Ultradwarf bermudagrass is the primary turfgrass used on golf course putting greens in the warmer regions of the United States. Improved varieties, including ‘Champion’, ‘MiniVerde’ and ‘TifEagle’ thrive in the heat of the summer and can provide a dense, consistent playing surface. However, one of the major weaknesses of the ultradwarf bermudagrasses is susceptibility to damage or death from cold temperature exposure.

The popularity of ultradwarf putting greens continues to expand farther north into the transition zone (Hartwiger, 2009). As these grasses have been planted in colder environments, moderate winter injury and even catastrophic loss of putting greens has been observed across the region (Richardson et al. 2014, Carolinas Golf Association 2018). When this occurs, it can put tremendous financial strain on a golf course as it may take weeks or even months to recover. Golfers and golf course superintendents in the Southeast and Central regions will remember devastating winters in 2014, 2018 and 2021 with varying degrees of turf loss occurring at many facilities. Proper management of ultradwarf putting greens is critical to preventing winter injury. Although winter injury can be associated with factors such as diseases, this article will focus on winter injury that results from either low -temperature exposure or desiccation and the best management practices for prevention.

Summary of Best Management Practices

  • Raise the height of cut on ultradwarf putting greens by 25% heading into the fall. This needs to be done while the grass is actively growing to realize benefits. Raise the height of cut by another 25% heading into winter dormancy.
  • Manage soil organic matter with sand topdressing, vertical mowing and core aeration during the growing season to prevent excessive thatch accumulation and improve ultradwarf rooting.
  • Provide ultradwarf putting greens with adequate nitrogen fertility for growth and recovery through the growing season and into the cold-acclimation process. Fall applications of nitrogen have shown to be beneficial for fall turfgrass quality and improved spring greenup.
  • Follow recommendations for adequate plant protectant programs in the shoulder seasons and dormancy periods for ultradwarf putting greens. Research at Virginia Tech has shown that fall and winter fungicide programs – including the use of phosphonates and contact fungicides – have improved ultradwarf bermudagrass winter quality and spring greenup.
  • Experiment with trinexapac-ethyl (Primo Maxx) applications in the fall and winter on ultradwarf putting greens at 1.0-3.0 fluid ounces per acre every two weeks. Anecdotal evidence and research at Virginia Tech have shown that trinexapac-ethyl can prevent early cold deacclimation and improve fall quality of ultradwarf putting greens. These programs will likely be more successful in temperate regions where ultradwarf putting greens do not go fully dormant.
  • Allow natural cold acclimation – i.e., hardening-off – of ultradwarf putting greens in the fall. The cold-acclimation process is critical to the plant’s natural defense mechanisms and can be interrupted if frequent unnecessary covering is performed in the fall.
  • Every golf course with ultradwarf greens located in a region with potential freezing temperature exposure should own at least one set of covers. 
  • Install black, woven polypropylene covers when air temperatures are forecast to be below 20 F. 
  • Install two black, woven polypropylene covers when air temperatures are forecast to be below 15 F.
  • Install covers when low temperatures are forecast to be below 30 F to protect young ultradwarf putting greens or ultradwarf putting greens in marginal growing environments – including areas with heavy shade or very exposed sites.
  • Prior to initial cover installation, apply a wetting agent at the labeled rate followed by heavy irrigation to maximize soil moisture content.
  • Manage soil moisture throughout the growing season and winter months with hand-held moisture meters and supplemental irrigation to prevent wilting or winter desiccation.

Cultivar Selection

New ultradwarf cultivars – including ‘G12’, ‘Sunday’ and ‘Mach 1’ – have recently been introduced into the market, but less is known about their cold tolerance and winter survival compared to ‘Champion’, ‘MiniVerde’ and ‘TifEagle’. Over six years of testing in Fayetteville, Arkansas, ‘Champion’ has the least winter tolerance of these varieties, while ‘TifEagle’ and ‘MiniVerde’ have performed similarly in most years. Dr. Aaron Patton, in the 2013 NTEP warm-season putting green trial in Bloomington, Indiana, reported that ‘Sunday’ and ‘TifEagle’ were slightly more winter hardy than ‘MiniVerde’ but new experimental varieties showed improved cold tolerance. A complete copy of the Indiana dataset is available here.

Mowing Practices

Ultradwarf greens are often maintained below 0.100 inch during the growing season. Low mowing increases stress on bermudagrass plants and exposes the growing points to increased traffic and temperature fluctuations. Winter injury is more likely on intensively managed ultradwarf putting greens during slower periods of growth as the grasses enter and exit winter dormancy. Raising mowing heights prior to winter dormancy and reducing mowing frequency can reduce injury of ultradwarf bermudagrass putting greens (O’Brien and Hartwiger, 2007; Lowe, 2013).

Covering Methods and Strategies

Temperature Thresholds and Costs

Turfgrass covers are known to effectively increase soil temperature compared to uncovered greens and provide protection against lethal low temperatures (Goatley et al. 2007). The most popular covers for winter protection of ultradwarf putting greens have been black, woven polypropylene covers. These covers are relatively lightweight, easy to install, and perform well for temperature modification compared to other covers (Goatley et al. 2017).

As ultradwarf greens became popular, USGA agronomists developed a conservative recommendation that greens should be covered when air temperatures were predicted to fall below 25 F (O’Brien and Hartwiger, 2013). However, deploying covers when cold weather is expected and removing covers when temperatures are warm enough for golf to resume can present some cost and logistical issues to the golf course. During winter months, the golf course maintenance staff may be significantly reduced and having adequate labor to place and remove the covers can be a challenge.

In 2015, a series of USGA-funded studies at the University of Arkansas were designed to investigate various strategies to prevent winterkill on ultradwarf greens. One of the first studies tested varying low temperature thresholds to see if protective covers could be installed less frequently in more northern environments. Black, woven polypropylene covers were applied on ‘Champion’, ‘MiniVerde’ and ‘Tifeagle’ surfaces based on a low temperature prediction of 25, 22, 18 or 15 F and those covering treatments were compared to an uncovered control. Over the three years of the trial, ultradwarf putting greens in the region experienced significant winterkill in two of the years and the uncovered control plots in the trial experienced complete winterkill. However, good protection was observed with all covering treatments, whether applied at 25 F or as low as 15 F.

It was predicted that a golf course would have saved approximately $9,000 in labor costs by applying covers at 15 F instead of 25 F without impacting the winter survival of the turf (DeBoer et al., 2019). This was modeled for other locations and it was predicted that areas like Dallas and Atlanta could avoid up to 10 covering events each year by adjusting the temperature threshold from 25 F to 15 F. An added benefit of reducing the cover temperature threshold is that the golf course would be more available for regular play, effectively increasing revenue and reducing costs with one management change. While many courses may not be comfortable moving from a 25 F to 15 F temperature threshold, covering at 20 F can mitigate the risk associated with cold temperature exposure while reducing the total amount of coverings.

Air Gaps and Double Covers

Superintendents were some of the first to consider placing an air gap under a cover or to use two covers to further protect ultradwarf greens from cold temperatures (O’Brien, 2017). Research in Arkansas and Virginia has examined alternative covering strategies with black, woven polypropylene covers. An air gap was created by installing drainpipe or other lightweight materials beneath the cover to keep it elevated over the ultradwarf putting green.

An initial study at Virginia Tech evaluated the impact of various covering strategies on turfgrass canopy temperature over ‘Patriot’ hybrid bermudagrass at fairway height. Treatments included 1) uncovered 2) single cover 3) single cover with air gap 4) double cover and 5) double cover with air gap. The results of this study concluded that double-cover treatments were superior to single-cover treatments but there were no differences between cover treatments with an air gap and cover treatments without an air gap. In fact, when average overnight temperatures were less than 30 F, the single cover with air gap was never superior to the uncovered treatments. The data supports the use of double covers instead of single covers but did not statistically support using an air gap between the cover and the turfgrass.

Subsequent USGA-sponsored research at Virginia Tech and the University of Arkansas have evaluated the impact of air gap methods and strategies on canopy and soil temperatures on ultradwarf putting greens. These studies included different air gap heights – i.e., the distance between the cover and the ground – and different materials for creating the air gap – e.g., straw or synthetic batting. Once again, no significant differences were observed between using covers alone or covers with an air gap.

Over three winters in Fayetteville, Arkansas, injury was not observed under any of the covering treatments while significant winterkill was observed in the uncovered control plots (Walton and DeBoer, unpublished). An extremely cold period occurred in February 2021, when air temperatures reached as low as -20 F in Fayetteville and did not exceed 32 F for a period of 10 days. The difference in the 1.0-inch soil temperature between the cover treatments and the uncovered plots was as much as 10 F.

However, there were minimal differences in soil temperature between the various air gap treatments and the cover alone and no differences in spring greenup between any of the cover treatments. Using a straw blanket under a cover enhanced soil temperatures compared to other treatments but would come with substantial labor, storage and material costs. The labor required to deploy these heavier materials prior to installing the covers will be a significant addition to the cost of covering greens and the extra material requires additional storage. If a golf course is considering using an air gap, it is recommended that they only focus on using straw blankets beneath covers on putting greens in poor environments or with a history of winterkill.

The results of these studies point toward using black, woven polypropylene covers when air temperatures are forecast below 20 F, using two black, woven polypropylene covers when air temperatures are forecast below 15 F, and using a straw blanket beneath covers under extreme conditions.

"The results of these studies point toward using black, woven polypropylene covers when air temperatures are forecast below 20 F, using two black, woven polypropylene covers when air temperatures are forecast below 15 F, and using a straw blanket beneath covers under extreme conditions."

Soil Moisture and Wetting Agents

Dehydration of the turfgrass growing points – i.e., desiccation – during extremely cold weather is another form of winterkill. One of the challenges for a golf course superintendent in more northern environments is detecting desiccation since the typical signs of wilt are not apparent on a dormant grass. Although the growing points of a dormant putting green are in a resting state and do not require significant amounts of water, they must remain hydrated to survive.

Preventing winter desiccation can involve several strategies – including the use of covers to reduce moisture loss from the soil, periodic application of water to the greens using the irrigation system, and the use of wetting agents to maintain soil hydration. Superintendents should continue using a soil moisture meter during the winter months to determine when irrigation might be needed. Although critical winter soil moisture thresholds have not been adequately studied or defined, the soil moisture readings that a superintendent uses during the summer months can certainly be a guide for maintaining winter soil moisture. If a colder golf course location chooses to use ultradwarf bermudagrass on their putting greens, they should also consider that the irrigation system may need to remain charged during the winter months to supplement moisture when needed.

"Superintendents should continue using a soil moisture meter during the winter months to determine when irrigation might be needed."

Wetting agents are often used in the growing season to combat localized dry spots and provide consistent soil moisture in putting greens. The University of Arkansas has conducted studies to determine if applying wetting agents while the turf is dormant might provide some protection from winter desiccation and winter injury on ultradwarf greens. Although the benefits of wetting agent applications during dormancy were not consistent from year to year, a positive effect was observed during drier winters, suggesting that a wetting agent application would be a cost-effective way to reduce the risk of winter desiccation – especially in cases where the irrigation system is shut down for the winter. In this research, positive effects were observed from a single application of various products at label rates made when covers would be initially used – e.g., mid-December in Fayetteville, Arkansas.

Fungicide Programs

Fall and early winter applications of multi-site contact fungicides, QoI fungicides and phosphonates have been shown to improve winter turfgrass quality and spring greenup in research at Virginia Tech. These fungicides protect the dormant ultradwarf bermudagrass putting greens from secondary pathogens and complement fungicide programs for root-borne and pythium diseases. The most successful treatments included the use of a pigmented phosphonate – e.g., Appear II. Visual turf quality and density as well as disease control were superior to untreated control plots during fall and winter months on unpainted ‘MiniVerde’ ultradwarf bermudagrass putting green turf.

Trinexapac-ethyl (Primo Maxx)

Cold acclimation and winter dormancy are the best defense against damage on ultradwarf putting greens from cold temperature exposure. Promoting cold acclimation and a consistent winter dormancy period may improve the cold tolerance of ultradwarf putting greens. On golf courses throughout the Southeast and in research over two winters at Virginia Tech, trinexapac-ethyl (Primo Maxx) has inhibited early dormancy break during periods of warm winter weather. In several studies at Virginia Tech, Primo Maxx improved ultradwarf quality and density coming out of winter.

Preventing an early dormancy break during warm periods in the winter allows ultradwarf bermudagrass to conserve carbohydrates until soil temperatures consistently warm up in the spring. Decreasing the fluctuation of greenup during the winter months prevents putting greens from wasting stored carbohydrates with unnecessary growth. Trinexepac-ethyl may have different impacts on the winter injury risk to ultradwarf putting greens depending on cold temperature exposure. Many golf courses in temperate regions have seen benefits from using trinexapac-ethyl programs in the fall and winter months.

It should also be noted that greenhouse research has shown detrimental effects of trinexapac-ethyl on ultradwarf cold tolerance prior to cold acclimation and under extreme temperature situations – i.e., 10 hours of exposure to 15 F. Golf course superintendents should experiment with low rates of trinexapac-ethyl in the late fall and winter on small areas of their putting greens before implementing course-wide programs.

Conclusion

In summary, there are many practices that should be considered when ultradwarf bermudagrass greens are grown in locations where winter injury is a potential concern. As these grasses are moved farther north into the transition zone, all of these strategies may be needed to minimize winterkill.

Dr. Mike Richardson is a professor in Horticulture at the University of Arkansas
Jordan Booth is an agronomist in the Southeast Region

References

Anderson, J.A., C.M. Taliaferro, and D.L. Martin. 2002. Freeze tolerance of bermudagrasses: Vegetatively propagated cultivars intended for fairway and putting green use, and seed-propagated cultivars. Crop Sci. 42:975-977.

Carolinas Golf Association. 2018. Turf advisory: Winter kill emerging on courses. carolinasgolf.org.

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 Mgmt. 5:190019.

DeBoer E.J., D.E. Karcher, J.H. McCalla, and M.D. Richardson. 2020. Effect of late-fall wetting agent application on winter survival of ultradwarf bermudagrass putting greens. Crop Forage & Turfgrass Mgmt. 6:e20035. 

Goatley, J.M., Jr., J.P. Sneed, V.L. Maddox, B.R. Stewart, D.W. Wells, and H.W. Philley. 2007. Turf covers for winter protection of bermudagrass golf greens. Applied Turfgrass Science. 4:1–9. 

Goatley, M., W. Askew, S. Askew, J. Dickerson, and D. McCall. 2017. Turfgrass cover sources vary in temperature, light and moisture penetration, and weight. International Turfgrass Society Research Journal. 13(1):297-304.

Hartwiger, C. 2009. The heat is on. USGA Green Section Record. March/April. 47(2). 

Lowe, T. (2012). Changing times in ultradwarf bermudagrass putting green management. USGA Green Section Record. June 22. 50(13).

O’Brien, P. and Hartwiger, C. 2007. Ultradwarfs in the off-season–a winter wonderland. USGA Green Section Record. November/December. 45(6).

O'Brien, P. and C. Hartwiger. 2013. Covering guidelines for ultradwarf bermudagrass putting greens. USGA Green Section Record. January 9. 51(1).

O’Brien, P. 2017. Stop the cold with a “dead air gap”. USGA Green Section Record. February 3. 55(3).

Patton, A. 2020. Personal communication: Winter hardiness of bermudagrass cultivars in NTEP evaluation.

Richardson, M. D., Brosnan, J. T., and Karcher, D. E. (2014). Turfgrass winterkill observations from the transition zone. Applied Turfgrass Science. 11(1):1-4.