Putting Greens: Keeping Cool with the World’s Smallest Swamp Cooler
Swamp coolers provide excellent cooling effects by using two simple ingredients – wind and water – to create evaporative cooling. These same two ingredients create transpirational cooling on leaf surfaces and keep plants cool in high temperatures. Imagine for a moment the plight of a bentgrass plant growing on a putting green mowed at 0.125-inch (3.1 mm) on a 100-degree (38-degree C) day. The daily mowing limits the plant’s leaf surface area to a matter of millimeters! And believe me, you don’t get much cooling power out of millimeter-sized swamp cooler. So how do you mitigate heat stress under these challenging conditions?
Defining Heat Stress: The Proper Starting Place
First, let’s define heat stress on plants. Anthony Hall of the Botany and Plant Sciences Dep. at Univ. of California-Riverside, defines heat stress as “where temperatures are hot enough for sufficient time that they cause irreversible damage to plant function or development. Plants can be damaged in different ways by either high day or high night temperatures and by either high air or high soil temperatures. Also, crop species and cultivars differ in their sensitivity to high temperatures.”
Considering this complex definition, let’s look again at our diminutive bentgrass plant on a putting green. The frequent low mowing exposes the turf and soil surface to more solar radiation and increases the surface and soil temperatures. In addition, during times of high stress the plants close their stomata to help conserve water, which also reduces the cooling effects and increases temperatures even more. Combine high daytime temps with high nighttime temperatures and you can see why greens are at such high risk of succumbing to heat stress in the summer months.
A Better Way Forward
There is a way to improve transpirational cooling on greens. Syringing greens – applying light amounts of water to cool the canopy - has been a practice used by superintendents for many years, but how effective is it, and when is the best time to do it? Researchers Dr. Beth Guertal and Dr. David Han of Auburn University have conducted several studies to answer these types of questions. Their results were summarized in the June 1, 2018 edition of the USGA Green Section (Cooling Creeping Bentgrass Putting Greens, USGA.org):
- Fans reduce soil temperature and improve creeping bentgrass rooting during summer, even without syringing.
- Syringing improves turf quality during summer when fans are used but can decrease rooting without fans.
- Irrigation timing – i.e., morning or afternoon – may not affect rooting and turf quality if fans run continuously.
- Without fans, morning irrigation reduced soil temperatures more than afternoon irrigation, but there has been variability in this response from year to year.
Additional Tips
Other tips to improve greens during the hottest months (from an article published by from Dr. Jack Fry and Dr. Bingru Huang in 2000, “Help Bentgrass Beat the Heat”):
1. Mow at the highest acceptable height to increase grass plants' ability to produce food.
2. Irrigate to match water needs - not by a rigid schedule - to reduce stresses on the grass plants.
3. Fertilize adequately to increase plant health and vigor.
4. Select heat-tolerant bentgrass cultivars.
5. Construct greens with proper techniques and correct any underlying soil problems on existing greens.
