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Turfgrass Science

Winter 2014 will be remembered for a few bitterly cold periods as well as it being the winter that just wouldn't end. The coldest temperature recorded in Lexington was in early January and we reached -6 F. The question on everyone’s mind is how are the grasses handling this winter’s temperatures. I suspect our cool-season grasses will all be fine. Kentucky bluegrass and creeping bentgrass both have excellent cold tolerance and tall fescue has fair cold tolerance.  The question isn't so much about our cool-season grasses but really what’s going to happen to our warm-season grasses. The answer, unfortunately, is not a simple one. Certainly cold temperatures can lead to warm-season grass loss, but the length of the cold snap is a critical part of the puzzle. Soil is an excellent buffer of temperature. It takes a long time to warm up in the summer and a long time to cool down in the winter. The good news about this is that rhizomes are encased in this medium that is warmer than the air temperature. The issue with prolonged cold air temperatures is that the soil will eventually get cold and the concern of sub-freezing temperatures being in contact with the rhizomes becomes a reality. We only stayed extremely cold for a few days so we may be safe if we are just talking air temperature. We also had snow cover for several days throughout the winter and snow is an excellent insulator as it slows the loss of heat from the soil. Other factors come into play with winterkill. Wet and dry soils, wind causing desiccation, north facing slopes, excessive thatch, shade, prolonged ice cover, warming and cooling periods, cultivar, and excessive traffic are all factors that can contribute to warm-season grass loss during the winter. The more of the above factors you can check off for your stand of turf, the greater the chance that you will experience winterkill.

for post-emergent crabgrass control are quinclorac or MSMA (golf and sod only). If you are planning on vegetatively planting, the only pre-emergent herbicide available is oxadiazon. Most of the other common pre-emergent herbicides will cause root pruning and hinder the spread of warm-season grasses.

What it all boils down to is that we live in the heart of the transition zone. There is never a guarantee that warm-season grasses will make it through our winters. It’s been since 1998 that we had any significant lose of bermudagrass in Kentucky, so we tend to forget that it happens. However, if you are growing warm-season grasses in the transition zone, you will likely experience winterkill at some point during your career. Choosing an appropriate cultivar and properly managing the stand before and during winter will significantly reduce the chance of loosing grass during the winter.

The effect of the cold winter on grasses in Kentucky

We pulled various warm-season samples in early February to see where we were in terms of winterkill. If you are concerned about whether you have lost grass this winter, the steps for checking are quite simple. Remove a few cup-cutter samples and bring them inside (Figure 1). Place them in pots and either place them in a warm and sunny win-dow or place them below a 100 watt bulb where they will experience the heat and light from the bulb. Wait a couple weeks and you should have a good indication of the health of the grass. It appears from our test that the fairway/sports field type bermudas should be in pretty good shape (as is evidenced by the green shoots appearing in Figure 2). The greens bermudas and zoysia may be in trouble. However, none of the grasses shown in the images were covered. Covers will always help warm-season grasses survive harsh winters. Dr. Powell wrote a good article on winter covers and it is available by clicking here. We have the light-weight black woven covers on some of our warm-season greens grasses (Figure 3). We placed a temperature sensor under the cover to measure the difference between air and cover temperatures. When the air temperature hit -6 F, the under cover tempera-ture was 16 F. This is a huge difference but 16 F is still plenty cold enough to kill bermuda-grass. Tony Whitmer in Bowling Green covered his ‘Champion’ bermudagrass greens and his tests of greenup have shown good success (Figure 4). He tells me that it appears the only place he may have lost grass was where he didn’t have covers.

If you lose grass this winter, there are a few things to keep in mind for re-grassing. First, if you plan on seeding, do not apply a pre-emergent herbicide. These herbicides work just as well on desirable grasses as non-desirables. Your choices

Figure 1. Taking samples for testing of winter survival only involves a cup-cutter or plugger.

Figure 2. Greenup after 2 weeks at 85 F and constant light. Clockwise from top left: Champion, Riviera, Tifeagle, Tifway, Latitude 36, Diamond zoysiagrass, and an experimental bermudagrass.

Figure 3. Black light-weight woven covers are easier to install/remove and appear to offer good protection from cool temperatures.


woven covers.

Figure 4. A plug of Champion bermudagrass from Bowling

Green, KY. These greens were covered with black light-weight covers.

woven covers. Photo courtesy T. Whitmer.