The recent cold snap has raised a number of questions about survival of warm-season turfgrasses, especially given the relatively recent popularity of bermudagrass for fairways and athletic fields. Though winterkill is highly variable, all turfgrasses are most susceptible to cold temperatures very early and very late in the winter and thus we wouldn’t expect much winterkill from the last few week’s temperatures. Furthermore, snow cover will help insulate the turf and moderate temperature swings that can damage the turf. Winterkill in cool-season grasses is dependent on many of the same factors as it is in warm-season grasses and following following is more information on winterkill in warm-season grasses from a 2007 Turf Tip.
Winterkill in warm-season grasses is highly variable and difficult to study by scientists. Because of this, our understanding of winterkill is based on science as well as practical experience and observation. Since winterkill in warm-season grasses can be affected by genetics, temperature extremes, and management factors, an endless combination of these factors can interact to cause highly variable damage. Any one of these factors may cause significant winterkill, but a combination of many of these factors will certainly increase the chances of winterkill. Factors contributing to winterkill of zoysia and/or bermudagrass include:
Temperature extremes: Most winter damage on bermudagrass occurs between 18 to 23F, while damage to zoysiagrass occurs at lower temps near 12F. Duration of cold temperature and timing of the cold will also affect survival. Since Indiana is on the northern edge of adaptation for warm-season grasses, low temperatures are usually an annual problem.`
Excess winter traffic: Cart, foot, or mower traffic can crush frozen or dormant crowns causing death. Traffic can also cause compaction, reducing soil oxygen and weakening the plant and thus increasing winterkill. This is why winter traffic should be avoided on warm-season fairways, tees, and athletic fields.
Genetics: Zoysiagrass is more winter tolerant than bermudagrass and within each species, cultivars exhibit a wide range of winter tolerance. Quickstand, Midlawn, Vamont, and Riviera are the most winter-tolerant bermudagrasses, while Meyer and Zenith are the most winter-tolerant zoysiagrasses. Our very recent zoysia data suggest genetically high levels of glucose, proline and dehydrin-like polypeptides improve winter-tolerance.
Overseeded ryegrass to maintain winter color will out-compete warm season grasses during cooler weather and enhances winterkill. Overseeding temporary winter turf into dormant bermudgrass should be avoided if possible.
Age of warm-season grass: Usually zoysiagrass or bermudagrassare are most susceptible to winterkill in the first winter after establishment because they have not maximizes stolon or rhizome formation, and these structures are key in winter survival.
Excess water/poor drainage can cause direct damage to crowns by freezing and thus killing the crowns. Plus too much water can reduce soil oxygen as mentioned previously.
Excess thatch can elevate the crown and since thatch is not as insulating as soil, the crown in thatch is exposed to colder temperatures compared to that in minimal thatch areas.
Dry soils/exposed windy areas can lead to dehydrated crowns causing winterkill. Snow cover or winter covers may help alleviate winterkill where practical.
Mowing height: Higher mowing heights tend to lower winterkill because of increased carbohydrate storage. Higher mowing heights going into winter are also thought to provide structural protection of crowns as well as potentially insulating the crowns.
Shaded areas tend to have the most winterkill because of overall lower temperatures, reduced photosynthesis and thus carbohydrate storage going into winter, and also competition from roots when the shade is caused by trees.