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Key Takeaways

  • Researchers at Oklahoma State used an objective method (LT50) to quantify and compare the relative cold tolerance of two industry-standard bermudagrasses with two experimental varieties.

  • ‘Tahoma 31’ and the experimental variety ‘OKC1406’ were the most tolerant of extreme cold, while ‘Tifway’ and another experimental variety were the least tolerant.

  • The most cold-tolerant variety had a 50% survival rate at 15.6 degrees F measured at a 1-inch soil depth.

  • Other experimental bermudagrasses being developed at Oklahoma State have exhibited the same or better cold tolerance than industry standards ‘Tahoma 31’, 'Latitude 36’, and ‘NorthBridge’ and show promise for the future.

  • The LT50 value (the mean low temperature at which 50% of the grass dies) is an effective way to compare the relative cold tolerance of different bermudagrass varieties.
     

The risk of winter injury is a serious concern for golf course superintendents managing bermudagrass in the transition zone. Bermudagrass thrives during the hot summer months in this region, but unpredictable winter weather can lead to turf damage or death from direct low-temperature injury, desiccation, and other stresses. Turf damage due to winter injury disrupts play, reduces visual quality, and can require a lot of work to repair in the spring, which is labor-intensive and comes with a significant economic cost to the golf facility. Fortunately, bermudagrass breeders have been hard at work for decades seeking to improve the cold tolerance of this versatile grass. For superintendents in the transition zone, understanding how different bermudagrass varieties perform under low-temperature stress is critical for selecting grasses that will hold up to harsh winters and reduce the risk of winter injury.

The goal of this study was to provide insights on how the tolerance to extreme cold of experimental bermudagrass varieties compares to varieties already on the market. Determining how experimental lines of bermudagrass stack up against industry standards can provide helpful information to guide current and future breeding work. This work also helps superintendents better understand the real and measurable differences in cold tolerance between bermudagrass varieties. Lastly, the study demonstrates the effectiveness of the LT50 value (the temperature it takes to kill 50% of the grass) for benchmarking the cold tolerance of different bermudagrasses.

Research Methods

Researchers evaluated two experimental varieties – ‘OKC1406’ and ‘OKC1873’ – and two widely used commercial bermudagrasses – ‘Tifway 419’ and ‘Tahoma 31’ – in a controlled laboratory environment in Stillwater, Oklahoma. The industry standards were chosen because ‘Tifway 419’ represents a variety that is considered more sensitive to low-temperature injury while ‘Tahoma 31’ has good tolerance to extreme cold. Replicates of each variety were vegetatively propagated in long, conical 1.5-inch pots and maintained at a 1.0-inch mowing height. Fertilizer, water and pest control products were applied as needed. After 13 weeks of establishment, the grasses were slowly acclimated down to a daytime temperature of 46 F and 36 F at night prior to being exposed to subfreezing temperatures. 

Several pots of each variety were placed in a freeze chamber at 27 F until acclimated and then subjected to 11 different freeze treatments, ranging from 7 F to 25 F. After the target temperature was reached, pots were removed and allowed to thaw. The pots were reacclimated and then placed in a warm and well-lit environment to encourage regrowth. The level of regrowth was visually evaluated after five weeks of recovery using binary values (1 = alive, 0 = dead).

The LT50 for each variety was estimated using logistic regression, and this analysis produced predicted survival percentages at each temperature, with the temperature where predicted survival reached 50% taken as the LT50. Because the freeze test was conducted three times, each bermudagrass variety had three LT50 values. These LT50 values were then statistically analyzed to allow researchers to compare the relative tolerance to different freezing temperatures among varieties. As mentioned earlier, this study reinforces earlier work demonstrating LT50 as a good measure of relative cold tolerance (Anderson et al., 1993, 2002).

What Does the Research Say?

‘Tahoma 31’ demonstrated the highest tolerance of subfreezing temperatures, with an LT50 of 15.6 degrees F, followed closely by the experimental selection ‘OKC1406’ at 16.2 F. These two were statistically similar, forming the most cold-tolerant group in the study. In contrast, ‘Tifway 419’ and ‘OKC1873’ had significantly higher LT50 values, indicating a lower tolerance to subfreezing temperatures. The distinction between these groups highlights meaningful differences in cold tolerance among bermudagrasses. The tight control of the growth chamber study also reduces environmental variability, making the data especially reliable for early stage screening in breeding lines.

Superior performance under controlled freezing conditions suggests that ‘Tahoma 31’ and ‘OKC1406’ are less likely to suffer catastrophic winterkill during cold snaps. In addition to its lower rate of winterkill, other field studies have shown ‘Tahoma 31’ to have superior post-dormancy greenup and recovery following prolonged freezing temperatures (Fontanier et al., 2020; NTEP, 2014).

‘Tifway 419’, a common warm-season standard, was the least tolerant to extreme cold in this study. In past research, findings regarding this grass’ exact level of cold tolerance have varied (Dunne et al., 2019; NTEP, 2014). These other studies were conducted in outdoor settings across the transition zone, which illustrates that a range of factors contribute to the winterkill susceptibility of a particular turf variety beyond simply what is measured in a controlled setting.

Practical Implications for Superintendents

This research highlights the fact that there are measurable differences in bermudagrass cold tolerance between different varieties. Superintendents in areas of the transition zone that regularly experience extreme cold should strongly consider choosing a bermudagrass with good tolerance to subfreezing temperatures – e.g., ‘Tahoma 31’ – when opportunities for regrassing arise. Although there are many other factors that can contribute to winterkill besides direct low-temperature injury, controlled-environment studies like this one help to quantify differences in cold tolerance, which is directly related to the risk of winterkill. In recent years, golf courses in the transition zone have seen several instances of severe winterkill on warm-season turf and the risk of this problem doesn’t seem to be going away. Increased genetic tolerance to extreme cold can make a big difference in this context.

This research also underscores that there is still room for improvement when it comes to cold tolerance in bermudagrass. The experimental selection ‘OKC1406’ performed as well as one of the most cold-tolerant commercial varieties. Breeders remain hard at work and the genetic potential is there to develop bermudagrasses with even greater cold tolerance than ‘Tahoma 31’ and other currently available cold-tolerant varieties.

A trial conducted at about the same time at Kansas State supports the finding that ‘OKC1406’ has good winter survivability. The Kansas study also showed several other experimental lines from Oklahoma State’s breeding program outperforming commercial varieties like ‘Latitude 36’ and ‘NorthBridge’ (Xiang et al., 2022). The USGA’s Mike Davis Research Program funded a portion of this research and has invested in breeding programs targeting bermudagrass cold tolerance and other improved characteristics for decades.  

Conclusion

When it comes to bermudagrass cold tolerance, which variety you choose matters. Some currently available varieties – like ‘Tahoma 31’ – are objectively more cold tolerant than others, and new experimental lines under development could raise the bar for bermudagrass performance in cold regions going forward. The risk of winterkill on bermudagrass will never be fully eliminated, but by choosing cultivars with strong tolerance to extreme cold, superintendents can stack the deck in their favor. It’s also important for superintendents to use management strategies that reduce the risk of winter injury on warm-season turf regardless of how cold tolerant the varieties on their golf course might be. This includes raising mowing heights prior to dormancy, improving drainage, reducing shade and using covers when needed.

References

Anderson, J.A., Taliaferro, C.M., & Martin, D.L. (1993). Evaluating freeze tolerance of bermudagrass in a controlled environment. HortScience, 28(9), 955-955.

Anderson, J.A., Taliaferro, C.M., & Martin, D.L. (2002). Freeze tolerance of bermudagrasses: Vegetatively propagated cultivars intended for fairway and putting green use, and seed‐propagated cultivars. Crop Science, 42(3), 975-977.

Dunne, J.C., Tuong, T.D., Livingston, D.P., Reynolds, W.C., & Milla-Lewis, S.R. (2019). Field and laboratory evaluation of bermudagrass germplasm for cold hardiness and freezing tolerance. Crop Science, 59, 392-399. https://doi.org/10.2135/cropsci2017.11.0667

Fontanier, C.H., Moss, J.Q., Gopinath, L., Goad, C., Su, K., & Wu, Y.Q. (2020). Lipid composition of three bermudagrasses in response to chilling stress. Journal of the American Society for Horticultural Science, 145, 95-103. https://doi.org/10.21273/JASHS04815-19

Gopinath, L., Moss, J.Q., & Wu, Y.Q. (2021). Evaluating the freeze tolerance of bermudagrass genotypes. Agrosystems, Geosciences & Environment, 4(2), e20170. https://doi.org/10.1002/agg2.20170

NTEP. (2014). 2013 national bermudagrass test. (Progress Report NTEP No. 15-2). National Turfgrass Evaluation Program. http://ntep.org/reports/bg13/bg13_15-2/bg13_15-2.htm

Xiang, M., Fry, J., & Wu, Y.Q. (2022). Winter survival of experimental bermudagrasses in the upper transition zone of the US. International Turfgrass Society Research Journal, 14(1), 708-712. https://doi.org/10.1002/its2.3