As we head toward the start of the new summer season there are several factors turfgrass managers need to be prepared for. Combinations of heat, drought, wear, and disease are responsible for most of the stress and injuries we must contend with during summer. Heat and drought stress are the two we’re concerned with today. These are major limiting factors in the growth and quality of turfgrasses and they are becoming more relevant as water is becoming increasingly limited for irrigation and temperature increases with global warming. These stress factors on their own can lead to a decline in turfgrass quality and detrimentally impact turfgrass health but when combined can have devastating effects. Heat and drought stress are more common with cool-season turfgrasses, but warm-season turfgrasses, as well as other drought-tolerant turf species can also be susceptible to injury.
Cool-season grasses grow most actively within the temperature range of 60 to 75 ° F (16 to 24 °C) and temperatures exceeding this can lead to a reduction in metabolic rate and a decline in turf health. Warm-season grasses tolerate a higher temperature range of 80 to 95 ° F (27 to 35 °C) but suboptimal temperatures can also limit their growth. In both turfgrass types, heat stress injury is associated with photosynthesis inhibition and various other physiological changes such as limited water and nutrient uptake and hormone synthesis. Wilting, heat tracking, and browning of turf can be observed, and the grasses can become dormant to conserve energy. While in dormancy, turf is more susceptible to weeds, pests and various diseases. However, even though it may appear dead, it will come out of dormancy and green up with regular watering once temperatures begin to drop.
High air temperature is not the only factor contributing to heat stress as excessive rootzone temperatures can also be very detrimental to turf health. For example, when soil temperatures exceed 70°F (21°C) root mass in cool-season grasses can be negatively impacted by more than 50%.
Both cool- and warm-season turfgrasses perform best at specific temperatures, and ensuring soil moisture is adequate is vital, as without moisture in the soil, there’s the risk of drought stress and the synergistic effect of heat stress and drought can prove devastating.
Drought stress goes hand in glove with heat stress and although warm-season grasses can withstand the heat of summer when compared to cool-season grasses, both are subject to drought stress and injury without water. Drought stress occurs when the amount of water lost in transpiration is greater than the amount of water available and will lead to a decrease in photosynthesis and plant respiration, and an increase in plant temperature. Combined with excessive temperatures, the risk of death is significant. Symptoms of drought stress can vary depending on the turfgrass species and the severity of water limitation. Typical symptoms would be turf discoloration, leaf blades folding lengthwise, giving the appearance of thinner turf, the cell walls losing elasticity resulting in foot printing.
That’s the problem, how can we limit the damage?
Phosphites and Silica have proven abilities in this area.
Phosphite: will prime stress defences in turfgrass enabling a better response to abiotic challenge.
Silica: There are numerous published data confirming the positive effect Silica has on reducing drought stress in many plant systems.
Plant growth regulators (PGR) research suggests that PGR treatment can enhance turf performance during prolonged periods of stress. The beneficial effects of PGR on turfgrass stress responses could be related to the maintenance of photosynthetic activities and greater level of cellular hydration.
Cultural practices can be amended to reduce turf stress, procedures such as reducing the mowing frequency and height of cut, increased use of lightweight rolling to maintain playing qualities, reduce or stop topdressing. Herbicides should be applied prior to periods of stress in the Spring, reduced weed populations will increase the moisture and nutrient availability for the desired turf plants in the Summer. Limit the amount of foot and machinery traffic, damage to the crown of the plant is more likely because the plant is not actively growing to recover from the wear.
Basically, don’t do anything that is going to add more stress.
Irrigation: Intelligent irrigation is important, ensure water is infiltrating efficiently and is evenly distributed in the rootzone. Despite the temptation do not over water! This can deprive roots of oxygen, increase susceptibility to disease and create numerous physiological problems for plants and soil organisms alike. Resist the urge to irrigate more when the heat index is high as turfgrass will struggle when the humidity is high.
Early morning is the optimum time for irrigation, as air temperature is usually coolest whereas in the afternoon or evening irrigation can lock heat in the soil increasing stress on the roots.
Watering deeply and infrequently is a good rule of thumb, ensuring the entire root zone has moisture, the use of moisture meters can be used to determine the next irrigation cycle.
Obviously ensuring your irrigation system works to the best of its ability is something you would have done early in the year!
Surfactants: key to optimizing irrigation and precipitation inputs is of course the use of surfactants. There is much supporting research showing the value of including a surfactant in your regular program. A key factor is ensuring surfactant inputs start well before any drought stress and are applied on a regular basis.
For turfgrass professionals a hot dry summer brings many challenges, but hopefully some of the points here will assist in preparing and managing the heat and lack of rain, while still allowing you to produce healthy turf.