Arkansas Turfgrass – Mike Richardson, Eric De Boer, and Thomas Walton – University of Arkansas
Last winter (2019-20) was a very mild one for the Natural State. In Fayetteville, our daily high and low temperatures were consistently above the 30-year average and, more importantly, we only had a handful of days when the temperature dropped into the teens and no days where we were in the single digits (Figure 1). However, don’t let one nice, mild winter lull you into thinking that you should not be doing everything you can to prepare for the winter ahead. This is especially true for golf course superintendents that are using warm-season grasses such as ultradwarf bermudagrasses on their greens.
Winterkill has been an ongoing challenge over my 20+ years of doing research in Arkansas and there are still no “magic bullets” or “super grasses” that will solve all of your problems. Although we usually think about those super low temperatures as being the main culprit, winterkill can also be the result of winter desiccation or low-temperature diseases such as spring dead spot. We have conducted countless field trials associated with winterkill at the UofA-Fayetteville over the past twenty years and have investigated everything from cultivars to fertility programs to various winter over-seeding approaches to the use of plant growth regulators (PGRs), wetting agents, fungicides, and protective covers. What have we learned? You should use the best available genetics, apply proper fertility, utilize PGRs, and possibly apply wetting agents, fungicides, and covers!! ALL of the tools in your toolbox should be in play when preparing for the dark days and cold nights of winter. Another factor that is always an issue when winterkill shows up is the overall health of the turf going into the winter. Remember, any area that has been weakened by shade, poor drainage, traffic, compaction, weed competition, herbicide injury, etc., is likely going to be an area that could be damaged by a hard winter.
Genetics are always a great place to start when thinking about avoiding winterkill. The development of cold-tolerant bermudagrass cultivars really ramped up back in the 80s and 90s and was led by the turfgrass breeders at Oklahoma State University, including Dr. Charles Taliaferro and now Dr. Yanqi Wu. Some of their landmark bermudagrass cultivars that have really “moved the needle” in terms of cold tolerance include early vegetative cultivars like Patriot and seeded cultivars like Riviera and Yukon. In recent years, a number of new hybrids such as Latitude 36, Northbridge, and Tahoma 31 have shown even more promise for protection against winterkill. The Arkansas Razorbacks installed ‘Tahoma 31’ bermudagrass in Reynolds Razorback Stadium in 2019 and that selection was primarily based on its superior cold tolerance. The Razorbacks have also converted most of Baum Stadium to ‘Latitude 36’ over the past 5-6 years. Over that time period, issues with winterkill have been almost non-existent with these new grasses. Although more energy has been focused on developing cold-tolerant bermudagrasses than other species, there are also some exciting new developments on the horizon for traditional lawn grasses like St. Augustinegrass.
When looking at the ultradwarf bermudagrasses (UDB) for putting greens, there are fewer options with regards to genetics, but we have certainly seen differences in our trials. We conducted a 3-year trial at Fayetteville that included the major UDB cultivars Champion, MiniVerde, and Tifeagle. Although the degree of winterkill that was observed each year was strongly influenced by the severity of the weather, we consistently saw less winter injury on MiniVerde and Tifeagle than we did on Champion (Figure 2). We recently published that work and would be happy to share a full copy of the paper (De Boer et al., 2019). There are also some new cultivars and experimental lines out there that appear to be promising, but it is too early to make a strong recommendation regarding their winter survival.
When it comes to management, some of our earliest studies looked at the effects of fall fertility programs and plant growth regulators, specifically Primo, on winter injury of bermudagrass. Historically, it was believed that nitrogen should not be applied in the fall to grasses like bermudagrass and you should switch to a “winterizer” fertilizer that was high in potassium to avoid winter injury. Studies over the last 20 years from Arkansas, Kentucky, and Virginia have consistently shown that applying nitrogen in the fall is not a contributing factor to winter injury, as long as potassium is not deficient. Our current recommendations are to continue applying some nitrogen into September and October, but the rates should be reduced just because the grass is not growing as much and does not need as much nitrogen. We have also seen some positive benefits of applying Primo (trinexapac-ethyl) going into the fall, but all of that work was done on golf course fairway turf, so we’re not sure if it is applicable to other species or surfaces. Both fall nitrogen and Primo have also shown a positive benefit on early-spring greenup in bermudagrass.
Over the last five years, much of our work has focused on managing winter injury on ultradwarf putting greens and most of those studies have revolved around the use of protective covers and wetting agents. The use of protective covers during the winter season is a proven strategy to combat winterkill in warm-season grasses. Covers enhance survival of bermudagrass by retaining more moisture in the crowns and maintaining soil temperatures above a critical threshold. Historically, it was recommended that ultradwarf bermudagrass putting surfaces should be covered when the predicted low temperature was going to be below 25 °F. Recent field trials at our location have demonstrated that the predicted temperature for covering greens can be lowered to 15 °F with no reduction in winter survival (DeBoer et al., 2019a). This strategy can be financially beneficial to a golf course as the labor required to cover and uncover greens is significant and this reduction in the covering temperature can significantly reduce the number of covering events during the winter. In addition, reducing the number of coverings can also allow the course to be open for play more in the winter, which can impact revenue for the club.
Unfortunately, even when covers are used, winterkill can still be observed on putting greens in more northern locations (Figure 3). Another strategy that we are currently studying is the inclusion of an “air gap” under the cover to further reduce extreme low temperatures on the greens. An air gap prevents the cover from coming directly in contact with the surface of the putting green and could provide additional insulation and warmer temperatures than covers alone. Things like straw or pine needles have been used for decades, either alone on the surface of putting greens or underneath tarps, to retain heat and reduce fluctuations in temperature more efficiently than covers alone. In recent trials conducted at the University of Arkansas, (De Boer et al., 2019b; Thomas Walton, unpublished) we have experimented with the inclusion of alternative air gap treatments under covers such as synthetic “batting” material (Hendrix Batting, High Point, NC), drainage pipes, and straw erosion blankets (Figure 4). The results so far have been promising, as air gap treatments can raise soil temperatures a few degrees compared to covers alone. It is also clear that the use of air gap products will add cost and labor to the process and should only be considered in very difficult locations on a course that are more prone to winter injury, such as a shaded putting green.
Another approach that we have seen promising results with is the use of wetting agents in the winter to reduce the likelihood of desiccation injury. Hydrophobic sands and localized dry spot are common problems on putting greens and the use of wetting agents (surfactants) is now considered as essential to putting green management as mowing and fertilizers. In the winter months, when greens are dormant, the presence of dry conditions are not as easy to diagnose and may be overlooked by the superintendent. We have conducted a number of studies looking at a single application of a wetting agent in early winter (December) and have seen a positive effect of those treatments on winter survival (De Boer, 2019a; De Boer, 2020). Although the benefits have not been observed from year to year, when we have had an unusually dry and cold winter, the results have been very evident. Since the cost of these products is not a major hindrance, we consider their use to be a good insurance policy to help prevent winter injury caused by desiccation.
Finally, all turfgrass managers should be aware of the disease problems that can exist in turfgrass systems during the winter months. Spring dead spot is considered a low-temperature disease of bermudagrass and is often more problematic during very cold winters. On high-value turf areas such as putting greens, golf course fairways and tees, athletic fields and even some lawns, managers should consider the use of fall-applied fungicides to minimize the occurrence of this disease. We discussed a number of strategies for reducing and controlling spring dead spot in a previous edition of this magazine (Richardson, 2019).
Managing turfgrasses in Arkansas to minimize winter injury can be a complex process, but it is important for managers to look at their entire program and utilize all of their tools to best address this issue.
De Boer, E.J., M.D. Richardson, J.H. McCalla, and D.E. Karcher. 2019a. Reducing ultradwarf bermudagrass putting green winter injury with covers and wetting agents. Crop, Forage, and Turfgrass Management 5:190019. https://doi.org/10.2134/cftm2019.03.0019.
De Boer, E.J., M.D. Richardson, and J.H. McCalla. 2019b. Increasing winter soil temperatures with air gaps on ultradwarf bermudagrass putting greens. ASA-CSSA-SSSA International Annual Meeting, San Antonio, TX.
De Boer, E.J., M.D. Richardson, J.H. McCalla, and D.E. Karcher. 2020. Effect of late-fall wetting agent application on winter survival of ultradwarf bermudagrass putting greens. Crop Forage and Turfgrass Management https://doi.org/10.1002/cft2.20035.
Richardson, M.D. 2019. Spring dead spot and large patch – spring diseases that need fall attention. Arkansas Turfgrass, Fall 2019, pp. 12-14.