Cover Story
Weathering the
Storm
– Bermudagrass Winterkill 2010–11
By Mike Goatley Jr., Ph.D., Whitnee Askew and Erik Ervin, Ph.D., Virginia Tech
T
urfgrass scientists look at environmental extremes as blessings in disguise. As researchers, the Virginia Tech team is pleased when environmental extremes allow for some “separation” in turfgrass-variety performance. At the same time, we hate to see these challenges posed to our industry colleagues whose livelihood is managing these same grasses that might have perished during one of these extremely challenging periods. The winter of 2010–11 at the Turfgrass Research Center in Blacksburg provided some important new information in bermudagrass winter hardiness for our transition-zone climate.
What is winterkill?
One of the most pertinent descriptions of winterkill overheard in the past few years was when a fellow scientist from another land grant university said, “Winterkill is like pornography. You don’t know how to explain it, but you know what it is when you see it.” This no doubt leaves you scratching your head in how bermudagrass winterkill and pornography could ever be linked, but you get the point, don’t you?!
Contributing factors
The factors that somehow combine to create bermudagrass winterkill include the following.
Low temperatures
Although cold weather is the most obvious of the criteria, this is not always a clear-cut reason why winterkill occurs. Growth-chamber studies from Oklahoma State University reported that the lethal temperature for 50% of the bermudagrass stems tested ranged from 17°F for Midlawn to approximately 22°F for AZ Common. These studies exposed the stems directly to the low temperatures for specified time periods with no “insulating” effect of the soil (mineral, organic and moisture components) and any dormant foliage. Therefore, it is probable that bermudagrass stems can survive lower temperatures in the field due to the insulating value of the soil and the turf canopy.
Soil moisture
Either too wet or too dry is a likely contributor to winterkill. Wet soils require more energy to cool AND more energy to warm — they resist rapid changes in temperature, so
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this can both work for you and against you in terms of winterkill. Dry soils will have significantly greater variation in temperature, both highs and lows. Many people attempt to mediate winter damage by manipulating soil moisture, especially during a dry year when Mother Nature allows a turf manager some level of control. Desiccation (moisture evaporation from the plant’s leaves) is likely a very significant contributor to winterkill, and it is a factor that is often forgotten in the dead of winter, especially when most irrigation systems are “winterized” for the season.
Soil compaction/traffic
A compacted soil will likely already have restricted root growth and overall poor health entering the winter season. Compacted soils are often also poorly drained soils. Soil compaction should be addressed during the active growing season so that the turf can properly heal before winter dormancy.
Exposure
The weakest areas on an annual basis will be those with northeast