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Mississippi Turfgrass Association – What You Need to Know about Winter Kill
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What You Need to Know about Winter Kill – Jay McCurdy, PhD Assistant Professor and Turfgrass Extension Specialist
Contrary to popular belief, there are things that can be done proactively, and from hence forth, to reduce potential problems with winter kill.
“Winter Kill” is a common phrase used to describe the effects of harsh winter conditions upon warm- and cool-season turfgrasses alike. Warm-season species in Mississippi and the rest of the southeast include bermuda, zoysia, St. Augustine, centipede, bahia, and carpet grasses. All are susceptible to freeze damage. But damage is not solely linked to cold weather. For instance, not all properties are equally affected. It is not atypical to see a dead lawn adjacent to a lawn that greens up well in the spring. Similarly, certain species seem to suffer more at different latitudes and due to sometimes small differences in growing conditions. There’s not much that can be done to stop the effects of cold weather, but there are some proactive strategies that help decrease the chances of winter damage.
Pro-Tips
1) Pre-season cultural practices. The condition of grass in the spring is determined by how healthy the grass was going into winter. Proper conditions during the growing season allow turf to grow roots and store up carbohydrates needed during the winter/spring. If turf is unhealthy due to improper cultural practices or conditions, the grass will be more susceptible to winter-injury.
Mowing height. This is possibly the most important yet overlooked cultural practice. Superintendents know intuitively to raise mowing heights in the fall to allow greens to recover. That means slower ball roll, but that is cheaper than replacing greens during May and June the following season. Likewise, homeowners and other professional turf managers should increase mowing heights before cooler fall temperatures. Some possible exceptions are 1) in overseeded scenarios or 2) on high-end sports fields where athlete performance is a consideration.
Perform aerification and sand-topdressing. Root health depends upon adequate soil drainage and gas exchange. There are many lawns, if not most, that would benefit from aerification to relieve compaction and promote root growth. Most golf courses try to core-aerify greens at least once per year and perform other small-tine or solid-tine practices several other times per year. This is almost always combined with frequent and light sand topdressing. Sports field managers should at the very least be aerifying and topdressing high traffic areas, such as between the hash marks on American football fields and at goal mouths of Soccer pitches. Aerification of lawns is a less common practice unless they are highly compacted. I argue that most homeowner rented aerification tools are probably inadequate to perform the type of soil disruption necessary to decrease soil bulk density and increase turf health. Timing of aerification is also critical – too early, and turf is slow to emerge from dormancy; too late, and turf health will be compromised prior to winter. Ideal aerification timing for warm season grasses is approximately May to July. This doesn’t mean you can’t vent or solid tine greens, but it does mean you need to think critically about why you’re doing these practices and whether grass can recover from the stress.
Regardless of aerification, there is no substitution for proper construction of the soil medium. This means proper drainage and nutrient holding capacity. Shoddy soil construction is all too frequent in modern suburban lots and on school playing fields. Even the best turf professionals cannot solve this problem without heavy equipment.
Reduce tree root competition. Trees are an important part of our landscapes, but the turf under them often suffers from drought and limited nutrients due to competition from trees. Root pruning or tree trimming are sometimes the best option to increase turf cover under trees.
Irrigate correctly during the growing season. Over watering is a common mistake. Proper soil moisture management means irrigation cycles should only occur when needed. Irrigation can be a double edged sword. In some instances, irrigation mitigates drought stress, but in excess, it causes disease, shallow roots, and scalping. Excluding golf greens, typical irrigation of native soils should be approximately one inch of water applied once per week (not every day) in lieu of natural precipitation (a rain gauge or coffee can works fine for measuring amount applied).
2) Let the grass go dormant. Dormancy is an avoidance mechanism for tough winter months (when plants can suffer from cold damage, saturated soil conditions, and even desiccation due to dry winds). Fall nitrogen fertilization to enhance or prolong green color is a risky decision. Fall applications of potassium are frequently suggested. In general, if soil test results suggest that a soil contains “medium” or less potassium, then it’s safe to assume potassium applications could be beneficial. There is no substitute for a soil test! Period. Keep in mind that potassium uptake is minimal when soil nitrogen is limiting, so an all-around well balanced fertility program (throughout the year) is necessary.
Likewise, don’t rush spring emergence. Fertility applications to lawns and fairways prior to the historic last frost date are risky.
3) Don’t go overboard with herbicide treatments during fall and spring transition! Herbicides applied in fall and spring can affect rooting of turf, especially when weakened by environmental factors listed previously. Many herbicides affect plant roots of weeds AND desired turf alike. In isolated incidences, weakened roots may be more susceptible to herbicide injury than normal. I have seen several bermudagrass lawns injured by inopportune fall and spring applications of 2,4-D containing products, especially when applied in conjunction with preemergence herbicides.
What to Do Now?
Let me reiterate: Do not rush spring time applications of herbicides following a harsh winter. Even if grass is 100% dormant, use caution and rationale when applying non-selective herbicides (like glyphosate or diquat).
The same is true of selective herbicides. Avoid application of selective herbicides during transition from dormancy to green cover. For instance, auxin-mimicking herbicides, such as 2,4-D and mecoprop, are labelled for use on bermuda and zoysia. Simazine is a common “go-to herbicide” for centipede and St. Augustine during winter months. But for some reason, even these “safe” and “labelled” herbicides can delay spring green up when applied at improper timings when turf is weakened by winter.
When possible, avoid auxin-mimicking herbicide applications on St. Augustine and centipede grasses during fall and spring transition. Even if you think these two grasses are dormant, they are not!
Avoid high rates of preemergence herbicides. This is a tricky decision. Our spring crabgrass/goosegrass control programs depend upon preemergence applications at proper timings. That initial preemergence timing is roughly late February through mid-March for most of the mid-South. Follow label directions and use conservative rates when appropriate.
To Conclude
There are many factors that contribute to winter kill. Attention to proper cultural practices remains key to managing turfgrass.
A previous version of this article appears as “Proactive steps to prevent winterkill on turfgrass” in the GCM online magazine: https://www.gcmonline.com/course/maintenance/news/prevent-winterkill
Fig 1. Winter kill injury of Ultradwarf bermudagrass greens manifests itself differently depending upon a number of environmental conditions. Here, thin spots are weakened and lack full turf canopy. If not quickly nursed back to health, these areas become open sores that won’t heal without sodding.
Fig 2. Trees are a fundamental part of urban infrastructure and design, but they exacerbate winter kill, especially on golf course putting greens.
Fig 3. Traffic, such as from carts or sports competitions, also exacerbates winter injury.
Fig 4. Bermudagrass regrowth depends upon carbohydrate and nutrient reserves laid as a foundation in the fall and early winter.
Fig 5. Water confluence areas that stay saturated throughout dormancy tend to be slowest to recover.
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