Gil Landry - The University of Georgia

So what does the very cold, record cold, December mean??? ..... Let's start by finding out how cold it was. In middle Georgia (Griffin) the average temperature for the month was 11⁰F below 1999 or 36⁰. That was 13⁰ below 1998 and 7⁰ below 1994, a year of significant winterkill.

Are you sick yet? Well, how about the average low for the month being 26⁰? How about nightly lows of 10⁰ to 21⁰ for seven consecutive nights before Christmas? OK, enough torture. At this point, winterkill is still speculation. So let's review the literature and past experiences.

Winter injury is most commonly caused by direct low temperature stress, winter dessication, low temperature fungi, and traffic (Beard, 1985). Winter injury generally also results from the interaction environmental, soil, and cultural factors. Rapid temperature drops below 25⁰ F can result in injury to some warm season species. Fry, 1991, provided the following killing temperatures of plant tissue as follows:

Obviously, turfgrasses with rhizomes have the added insulation of the surrounding soil and observations suggest there is significant variation between cultivars within some of these species . . . In other words these numbers are far from absolute.

Beard, 1973, also points out that more winter injury tends to occur in late winter as plant hardiness declines. Winter injury is also more common to the exposed and less insulated areas which suffer dessication. Injury may also reflect an accumulation of stress over an entire season leading to a less hardy plant. Obviously, drought stress from last year and any below normal rainfall now may contribute to injury. Finally, significant injury after greenup begins is another problem when mild 70-degree temperatures are followed by a freeze.

Optimistically, we had fairly good moisture during the cold temperatures of December and most grasses were hardened off before the extremely cold temperature.

At this stage, about the only practices that can help minimize injury include preventing dessication with irrigation and minimizing traffic until after spring greenup. So maintaining good soil moisture, minimizing traffic and cultural injury from verticutting, scalping, topdressing or aerification should help. Properly timing that first fertilizer application and minimizing the risk of injury from herbicide applications might also be important. Finally, spring disease control should also help recovery.

The following were observations made after the winter of 1994-1995, the last time we had significant winter injury. First it was really interesting to see the relatively minor differences that separated some dead and live areas.

The following factors are ranked in their general order of influence on the severity and amount of winter injury observed mainly on golf courses.

1. Traffic/soil compaction. These factors are presented together because most soils of north Georgia are naturally hard. This "natural" hardness combined with other factors like slope severity and direction often results in injury. The most common example showing the effects of traffic and soil conditions on winter injury were irrigation trenches where the grass growing on the more friable disturbed soil survived while the grass around it did not.

Most traffic related injury was on tees, tee and green approaches, and in golf car traffic areas on fairways. Newer golf courses established with minimum soil preparation or little topsoil spreading tended to have more extensive injury. There were also many dead areas where rock was present two to four inches below the surface.

Around many greens with sandy soils, injury was high because of the lower insulation and greater droughtiness of the coarser soil. Of course these areas also were mowed lower and received more traffic. In some cases, mower equipment tracks on fairways and roughs were obvious.

2. Mowing height. In most cases where there was a comparison of mowing heights, like rough-to-fairway lines, tee top-to-tee slope, and green collar-to-green apron, the lower mowed area was more injured. The most interesting mowing effect was with small undulations of less than one-half inch. The high part was dead, the low alive. These minor depressions probably got more water and fertilizer than the higher areas.

3. Slope direction and severity. Most injury was on north facing slopes or other slopes that receive little direct sunlight during the winter. Injury was also more common on steeper slopes and small mounds in fairways. Besides being cut slightly lower, these small areas probably got less water and fertilizer during the year because of runoff, etc.

4. Trees and shade. Obviously areas shaded during the winter generally get colder and stay colder for longer periods of time. However tree roots also compete for water and nutrients and that extra competition during low rainfall periods might also have had an effect. Often in really densely shaded areas grass survived in trenches, small depressions, and at a higher mowing height.

5. Moisture. Non irrigated areas generally had significantly more injury than irrigated areas. Injury related to moisture seemed to be as common in low, wet areas as in high, dry areas. However, areas not receiving water for probably 10 to 14 days during the spring were much slower to recover.

6. Cultivars. In many areas, coarse-textured, common-type bermudagrasses were more injured than the hybrids. This was more common on older, established turf areas.

7. Minor stresses. Stresses that normally are tolerated may have had more influence on turf vigor because of colder and drier than normal conditions. Examples include a patch-type symptom similar to rhizoctonia, insects like white grubs, and many areas suggested herbicide applications which would normally be safe may have contributed to some of the injury. Finally, in some areas, excess thatch was another factor.

So, you want to check out your grass. Take plant samples, including the roots and put them in a greenhouse or 18 inches below a 100 watt bulb in a protected area and keep them moist. If you want results in two to three days, remove all the soil from several stolons and place them in a clear plastic sandwich-type bag with a saturated paper towel. Leave the bag open and place it under the light. In a few days you should see new tillers forming at the nodes.

References

Beard, J.B. 1985. Winterkill of Warm Season Turfs: Diagnosis, Causes, and Preventions. Quarterly Bulletin of the Southern Turfgrass Association.

Beard, J.B. 1973. Turfgrass: Science and culture. Prentice-Hall Inc.

Fry, J. 1991. Freezing resistance of southern turfgrasses. Lawn & Landscape Maintenance.