Arkansas Turfgrass – Thomas Walton, MS Student and Mike Richardson, Ph.D, University of Arkansas

Widespread record low temperatures were experienced throughout much of Arkansas and the surrounding region in the late winter of 2021. At the Drake Field weather station, just south of Fayetteville, -20 °F was recorded on the morning of February 16^th! However, it was not just the extreme temperatures that caused many warm season turfgrass managers to lose sleep, but it was the sustained amount of time temperatures remained below freezing. For a 10-day period from February 9^th through the 19^th, the air temperature did not exceed 32 °F. Sounds like a pretty good recipe for winterkill! One small silver lining during the cold February was a small blanket of snow during the middle of the cold stretch, a total of about 7 inches from February 14-17, correlating with the record low temperatures. Unfortunately, significant winterkill was reported throughout Arkansas and the surrounding region, particularly in Oklahoma and Texas. Many golf courses in north Texas with ultradwarf bermudagrass putting greens do not own protective covers because winterkill has historically not been a concern, were hit the hardest. Numerous golf courses required complete reestablishment of putting greens and large acreage of fairways, tees, and collars. Southern Hills Country Club in Tulsa, host of the Senior PGA Championship in the spring, had to quickly resod more than four acres of short-cut bermudagrass in preparation for the tournament, with most damage occurring on north facing slopes (MacLeod, 2021).

We have been conducting field trials at the University of Arkansas over the past two winters with a goal of improving management strategies used to protect ultradwarf bermudagrass putting greens. The use of protective covers has been a tried-and-true strategy to prevent winterkill and covers are often deployed when the temperatures begin to reach the mid to low 20’s. A recent field trial conducted at the University of Arkansas determined that greens could be covered when there are predicted low temperatures of 15 °F with no reduction in winter survival (DeBoer et al., 2019). Unfortunately, golf courses in the region can still experience winterkill underneath protective covers. A frequent observation of winterkill underneath covers is a rippling pattern of healthy grass and enhanced survival in the thick, stitched seams. These observations suggest that the air underneath the cover is providing additional insulation (think double-pane windows). Therefore, the goal of this trial was to investigate a few potential materials to create an air gap by placing the materials on the putting green and pulling the covers over the top. A similar trial was conducted in Canada in 1999 on annual bluegrass putting greens, although an impermeable cover was used rather than the typical permeable covers used to protect bermudagrass putting greens. Air gap treatments consisting of straw, curled wood shavings, and 2 inches of air space around a wood frame were tested and provided warmer minimum soil temperatures and less temperature variation than covers alone (Dionne et al., 1999). Although the use of an air gap sounds promising, air gaps have not demonstrated protection against winterkill of ultradwarf bermudagrass putting greens during an adverse winter or enhanced spring green-up so it is important to test potential materials to determine if air gaps are worth the investment.

This field trial was conducted on our ultradwarf bermudagrass putting green during the winters of 2019-20 and 2020-21. The putting green includes the three most popular ultradwarf cultivars of ‘TifEagle’, ‘MiniVerde’, and ‘Champion’. Three different air gap materials were assessed including a straw material, synthetic batting, and drainage pipe, which were compared to a cover alone and uncovered control. The cost and physical properties of each treatment are described in Table 1. Soil temperature was continuously monitored at a 1-inch depth and spring green-up was assessed visually and using digital image analysis to determine the percentage of green coverage throughout the spring. In order to keep the trial relevant to golf course superintendents, the putting green was managed with the typical cultural practices found on golf courses throughout the region and was covered when the predicted low temperature was expected to drop below -20 °F. Covers were removed when the temperature was predicted to be above 50 °F to simulate a golf course being open for play for golfers.

The winter of 2019-20 was relatively mild and no winterkill was observed, but the 2020-21 winter provided a good test of these products. As mentioned previously, February was a very cold month in Fayetteville and the corresponding 1-inch soil temperatures of each treatment are shown in figure 1. The soil temperature in the uncovered controls fell to an average temperature of 19 °F. One method of determining cold hardiness of different grasses is laboratory testing which determines the temperature at which more than 50% of the grass is killed by the low temperatures. For ultradwarf bermudagrasses, this ranges between 19-23 °F (Anderson et al. 2002; Gopinath et al. 2021). For superintendents with built-in soil temperature sensors (i.e. Spio, Toro Turf Guards, ect.), these temperatures can give you an idea about when to be concerned about winterkill and further explain our observations this past spring. Widespread winterkill was observed on all the uncovered controls, regardless of cultivar (Figure 2). At times, the straw and batting material provided significantly warmer soil temperatures than the cover alone, however, did not provide any enhancement in spring green-up. If you take a closer look at figure 1, you can see that during our coldest stretch of the year, the cover alone was performing very similarly to all the air gap treatments and even outperformed the cover plus pipe treatment. All covered treatments maintained temperatures around 30 °F, safely out of the range of the soil temperature values previously discussed. At this time, we do not recommend the widespread use of air gaps on ultradwarf bermudagrass because of the high purchasing cost (Table 1), labor required for installation, and annual degradation of materials such as straw. If you have ample pine needles stockpiled from pine trees, you might avoid the upfront purchasing cost, however, it requires much more labor to apply and remove pine straw than the materials we tested (O’Brien, 2017).

Based on our results, we are not currently recommending widespread use of air gaps, but that’s not to say we don’t see the potential value of an air gap. Shaded portions of putting greens, north facing slopes, and areas exposed to the wind could benefit from the use of air gaps. One interesting observation made during this trial was that we did see a soil temperature enhancement which correlated with the snowfall we experienced. The lowest soil temperature for all treatments occurred on February 14^th, however, the air temperature continued to drop and record low temperature were experienced two days later, on February 16^th. This suggests that the snow did provide some insulation and produced a “natural air gap” and likely prevented more widespread winterkill in areas which were not protected by covers such as fairways and tees. Virginia Tech University has conducted a similar study and had positive results with the use of double covers, so be sure not to throw out your old covers when it comes time to replace them as these could prove to be valuable tools for problem greens (Booth et al., 2020)!

References

Booth, J., J. M. Goatley, D. S. McCall, and S. D. Askew. 2019. Impact of woven polypropylene covering strategies on bermudagrass canopy temperatures. Agron. Abr. p. 119519.

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, and Turfgrass Management 5:190019.

Dionne, J., P.-A. Dubé, M. Laganiére, and Y. Desjardins. 1999. Golf green soil and crown-level temperatures under winter protective covers. Agron. J. 91:227-233.

Gopinath, L, J.Q. Moss, Y. Wu. 2021. Quantifying freeze tolerance of hybrid bermudagrasses adapted for golf course putting greens. Hor. Sci. 56(4): 478-480.

O’Brien, P. 2017. Stop the Cold with a “Dead Air Gap”. USGA. Far Hills, NJ. Feb. 3: 1-3

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