The forecast for the next ten days looks dry and hot: average high air temps are currently at 85.5 degrees F and we are carrying a precipitation deficit of 2″ for the month of July (1″ actual precipitation versus 3″ normal). The average soil temperature at 4″ depth is currently 87.2F. These hot and dry conditions are far from ideal for cool-season grasses. This article will look at how cool-season grasses respond to heat and drought and how best to manage them to minimize turf injury.
The Principles of Heat & Drought Stress
Cool season turfgrasses perform best when daytime temperatures are in the 60-75°F range and soil temperatures are in the 50-65°F range, along with adequate soil moisture. By comparison, warm-season grasses can function better in warmer temperatures and are able to continue growing during the heat of the summer if there is adequate soil moisture. The most stressful time of the year for cool-season turfgrasses is typically June through August, when there are hot sunny days and temperatures routinely in the 80’s. With temperatures on the rise, it is a good time to remember what effect temperature has on turf. Solar radiation is the source of heat buildup in the turfgrass plant. Transpiration of water up through the plants and out of the stomatal pores dissipates the heat. The rate of transpirational flow is dependent on temperature, wind, solar radiation and humidity. It is regulated by the stomatal openings and by the static layer of air that covers the leaf blade (called the “boundary layer” and held at 100% humidity). Once the boundary layer is removed by windy conditions, the movement of water via transpiration is increased. Thus, the rate of transpirational cooling is much greater on a sunny, windy day than a cloudy, calm day.
Given the following conditions this is what the turf may be experiencing:
- Clear sunny day, no breeze, and adequate soil moisture – The canopy temperature will be 15°F higher than the air temperature
- Clear sunny day, slight breeze, adequate soil moisture – The canopy temperature will be within 1°F of the air temperature
- Cloudy, no breeze, adequate soil moisture – The canopy temperature will be the same as the air temperature
- If soil moisture is limiting, under sunny days the canopy temperature can rise 20°F above the air temperature
- Heavy sand topdressing left on the turf surface can increase canopy temperatures 23°F
The most common heat stress symptoms are a reduction in shoot growth and a stoppage or loss of a functional root system. In most cases heat stress alone does not cause turf death. The impact of heat is often associated with detrimental changes to the plant that increase the likelihood of traffic or wear injury. Turfgrasses vary in their tolerance to heat stress, with tall fescue withstanding considerably more heat stress than perennial ryegrass or rough bluegrass. Soil temperatures above the optimum are more detrimental than air temperature. When average daily soil temperatures exceed 70°F 50% or more of the root system of a cool season turfgrass can be lost. In a controlled study at Rutgers University, the researchers exposed creeping bentgrass plants to increasing soil temperatures while holding the air temperature constant at 68°F. Above 70°F they observed a decline in root mass, length and activity, which continued to decline with increasing soil temperature. In Ohio State rhizotron research, root activity and growth declined dramatically during summer stress.
While warm-season grasses perform much better during the heat of the summer than cool-season grasses, both are susceptible to drought stress and will go dormant in the summer if they do not get water.
How drought tolerant a turfgrass is depends upon:
- The severity and duration of the drought and number of drought exposures
- Being able to evade the stress. Turf may go dormant during the summer to evade the stress period. An example of this would be rough bluegrass (Poa trivialis).
- Being able to avoid the stress. Turf can avoid drought stress by having deep roots, or an abundance of root hairs, a dense sward, rolled or hairy leaf blades, thick cuticles and/or small leaf areas. An example of this would be tall fescue (Festuca arundinacea) which has deep roots.
- Genetic tolerance. Turf may be able to tolerate drought stress by having greater food and water reserves during the stress period. An example of this would be bermudagrass (Cyanodon dactylon).
The amount of water a turfgrass system needs is based upon its “water use rate” (WUR), which is calculated by the evapotranspiration (ET) rate. Compared to warm-season grasses like bermudagrass, cool-season grasses have high water use rates during the summer, typically between 0.25 – 0.35 inches per day. ET rate is commonly used as a guide for the amount of supplemental irrigation needed each week if there is no rainfall. A ball-park figure would be that turfgrasses require 1-inch of water per week. Lower amounts than the weekly ET rate could also be applied if there was a severe drought and the irrigation water was scarce or too expensive. It is not uncommon to see irrigation strategies of replenishing only 50-80% of the calculated ET rate, in order to conserve moisture. If turf is allowed to go dormant in order to conserve moisture, it is important to regularly check the crown (base) of the grass plant to make sure that is stays hydrated or the turf may die. Turfgrasses with rhizomes or stolons are more able to withstand periods of drought than shallow-rooted, bunch type grasses. These latter grasses, like annual bluegrass, (Poa annua) have a short wilt phase and die out quickly under summer stress.
Exposing turf to drought stress prior to the summer stress period (called drought preconditioning) can enhance the turf’s ability to withstand heat and drought stress. Research by Bingru Huang has shown that drought preconditioning can promote much deeper and extensive root systems in turf like Kentucky bluegrass. In addition to deeper roots, preconditioned plants maintained greater leaf water content and enhanced stomatal conductance and transpiration rates. Preconditioning is done by exposing the turf to mild drought stress in the spring, which can be a challenge if there are heavy rains events throughout the spring months. It is essential during the spring months to promote as many roots as possible by alleviating soil compaction and by being judicious with both water and fertilizer. Spring is the best root growth growing period for cool-season grasses. Cultural practices to encourage root growth at this time of year will provide great dividends during summer stress.
Now that we have discussed some of the underlying principles of heat and drought stress on cool-season athletic fields, let’s look at some of the turfgrass management practices that can help to alleviate some of that stress.
Pre-stress practices in the spring: Leading into the summer stress period, it’s important to encourage as much root, rhizome and stolon growth as possible. This is done by carrying out aggressive aeration and topdressing programs in the spring. In addition, turf should be exposed to mild drought conditions to harden-off or precondition for the stress period. Excessive soil compaction and saturated soils should be alleviated at all costs since the goal is to promote roots!
Excessive amounts of nitrogen applied in the spring will favor top growth and be detrimental to root growth, so apply light rates of fertilizer. Potassium is the nutrient most associated with water regulation within the turf plant and it has been linked with increased stress tolerance and improved injury recovery, particularly in warm-season grasses. Excessive top growth could also be caused by using growth blankets and covers during the spring, which may be at the detriment of root growth. It is important to take covers off and harden the turf off several weeks prior to the summer stress period.
General guidelines for managing turf during summer heat & drought stress:
- Apply irrigation early in the morning. Do not over-water during periods of high temperatures as this could cause wet wilt and also encourage diseases like brown patch and pythium. Irrigate deeply & infrequently. If rooting depth has been greatly reduced, lighter, more frequent irrigation should be applied. Make sure water application rates are not greater than soil infiltration rates to avoid run-off. Keep in mind that turf growing in shade needs about half as much water as the same grass growing in full sun. Monitor turf closely for signs of wilt: purple color, foot printing, and rolled leaves.
- On sand-based and other high stress fields, lightly syringe (cool down) the turf in the afternoon by spraying turf with a hose or rotation of the sprinkler heads.
- On non-irrigated fields, use warm-season grasses, tall fescue, or mixes of tall fescue and Kentucky bluegrass. Choose stress tolerant cultivars wherever possible. Check the National Turfgrass Evaluation Trials for the best cultivars within a species for your area (NTEP.org).
- Kentucky bluegrass fields not in use during the summer can be eased into summer dormancy, whereby the top growth stops and goes brown but the crown remains hydrated and viable. Kentucky bluegrass can survive in this dormant state for a couple of weeks and then recuperate when environmental conditions are more favorable. Monitor the crown to make sure it stays hydrated, or the plant will die.
- Raising the height of cut slightly will increase the wear tolerance of the turf. Municipal fields and tall fescue fields should be mowed at 3-inches. A denser turf will provide better protection to the growing point and also help minimize temperature buildup at the soil surface.
- Minimize the frequency of mowing. For example, if double cutting prior to games is a standard practice, reduce the frequency to single cutting. Avoid mowing during the heat of the day. Avoid mowing wilted turf. Minimize repetitive mowing patterns.
- Disperse or stagger the entering and exiting patterns and practice drill areas to minimize and reduce the concentration of wear. Keep traffic off dormant fields.
- Reduce the frequency of practices like verti-cutting, core aeration and sand topdressing.
- Avoid applying DMI fungicides and plant growth regulators.
- Avoid the use of covers and tarps.
- Apply nitrogen fertilizer during the stress period to suppress leaf senescence during periods of high temperature and to promote recovery. Use a slow-release source of N.
- Monitor the turf closely for damage from white grubs in May and June, since they damage turf roots. Fields with a history of grub problems need to be protected with a season-long control product. Bluegrass billbugs have been a major problem on fields the last two summers but damage has been misdiagnosed as mild drought. Check for insect activity if turf looks droughty.
Authors: Dr. T. K. Danneberger, Dr. J. R. Street & P. J. Sherratt