Phil Busey Agronomy
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St. Augustinegrass: Shade

  

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Turf roots in water table  
   
 
   

Shade is a problem in growing St. Augustinegrass.  While the Floratam variety is notoriously poor in the shade, dwarf St. Augustinegrasses such as Seville, Delmar, and Jade have a degree of tolerance (Table 1).   Palmetto St. Augustinegrass also looks very intriguing for the shade, but there is no known scientific comparison data.  Whatever variety of St. Augustinegrass is used, a major problem is in knowing what we mean by "shade."
 The closed canopy of broad-leafed trees (e.g., black olive, citrus, ficus, and oak) transmits only 1 to 6% of sunlight, but turfgrasses generally need a minimum of 12 to 25% relative illumination (compared with full direct sun).
 You can use your camera as a relative light meter.  A single-lens-reflex camera normally has a light meter in the view finder.  Assuming that you keep a constant ASA setting (film "speed") and exposure (e.g., 1/30 second), the meter will usually tell you the F-stop, that is, the size of the diaphragm opening, and hence the shadiness.  F-stops are measured in unusual units, e.g., 2.8; 4.0; 5.6; 8.0; 11; and 16.  Every jump to a larger full unit, say from 4.0 to 5.6, involves a 50% smaller opening or aperture, because the camera has determined that there is 200% more light .  So in the bright sun your camera determines that a smaller opening (larger number) is needed compared with the shade.
 All you need is to find the number of F-stops difference between sun and shade.  So if the camera says 5.6 in the sun, and 4.0 in the shade, that's one F-stop difference, or 50% shade.  Two F-stops difference is 25% shade.  To do this properly, point your camera down to a constant background, such as the same piece of cardboard, placed under the trees, then move the cardboard and measure down on it again in full direct sun.  This way you will be measuring what light would hit the ground.  To be more precise, do this on a cloudy day, using as your "sun" area a clearing away from the trees.  By measuring on a cloudy day, you will not be bothered by light flecks.   If you find three F-stops difference, your illumination is only 12.5% relative illumination, compared with full direct sun, and you should probably not attempt to grow warm-season turfgrasses.   This procedure ignores light quality differences in the shade, but is close enough for an estimate.

Variety Turfgrass quality
Delmar 6.8
Seville 6.0
Jade 5.3
Bitterblue 4.3
Raleigh 4.0
Floratam 2.3
Means of two observations. 10=best, 7=acceptable, 1=worst
Palmetto was not available when this test was conducted.  The experiment was done under 83% shade in a mixed hammock of live oak and sabal palm at R & D Sod Farms, Okeechobee County, Florida.  (Data from Busey and Davis, 1991.)

There's more going on in the shade than reduced light.  Leaves grow long and are damaged by mowers.   There is less breeze and less need for water, but more chance for fungus and caterpillars.  The grass grows more slowly and needs less fertilizer.  If you are planting sod which was grown in the sun, it may take several weeks for all the old sun leaves to die off, and for the turfgrass plants to acclimate to their new location.   As sun leaves begin to die they will be eaten by fungus.  Excessive fertilization and watering of new sod make the leaves tender, and a weak fungus such as Pythium can be encouraged to move onto the live turf, destroying it.
 Besides gradually raising the tree canopy, which will be beneficial to the turf, the strategies for growing grass in the shade are to raise the height of the mower, reduce fertilizer, and carefully reduce irrigation.  Growing turfgrass in the shade is often not realistic, but many people want to try to have a nice carpet at their feet, and tree shade overhead.

 

References

Busey, P. and E. H. Davis.  1991.  Turfgrass in the shade environment.   Proc. Florida State Hort. Soc. 104:353-358.

Barrios, E. P., F. J. Sundstrom, D. Babcock, and L. Leger. 1986. Quality and yield response of four warm-season lawngrasses to shade conditions. Agron. J. 78:270-273.

McBee, G. G. 1969. Association of certain variations in light quality with the performance of selected turfgrasses. Crop Sci. 9:14-17.

McBee, G. G. and E. C. Holt. 1966. Shade tolerance studies on bermudagrass and other turfgrasses. Agron. J. 58:523-525.

Peacock, C. H. and A. E. Dudeck. 1981. The effects of shade on morphological and physiological parameters of St. Augustinegrass cultivars. p. 493-500. In R. W. Sheard (ed.) Proc. Fourth Int. Turfgrass Res. Conf., Guelph, Ontario, Canada. 19-23 July 1981. Ontario Agric. Col., Univ. Guelph and Int. Turfgrass Soc., Guelph, Ontario.