During prolonged periods of drought, high temperatures and wind, there is the strong likelihood of heat stress on your turf. Not only can it be aesthetically unpleasant, but it can also be a starting point for other issues such as a higher incidence of weeds, an increase in water runoff, evaporation and enhanced soil erosion. It appears as irregular-shaped dry patches or localised dry spots where the turf will first wilt then brown off all vegetative growth.
In addition to weather conditions other causes may include compacted soil, excessive thatch, poor irrigation coverage and the complete drying out of the soil causing hydrophobic conditions. Hydrophobic soils are water repellent and prevent water from infiltrating into the soil matrix; these soils don’t wet up uniformly following rain or irrigation, creating localised dry spots. As the turf becomes stressed evapotranspiration slows, reducing the plant’s own cooling system and allowing the leaf temperatures to rise—caused primarily by the plant transpiring more water than the roots can uptake. Excessive heat build-up in the soil is also a concern with the turf being able to handle high daytime temperatures, however, if the nights are also warm, soil temperatures remain high. This, coupled with excessive transpiration leads to the plants using up carbohydrate stores. These issues cause the roots to shrink affecting the water uptake of the plant. Heat hardiness is also reduced by high nitrogen levels and acidic soils which can affect nutrient availability and the balance of potassium and phosphorus in relation to the nitrogen.
When identifying heat stress make sure the symptoms are not caused by soluble salts, insects or fungal diseases, and be sure to check irrigation coverage. Ways to manage heat stress include syringing (applying small amounts of water an hour before maximum temperatures in low humidity situations to help with turf cooling by evaporation), minimising thatch, maintaining mowing frequency, develop healthy root systems and avoid high nitrogen applications over summer months. The most successful reactive and preventive approach is to apply wetting agents that help water penetration, increase soil moisture retention and enhance irrigation uniformity. There are several products on the market that have different attributes for all situations.