Wednesday, March 12, 2014

Warm-season Turf Winterkill 2014: What Can you Expect and NOW WHAT?


With colder than normal temperatures in Indiana this winter, we are anticipating some minor winter damage. To help prepare for this we are publishing a three part series on this topic to help turfgrass managers prepare for what may await them in the spring. Look forward to the following topics over the next week.

Part I: Warm-Season Turf Winterkill 2014: What Can you Expect and NOW WHAT?
Part II: Cool-Season Turf Winterkill: Potential Losses and a Pathway to Recovery
Part III: Snow Molds in the Winter of 2013-14

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Warm-season Turf Winterkill 2014: What Can you Expect and NOW WHAT?
Predicting winterkill is a difficult task because turf can suffer low temperature injury in a variety of ways. The three most common ways that turfgrasses sustain low temperature injury are:
  1. Sustained low temperatures
  2. Low temperature spikes (freezing/thawing/freezing cycles?)
  3. Unseasonably warm temperatures followed by freezing temperatures in late winter or early spring
Sustained low temperatures. Recently we fell in this category when unseasonably cold temperatures occurred. During this period, numerous nights less than 0 °F with average temperatures that were 10-20 °F below “normal”. Many locations have, however, had snow-cover which can insulate the turf and protect it during these very low temperatures.

Meyer zoysiagrass fairway in southern Indiana. Photo credit Andy Eble.


Low temperature spikes. In many cases turfgrasses can survive one or two nights of very cold temperatures because soil temperatures are more highly buffered than air temperatures and do not change as rapidly. Some turfgrass species such as zoysiagrass and bermudagrass have underground stems (rhizomes) which are insulated from the air temperatures and may allow them to withstand sudden drops in air temperatures. Snowfall helps to insulate soils and protect against these temperature spikes but southern Indiana lacked snow cover during our first set of cold spikes in early January and during our recent cold spikes.

Unseasonably warm temperatures followed by freezing temperatures. Warm-season grasses such as bermudagrass enter winter dormancy in late October and remain dormant until April/May. If their winter dormancy in interrupted by unseasonably warm temperatures in March/April which is then followed by freezing temperatures, winter injury can result. It is these conditions that still have the potential for more winterkill conditions in the upcoming late winter/early spring months in our region.
Plant cells are protected from freezing stress by many mechanisms, including the accumulation of certain proteins and enzymes as well as certain sugars and amino acids. When winter dormancy is interrupted by a period of unseasonably warm temperatures, the balance of these protectants in the plant cell changes and predisposes them to greater injury if freezing temperatures follow.

Which areas are more likely to suffer winterkill?
  1. “In general” bermudagrass (particularly older cultivars) is more likely to winterkill than zoysiagrass
  2. North facing slopes
  3. Heavily shaded areas
  4. Poorly drained areas
  5. Heavily thatched turf
  6. Areas planted with poorly adapted cultivars
  7. Areas heavily trafficked during winter
  8. Areas with deficient levels of soil potassium (K)
  9. Areas lacking snow cover to insulate the soil
How can we estimate our losses? To help determine just how much winter injury you might have following these steps.
  1. Collect samples using a cup cutter, shovel or trowel from areas that you feel may be damaged (low areas, shaded areas, uncovered areas) as well as from other areas not likely damaged (full sun, south facing slopes; areas planted with a cold tolerant cultivar/specie) or areas with a history of early spring green-up. Make sure that the sample you collect is at least 3 inches in diameter and is at least three inches deep.
  2. Clearly label each sample denoting the date and location where it was collected.
  3. Place the samples in a warm area (room temperature or above). Provide lighting and water the plugs when necessary.
  4. Observe the plugs over the next two-three weeks. Alive plants should start to green-up (initiate new leaf growth) within 14 days. Differences in green-up between plugs collected from different areas should provide information on whether the plants survived the freezing temperatures. This will help as you begin to make decisions and plan for recovery.
Preparation and recovery: What should you do or not do this spring to help your turf?
Spring management practices including whether or not to apply spring preemergence herbicide applications should be based upon your winterkill likelihood estimates.
  1. Damage not likely (minimal)
  2. Moderate damage expected
  3. Severe damage expected
Damage not likely. Some may have little fear of winterkill including those who have ‘Meyer’ zoysiagrass, a cold-hardy cultivar of bermudagrass (Riviera, Patriot, Northbridge, Latitude 36, Yukon, Quickstand), or those predominantly growing cool-season grasses such as turf-type tall fescue or Kentucky bluegrass. If you fall into this group consider yourself lucky. While we don’t have as much data on newer bermudagrass cultivars like Latitude 36 and Northbridge, we anticipate that they will have good survival. Particularly if not heavily trafficked late in the season. Those who have little fear of winterkill may go ahead as usual and make their scheduled preemergence herbicide applications this spring.

Moderate damage expected. Those who manage areas of bermudagrass that are partly shaded and common bermudagrass stands known to thin in prior difficult winters should expect some turf loss. It is too early to tell whether this damage will be minimal, moderate, or severe, but some level of winterkill should be expected. See below for preemergence herbicide strategy for these areas.

Severe damage expected. Common bermudagrass in southernmost Indiana like Evansville, Corydon, and New Albany that historically thins in winter or is slow to green-up. These areas did not have snow cover to insulate the soil and direct low temperatures injury may be expected.  See below for preemergence herbicide strategy for areas anticipating winterkill.

Preemergence herbicide strategy for areas anticipating winterkill. Early spring preemergence herbicides are often necessary in Indiana to prevent troublesome summer annual grassy weeds such as crabgrass and goosegrass. Most preemergence herbicides work to kill weeds by preventing cell division causing death to weed seedlings shortly after they germinate. These products can also affect the rooting of established turfgrasses, especially when new stolons begin to initiate roots. Ronstar (oxadiazon) is the only preemergence herbicide that will not affect the rooting of stolons that are trying to recover following winterkill.  Additionally, all preemergence herbicides work to prevent the emergence of turfgrass seeds as well as weed seeds, so do not reseed areas treated with a preemergence herbicide or do not apply a preemergence herbicide if you plan on seeding.
Therefore, if you expect some level of winterkill then it is advisable to skip or delay your preemergence herbicide application this spring. Focus on letting your turf to green-up this spring, and perhaps consider a post-emergent option.

If little winterkill is noticed, then you may go ahead and apply your preemergence herbicide application after green-up. You may consider tank-mixing a postemergence herbicide in the tank as well to control any emerged crabgrass or just using Dimension (dithiopyr) since it has both preemergence and postemergence activity.

If moderate winterkill is noticed after green-up then you will either need to regrow these areas either by 1) waiting for the existing turf to recover (aided by increased “light fertilization” :0.2-0.4 lb. of actual N/1000) or by 2) reestablish these areas from sod, sprigs, or seed depending on the species and cultivar present. If you choose to wait for the existing turf to recover then use postemergence herbicides to control existing weeds. If you need to replant from sod, sprigs, or plugs then you should control weeds  with a postemergence herbicide or use Ronstar as your preemergent source (always follow label directions). If you wish to seed bermudagrass to help recover areas, consider reading this linked publication from the University of Arkansas.

If severe winterkill is noticed after green-up then you will either need to reestablish these areas from sod, sprigs, or seed depending on the species and cultivar present. If you need to replant from sod, sprigs, or plugs then you should control weeds with postemergence herbicides. Newly planted turf is more susceptible to herbicide injury than established turf, so please consult herbicide labels to see when these products may be applied after planting. For instructions on seeding. See the above link.
Another strategy to speed bermudagrass spring recovery is to dormant seed bermudagrass into areas where damage is expected.  The basic approach is to scarify the areas with a verticutter or core-arefier and apply seed while temperatures are still cold (March and April). When soil temperatures warm in May, those seeds will germinate and hasten the establishment. If there is no rainfall be sure to supply irrigation and control weeds using a postemergence herbicide after bermudagrass seedlings have emerged. Instructions regarding dormant seeding of bermudagrass can be found in the above linked publication.

Planning and planting improved cultivars for a better future.
Southern Indiana lies in the transitional climatic zone. What this means is that summer in Indiana is too hot for cool-season grasses to perform well all year and winters are often cold enough to periodically injure warm-season grasses. Unfortu­nately, maintaining turf in the transition zone is more difficult than in many other parts of the United States.

The genetics of the plant is the most important factor that will decide whether or not a plant can survive winter. It is extremely important to choose and plant a cultivar with good winter hardiness when planting a grass at the northern fringe of its adaptation zone. Bermudagrass cultivars like Riviera, Patriot, Northbridge, Latitude 36, and Yukon are reliably cold-hardy in southern and central Indiana (we have tested as far north as West Lafayette). None of the hybrid bermudagrass cultivars and many of the common bermudagrasses are not well-adapted to Indiana.  After this winter, we will learn more about which cultivars are best adapted to our region.

The majority of zoysiagrass used in Indiana is Meyer (Z-52, Amazoy) or Zenith (seeded variety) and should survive this winter just fine based on our previous research.

 

Practices to enhance winter survival in subsequent years.

Choosing a cold-hardy cultivar and keeping it healthy is the best thing you could do to enhance the cold hardiness of the turf you maintain.  Although certain cultural practices may affect winter hardiness, they do not affect cold hardiness nearly as much as cultivar selection.
Below is a list of maintenance practices or maintenance issues and their effect on winter hardiness.
  1. Nitrogen fertilization
  2. Potassium fertilization
  3. Plant growth regulators
  4. Mowing height/frequency (traffic patterns/clean-up passes)
  5. Soil drainage
  6. Shade
  7. Snow cover and blankets/covers
  8. Sand topdressing
  9. Traffic/soil compaction
  10. Disease
Late-season Nitrogen:
Recent research on bermudagrass found that “reasonable” late-season N applications prior to frost promotes fall color retention and does not have a negative effect on bermudagrass winter hardiness. Late-season fertilization is highly recommended for athletic fields and some newly established turf. The only downside to this practice is that it may increase winter annual weed pressure and may predispose bermudagrass to more injury from diseases like spring dead spot or large patch disease.

Late-season Potassium (K):
Potassium is thought to also improve winter hardiness in some situations. As a result, it is commonly recommended that a “winterizer” fertilizer containing a higher ratio of K be applied in autumn prior to winter dormancy. However, research shows that additional autumn K fertilization will not reduce winter injury “if” a soil test indicates that your soil has optimum levels of K.

PGR’s:
Plant growth regulators applied prior to winter dormancy are thought to possibly increase winter hardiness. The theory is that if the plant is not using the energy for increased growth, energy is going into storage. Researchers have tried to document this effect, but no increase in cold hardiness has been documented from applications of PGRs like Primo (trinexapac-ethyl) prior to winter. In summary, PGRs are not going to hurt the plant, and not enough evidence exists to show they help.

Mowing:
It is thought (not scientifically proven) that increasing the mowing height at the end of the season slightly by 0.25-0.5 inches (or skipping the last couple of mowing) will help increase winter hardiness. This should in theory increase the leaf area available for photosynthesis, allowing for more energy production and more energy (carbohydrates, proteins, etc.) storage. Additionally, the extra leaf area will also serve to increase traffic tolerance by providing more cushion above the turfgrass crown and the soil.

Improve soil drainage:
Grasses grown in poorly drained areas, or chronically wet soils are more likely to winter kill. Ice accumulation in these areas during the winter can cause direct kill. Make sure to correct/improve surface and sub-surface drainage in low lying areas of golf course fairways and other turf areas to reduce the likelihood of winter kill.

Shade:
Shaded turf areas are less productive in producing and storing plant foods, they also stay cooler in the winter months. These cooler temperatures allow the soil to stay frozen longer, ice, snow and frost to remain on the surface longer. As a result, these areas can stay 5 degrees (F) or cooler during the winter and lead to increased winter injury. Bermudagrass turf in shaded areas is more prone to winterkill. To remedy this situation, decrease the amount of shade if possible, renovate with a more cold hardy cultivar or species (like zoysiagrass or other cool-season grasses).

Covers:
Blankets serve to reduce winter drying (desiccation) and to help retain heat. Blankets are used in southern states for hybrid-bermudagrass greens when night time temperatures are below 28 degrees (F) to help protect the soil from getting too cold and killing the bermudagrass. A good snowfall also acts as a nice cover. Where chronic winter injury is expected covers or blankets should be utilized to protect your investment.

Topdressing:
A less expensive alterative to covers that will help increase retain soil temperatures is to apply a moderately heavy application (1-4 cubic feet per 1000 ft2) of sand topdressing sand immediately prior to the onset of winter. This topdressing helps protect the crowns and reduces desiccation (drying). The dark-color topdressing also helps attract additional solar radiation and usually will results in a minor increase in soil temperatures which could also help to reduce winter injury. This technique might help reduce winterkill somewhat (theoretical) but the impact is likely small compared to covers.

Traffic:
Traffic is a stress and excess mechanical damage can predispose a plant to winterkill or accentuate winterkill. Avoid all additional stresses on turf prior to entering winter dormancy. Also, remove traffic from areas during winter, especially when temperatures are at, near, or below freezing. If traffic is necessary on athletic fields after warm-season grasses enter winter dormancy, rotate traffic to reduce the level of injury. Keep carts on paths on golf courses to reduce injury potential. Do not open areas to play when soil temperatures are near or below freezing.

Disease:
Spring dead spot (SDS) is generally considered to be the most significant disease of bermudagrass. This disease becomes evident at spring green-up but the pathogen actually infects and damages the bermudagrass in the fall. Fungicide control is difficult and inconsistent, but factors like nutritional status and thatch depth do play a role in the severity of the disease. Research is ongoing at Purdue University to help provide solutions for this turf disease.

Aaron Patton and Cale Bigelow, Purdue Turfgrass Program
 

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