Cool season grasses lose a large proportion of their root mass when soil temperatures get above 75° (24°C). Root mass is also lost when soils become so wet that oxygen is depleted and the soil becomes "anaerobic".
One of the main functions of turfgrass roots is to absorb and translocate nutrients from the soil, into, and around the turfgrass plant. If the roots are not viable (healthy), they cannot uptake nutrients and the turf becomes nutrient deficient. This picture, taken this morning on perennial ryegrass turf, shows classic symptoms of nutrient deficiency, i.e. cholorosis and a general yellowing/browning in the turf canopy.
The only way to alleviate the problem and enourage root growth is to increase oxygen in the soil, either by merely waiting for the water to drain away, or by some form of soil cultivation (coring, deep tining etc.). Most importantly, do not irrigate already saturated turf - the practice of irrigating turns from a science into an artform during hot, humid weather. The ET rates are low and soils take a long time to dry, so careful soil moisture monitoring is needed.
Authors: Dr. John Street & Pamela Sherratt
Copyright OSU Buckeye Turf Program. Website manager: Dr. Tim Rhodus