Too hot for the grass?
What happens to grass during a heatwave, and when can the plant no longer withstand the heat? These are questions that arise now that the world is experiencing the hottest year since temperature records began. Heatwaves are often paired with periods of low rainfall. According to climate scientists, both weather extremes will occur more frequently in the future. What will happen to the grass on the golf courses in Germany, Italy, Arizona or Dubai? These are questions we asked Bernd Leinauer, who teaches as a professor at New Mexico State University and is involved in numerous turf research projects, including on golf courses in the southwest of the USA. At the same time, he is also involved in turf research in Europe.
“Generally speaking, cold zone grasses get too hot above 100F or 37C. They can no longer cope with the temperature,” he states.Cold zone grasses include all varieties found on golf courses in northern Europe or North America, but sometimes also in Italy or Spain. The common species that golfers are familiar with are Poa pratensis, Festuca or Agrostis varieties. “However, no strictly scientific study has determined heat stress independently of the water supply,” Leinauer notes. To date, the survivability of grasses in heat has always been investigated in combination with the water supply.
Drought stress in hot weather
This is ultimately also the issue on golf courses. The greater the heat, the more water the grass needs to cool down.
In this context, scientists like Leinauer speak of evapotranspiration (ET), which is simply explained as the evaporation of moisture from both the soil and the plant. “Even if a golf course or sports field has an unlimited supply of water, the storage capacity of the soil and the irrigation window is limited. The water in the soil is used up by midday or early afternoon, and the heat then leads to drought stress,” Leinauer explains the problem. “It has to be watered again, which is not always possible. Or grasses are used that require less water.”
Arizona, a state with around 400 golf courses, has experienced a heatwave this summer with more than 100 days of over 37.7 degrees Celsius. The warm zone grasses such as bermuda or zoysia can still cope with what is hard to bear for the population. For them, it only becomes uncomfortable between 43 and 46 degrees Celsius. This is also proven by a look at Dubai, where 49.9 degrees were measured on one day this summer and temperatures of around 45 degrees are not uncommon. Nevertheless, the golf courses there are in good shape. Their irrigation water is obtained from desalination plants.
In these hot regions, “the issue of water consumption is central and is being addressed,” explains Leinauer. “Golf courses will have to close if the water runs out,” which could be the case in the USA in particular, where irrigation is largely carried out using groundwater or even drinking water. For this reason, research has been carried out for decades into improving heat tolerance and reducing the water consumption of grasses through breeding. For Leinauer, however, it is clear that this is only one side of the coin: the other concerns “irrigation technology and determining irrigation volumes. There are considerable research shortcomings and implementation deficits in this area. There are major differences between what the grasses need and what is ultimately irrigated.”
Maria Strandberg (Foto: privat)