Summer soil temperature keys
I was clicking through GreenCastOnline.com
and came across the “tools” section, which lists a number of helpful weather related monitoring sites. The ones I focused on were the ones related to temperature, specifically the soil temperature maps
Figure 1. Soil temperature map of Columbus, Ohio on March 4. The type of information provided in the soil temperature map tool on GeenCastOnline.com.
Temperature is the driving force for all biological activity. It defines the optimum range for turfgrass growth. Outside those ranges turfgrass stress occurs. With summer temperatures arriving shortly, this is a good time to look at the impact of air and soil temperatures on turf health.
Solar radiation is the source of heat buildup in the turf. It helps explain why snow melts at temperatures below freezing on bright sunny days. Conversely, the processes of transpiration and conduction/convection dissipate the heat from solar radiation lowering the turf or canopy temperature.
Using solar radiation, transpiration and conduction/convection, we can predict turf canopy temperatures under various environmental conditions. Listed below are common scenarios that occur during summer.
- On a clear sunny day, with no breeze (still conditions) and adequate soil moisture (for transpirational cooling), the canopy temperature will be 15 °F higher than the air temperature.
- On a clear sunny day, with a slight breeze (ex. 4-5 mph) and when adequate soil moisture is present, the canopy temperature will approximate the air temperature.
- On a cloudy day with no breeze and adequate soil moisture, the canopy temperature will approximate the air temperature.
Soil temperatures greatly affect turfgrass health during summer heat. Soil temperatures are more important in determining the health status of turfgrass plants than air temperatures. When the average daily soil temperatures exceed 70 °F, root growth slows and the root system declines for cool-season turfgrass (ex. creeping bentgrass). As the average soil temperature rises above 82 °F, significant root loss occurs.
Figure 2. Soil temperature map of Dallas, Texas on March 4. The type of information provided in the soil temperature map tool on GreenCastOnline.com.
The “70 degree rule” is a good indicator that once soil temperatures reach this level, management practices, especially mechanical practices, need adjusting because the turf is entering a stress phase.
By knowing the impact of soil temperature on root growth, we can introduce management practices to delay the onset of root decline. Coring prior to the stress period may delay the rise in soil temperatures. At this time, nighttime temperatures might still be relatively cool. By coring, the cooler night temperatures may delay the increase in soil temperatures.
On warm-season turfgrasses, soil temperatures that average 75 °F can help define the start of optimum growing conditions. For example, the establishment of hybrid bermudagrass by sprigging coincides with 75 °F. Or bermudagrass recovery from spring dead spot starts when soil temperatures are above 75 °F.
Figure 3. Soil temperature map of San Francisco, California on March 4. The type of information provided in the soil temperature map tool on GreenCastOnline.com.
During summer, managing turf under temperature stress is difficult. Temperature, however, is predictable for providing us with a few hints of what it is doing to the turf.
About the author
Dr. Karl Danneberger is a professor of Turfgrass Science at The Ohio State University. Dr. Danneberger's contact information can be found here. You may also follow Dr. Danneberger on Twitter: