Understanding how cool-season grasses respond to summer conditions is essential for sports turf managers looking to keep their athletic fields healthy during the hottest months of the year. Generally speaking, cool-season turfgrasses will be at their best when air temperatures average from around 60°F up into the mid-70s. Optimal soil temperatures are a bit lower, coming in at around 50°F and reaching into the mid-60s (at two inches of depth and with proper moisture).

These are the best possible conditions for supporting strong growth and resilience, important for keeping the turf healthy and able to handle high levels of traffic and wear. This is the real world, however, and temperatures will fluctuate. Before we cover the issues that excessive levels of heat can cause, it’s important to know how plants cool themselves off in the summer.

Where Grass and Heat Meet

As the sun’s rays beat down on cool-season turf types, transpiration—the movement of water through a plant—helps the turf cool down. During this process, water moves upward through the plant and is pushed out of stomata, helping the plant fight the heat. The amount of water dispelled depends on the temperature, the sun’s rays, humidity, and wind. When it’s windy, for example, more transpiration is able to take place because the wind removes the blade’s boundary layer (which regulates transpiration).

How Hot Is Your Turf?

It may feel hot outside to you, but depending on conditions, your turf may be even hotter. Understanding how turf holds up under different circumstances can help sports field managers plan for heat stress and fight it. Here are a few examples:

  • On sunny days without cloud cover, breezy conditions will keep the turf’s temperature similar to the air temperature. Without breezy conditions, turf can be up to 15°F above air temperatures.
  • On cloudy days without a breeze, the turf’s temperature will be similar to the air temperature. 
  • When soil doesn’t have enough moisture on a sunny day, the turf can heat up to 20°F over air temperatures.

The Symptoms and Effects of Heat Stress

When turf gets too hot, it’s subjected to heat stress, which reduces shoot growth and damages the root system. Heat stress in and of itself is unlikely to kill turf, but it does make it more susceptible to traffic damage and wear. On a sports field that sees lots of heavy traffic, this is problematic.

The major culprit here is not how hot it is outside; it’s how hot it is below the surface. When soil temperatures reach 70°F and higher for days on end, cool-season turf is simply not able to withstand it, and those high levels can cause cool-season grasses to lose more than half of their roots.

Drought, Dormancy, and Hydration

Cool-season grasses can become dormant in the summer if water is scarce. Turf with deep roots, thick cuticles, and other signs of strength is less likely to suffer drought stress. Cool-season grasses use more water than warm-season grasses, especially during the summer. Without rainfall, it’s safe to assume that turf will need an inch of irrigation per week. In areas with water restrictions, that may need to be dropped (try not to go below ½-inch). If turf is allowed to go dormant, be sure to keep the crown hydrated.

Spring Preparations for Summer

The deeper the roots, the better the chance. Shallow-rooted turf types are less likely to survive droughts than their deep-rooted counterparts. Sports field managers should prepare by properly fertilizing, irrigating, and reducing soil compaction through aeration. When strong root systems are established in the spring, turf is prepared to handle summer heat and drought.

For more tips on protecting your cool-season sports fields this summer, reach out to your ATS rep. If you’re new to ATS, use our rep finder tool to find one in seconds.