to remain healthy, which leads to decline and death of the turf.
So the question everyone has is, just how much light do warm season grasses need to survive? First, we need to know how the light is measured. Light for our purpose is measured as Photosynthetic Active Radiation, or PAR.
What's vital is the red and blue wavelengths in the visible light spectrum. Light blocked by a tree or building make PAR light unavailable to the turfgrass.
In recent years as technology has come along, some affordable light measuring devices have come onto the market. For simplicity, we recomend the Daily Light Integration Meter. You can pick up three (3) of these for about $200.00, and they average the PAR value over a 24 hour period. While not quite good enough for high end university research, it is a good estimator of your light quality or Daily Light Integrals (DLI) .
Let's list some numbers here:
For bermuda grass mowed at 2" for healthy turf, you need a value of about 61, with decline of the turf begining with light levels at 42. By raising your mowing height 1/2" you can change your numbers for healthy turf to about 32 with decline begining around 26. So, by raising your mowing height in shaded areas you can actually help your bermudagrass function with a lower light requirement. Keep in mind though, bermuda is a full-sun plant.
With zoysiagrass , the number for healthy turf is about 30 with decline begining at around 21.
I hope this little blog is helpful in "shedding some light" on turfgrass needs when it comes to light quality requirements!
Thanks to Logan Patterson and Travis Russell for their work on this at the University of Arkansas turfgrass department.
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