What's the big idea?: The importance of transpiration

Give your turf a break by enabling this critical process for cool-season grasses


To get excessively hot during the summer months, wear a tight polyester windbreaker when it’s over 90 F with high relative humidity. The human body releases water through pores in the skin each day. Most often, we’re not aware it’s happening, and it’s referred to as insensible perspiration, which is evaporation of water through (microscopic) pores in the skin that keeps us cool (the average human has 5 million pores). If you wear the windbreaker, your body will heat up significantly, and perspiration will become sensible and quite noticeable — sweat!

Plants lose water insensibly as well. Microscopic “pores” in plants are referred to as stomates, which are the exit orifices for water to leave the plant. That water loss from the stomates is called transpiration (not perspiration!). Transpiration keeps the leaf cool, which is needed for all plants.

Most importantly, transpiration is critical for cool-season grasses, such as creeping bentgrass and annual bluegrass, during summer months so that the quality of putting green surfaces can be maintained. Stomate counts on Penncross creeping bentgrass were found to be around 170 per square millimeter on the top (adaxial) side of the leaf and about 100 per square millimeter on the back (abaxial) side in research done by Green, Beard and Casnoff in 1990 (HortScience 25(7), pages 760-761). Stomates are obviously extremely small but so important.

It’s critical that there be a sufficient gradient between the wet air inside the plant, usually about 100% relative humidity, and the drier air outside the plant. Water vapor moves (transpires) from wet to dry air (high to low relative humidity). The leaf surface is surrounded by a thin layer of still air, referred to as the boundary layer, which is resistant to the movement of water through it. During calm conditions with little or no wind, the boundary layer becomes thicker and more humid and resists transpiration.

Under these conditions, transpiring water does not leave the stomates fast enough, and cool-season grasses begin to suffer heat stress because evaporative cooling from transpiration is reduced.

Creeping bentgrass greens have been used frequently in Arizona, despite extreme summer high temperatures. The dry, desert air creates a strong gradient for movement of water from inside the leaf and across the boundary layer and allows water to transpire at a rate that enhances cooling of the plant.

Conversely, creeping bentgrass struggles during a Florida summer. Despite lower air temperatures in Florida than Arizona, the air is more humid, which reduces the gradient of wet to dry air from inside to outside the leaf, slows transpiration, and may create heat stress for the plant. This can be exacerbated by calm wind conditions, which thicken the boundary layer, slow transpiration even more and compound heat stress.

Helping cool-season grasses on putting greens stay cool during heat and high humidity is important, and there are a few approaches to lessen the boundary layer. Air movement across the putting green surface is important to mix drier air aloft with wetter air near the green surface, which disturbs the boundary layer and encourages more transpiration. This can be accomplished by removing or thinning trees to enhance air flow. If tree removal can’t be done or other factors are restricting air flow, fans can be used to enhance air movement and disturb the boundary layer. It’s important that fans operate efficiently to get air movement across the entire surface.

If you’re managing creeping bentgrass or annual bluegrass on putting greens during hot, humid summers, avoid having them experience what we experience with polyester windbreakers.

Insensible perspiration and transpiration are quite similar. Low relative humidity and good air flow will also keep us cooler and more comfortable during summer months. Likewise, consider ways to enhance the happiness of cool-season grasses on putting greens when midsummer conditions arrive.

Jack Fry, Ph.D., is a professor of turfgrass science at Kansas State University, currently working at the school’s Research and Extension Center in Olathe, Kan. He is a 25-year member of GCSAA and was recently awarded GCSAA’s Outstanding Contribution Award for 2022.