The introduction of ultradwarf hybrid bermudagrasses for putting greens has resulted in the grasses’ being pushed farther into chillier climes as superintendents consider whether this species is a viable alternative to creeping bentgrass. Not surprisingly, winter injury often affects this warm-season turfgrass in the transition zone.
Turfgrass researchers (Mike Richardson, Ph.D., and Doug Karcher, Ph.D., and their graduate student, Eric DeBoer) at the University of Arkansas decided to explore some methods to alleviate winter injury in the ultradwarf hybrid bermudagrasses, examining combinations of turf covers, wetting agents and selected temperatures at which the covers were placed on greens.
The study was conducted at the university for three years using the bermudagrass cultivars Champion, MiniVerde and TifEagle. The treatments were: 1) turfgrass cover (a black woven polypropylene; Xton Inc.) or no cover; 2) predicted low air temperature at which covers were placed on the plots (25, 22, 18 or 15 F; -3.9, -5.6, -7.8 or -9.4 C); and 3) application of a wetting agent (trade name Revolution) applied at 2 gallons/acre in a 75-gallon/acre spray volume (1.9 milliliters/square meter in 70 milliliters/square meter). This wetting agent was applied once in each of the three years, in the first week of December. All covers were removed whenever temperatures reached 45 F (7.2 C).
After realizing that spring frosts significantly affected green-up in the first year of the study, researchers changed the protocol for the following two years so that any treatment that received a cover also received that cover during spring green-up whenever a frost was predicted.
Soil temperature at a 1-inch (2.5-centimeter) depth was collected continuously using soil temperature sensors and data loggers. Percent green cover of each plot was determined using remote sensing during spring green-up, from March through May of each year.
Regardless of the soil temperature when covers were applied, soil temperature was always higher in covered plots than in plots that never had a cover. However, soil temperatures did not vary greatly because of the air temperature at which covers were applied. So, reducing the threshold temperature for covering greens to 15 F was just as effective in keeping an elevated soil temperature (when compared with an uncovered green).
And what about turf injury? Well, the first year of the study was a mild winter, and the uncovered plots had green turf cover equal to that measured in the covered plots. In the second and third years, there were some cold events — <5 F (-15 C) for multiple nights — and uncovered plots had fatal winter injury. When it got this cold, the protective covers were essential for protecting from winter injury and improving spring green-up.
The researchers noted two important items: 1) The threshold air temperature at which covers could be applied could go as low as 15 F without affecting green-up and winter damage, and 2) the spring green-up needed to be protected by applying the covers in spring, whenever a frost was anticipated.
For the studied cultivars, MiniVerde and TifEagle were consistently more cold-tolerant than Champion, even when the winter was considered mild. And, finally, the December application of a wetting agent proved inconsistent, with application in the first year improving spring green-up, and application in the second and third years providing no benefits. The authors noted that a single application of a wetting agent is relatively inexpensive, and a safety application could easily be warranted as a measure of assurance.
Source: DeBoer, E.J., M.D. Richardson, J.H. McCalla and D.E. Karcher. 2019. Reducing ultradwarf bermudagrass putting green winter injury with covers and wetting agents. Crop, Forage & Turfgrass Management 5(1).
Editor’s note: Read all of Beth Guertal’s recent Verdure columns.
Beth Guertal is the Rowe Professor of Soil Fertility in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and past president of the Crop Science Society of America. She is a 22-year member of GCSAA.