Winter Kill - 2010 - Part 4

Winterkill 2010?
With colder than normal temperatures in Arkansas this winter, we are anticipating that some of our turfgrasses around the state may suffer from winterkill. To help prepare for this we are publishing a four 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 four weeks.

Part I: Predicting the damage: What causes winterkill and how can we estimate our losses?
Part II: Preparation and recovery: What should you do or not do this spring to help your turf?
Part III: Planning and planting improved cultivars for a better future.
Part IV: Practices to enhance winter survival.


February 5, 2010

Part IV: Practices to enhance winter survival.

As we stated in Part III, choosing a cold hardy cultivar 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 affect on winter hardiness.

  1. Nitrogen fertilization
  2. Potassium fertilization
  3. Plant growth regulators
  4. Mowing
  5. Drainage
  6. Shade
  7. Covers
  8. Topdressing
  9. Traffic
  10. Disease

Late-season N:
Recent research on bermudagrass found that late-season N applications prior to frost promote fall color retention and do not have a negative effect on bermudagrass winter hardiness. Late-season fertilization is highly recommended for athletic fields and some newly established lawns. The only downside to this practice is that it will increase winter annual weed pressure and may predispose bermudagrass to more injury from spring dead spot and zoysiagrass and bermudagrass to more large patch.

Late-season 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 lawn soil has optimum levels of K.

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, it must be using the energy for increased storage. Researchers have tried to document this effect, but no increase in cold hardiness has been documented from applications of PGRs prior to winter.

It is thought (not scientifically proven) that increasing the mowing height at the end of the season (or skipping the last couple of mowing) will help increase winter hardiness. This should in theory increase leaf area allowing for more energy production and more energy (carbohydrates, proteins, solutions, 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 drainage:
Grasses grown in poorly drained areas are more likely to winter kill. Ice accumulation in these areas during the winter can kill these plants. Make sure to correct/improve drainage in low lying areas of golf course fairways and other turf areas to reduce the likelihood of winter kill.

Soil temperatures in shaded areas stay cooler in the winter months. These cooler temperatures allow the soil to stay frozen longer, ice to remain on the surface longer, and 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, a more cold hardy variety should be planted in shady areas or trees pruned to reduce shade.

In open areas (especially in plains states such as Kansas and Nebraska) winter kill often occurs from desiccation (drying out) due to low humidity and high winds during winter. To help prevent this from occurring, important turf areas are often covered with blankets to prevent desiccation. Blankets serve to reduce desiccation and to help retain soil temperatures. In Arkansas and other states in the southeast, blankets are often used for 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 “poor man’s” solution to preventing desiccation and to help increase soil temperatures is to apply a moderately heavy application of topdressing sand immediately prior to the onset of winter. This topdressing helps protect the crowns and reduces desiccation. The dark-color topdressing also helps attract additional solar radiation and usually will results in a minor increase in soil temperatures which can also help to reduce winter injury. This technique might be appropriate on bermudagrass golf course putting greens and bermudagrass athletic fields.

Traffic is a stress and can predispose a plant to winterkill or accentuate winterkill. Avoid all additional stresses on turf prior to entering winter dormancy. When possible remove traffic from areas during winter, especially when temperatures are at or near freezing. Rotate traffic on athletics fields to reduce the level of injury. Keep carts on paths on golf courses to reduce injury. Do not open areas to play when soil temperatures are near freezing.

Spring dead spot (SDS) is generally considered to be the most significant disease of bermudagrass. This disease becomes evident at spring green-up time during March or April in Arkansas.  Although several root-infecting fungi have been identified as being responsible for the disease in other regions, Ophiosphaerella korrae seems to be the causal fungus in Arkansas. The diseased area appears in the spring as well defined dead circular patches than can range in size from a few inches to more than 3 feet in diameter. Symptoms may sometimes be confused with winterkill and injury from soil insects such as white grubs. Although spring dead spot symptoms may occur on bermudagrass lawns of all ages, it typically appears 3-4 years after the turf has been established. This disease primarily affects the roots. Death of the plants is believed to occur following normal low winter temperatures. The fungi which cause SDS usually begin to colonize the roots, stolons, and crowns of bermudagrass in the late summer or fall and again in the spring when soil temperatures range from 50-70° F. Even though root and crown infections occur in the fall, foliar symptoms do not show up until green-up in March and April of the following year.  Excessive nitrogen fertilization during the late summer months tends to enhance symptom development during the following spring season. To reduce the likelihood of SDS in areas with a history of this disease, heavy applications of fast release nitrogen fertilizers should not be made in late summer and fall.  There is a close correlation between cold hardiness of bermudagrass varieties and disease susceptibility.  Cold hardy cultivars are more tolerant to the disease because they are not as easily winterkilled from the root infection. Fungicides which containing fenarimol, propiconazole, or myclobutanil are most effective.  For maximum effectiveness, these materials need to be applied according to label directions at least twice in the fall when the fungus becomes active. 

Dr. Aaron Patton