Winter Injury of Warm-Season Turfgrasses

Louisiana has a humid subtropical climate with mild winters. However, every few years, winter temperatures will reach below 55 degrees Fahrenheit for extended periods and even below freezing — lower than 32 degrees Fahrenheit — for short periods. In northern Louisiana, icy precipitation is more likely than southern Louisiana. Depending on the temperature and temperature duration, turfgrass can be injured. This injury is typically not visible during the cool periods but becomes visible in spring when patches of dead turfgrass are surrounded by green and growing turfgrass. Understanding the causes of winter injury can help homeowners select more cold-tolerant species and develop management strategies to decrease winter injury incidence.

Periods of increased susceptibility to winter injury

Warm-season turfgrasses are most susceptible to chill (32 to 50 degrees Fahrenheit) or freeze (below 32 degrees Fahrenheit) injury prior to acclimation in the fall or just after exiting dormancy in late winter or early spring. As the hours of daylight and temperatures decrease in fall, plants receive environmental cues as signals of changes in season that lead to complex reactions occurring in the plant. This period of acclimation is extremely important in preparing the plant for survival during unsuitable growing conditions. For warm-season plants, growth slows, energy reserves are funneled downward in the plant and dormancy can be induced. If severe cool temperatures suddenly occur before the plant is acclimated, there is less tolerance to winter injury. This is similar to what happens during late winter or early spring when the turfgrass is exiting dormancy and resuming growth. Stored energy is converted for use by the turfgrass to grow leaves and roots. These new tissues are tender and more prone to environmental stresses such as cool temperatures. Optimal temperatures for warm-season turfgrass shoot growth are 80 to 95 degrees Fahrenheit.

The basics of winter kill

• Winter desiccation: Most of the time we associate drought with warm temperatures, but winter can also be a time when droughty conditions exist. Dry winter winds can lead to greater evaporation from the soil, especially for open areas with higher topography. Although dormant warm-season turfgrasses do not require the same moisture as when they are actively growing, dormant turfgrasses do require adequate soil moisture. Make sure the soil has sufficient moisture every few weeks to ensure the turfgrass does not suffer from winter desiccation. This is especially important if sod is installed during the winter months.
• High moisture areas: Winters in Louisiana can bring periods of intense rainfall. Areas that have excess moisture are more prone to injury from temperature modulation when freezing temperatures occur. Areas that tend to have greater soil moisture include poorly draining turf and shaded turf. Increasing drainage is not only advantageous during the growing season but can help drain water away during the winter.
• Poor potassium fertility: Potassium regulates water movement within the plant. Just like salts that are applied to driveways or steps to prevent ice formation, potassium can depress the freezing temperature in the plant. When water freezes in plant cells, expansion of the ice can rupture cells, leading to cell death and ultimately plant death. By taking routine soil tests, fertility levels can be adjusted when the turfgrass is actively growing. This will encourage a healthy, more resilient plant prior to winter.
• Late-season nitrogen: It is tempting for homeowners to apply nitrogen late in the growing season to keep their lawns green. However, late-season nitrogen applications can cause the turfgrass to surge in growth during a period when it should be acclimating to shorter days and cooler temperatures. Once temperatures are consistently cool, many warm-season turfgrasses enter dormancy and cease growth. Dormancy provides a survival mechanism for the turfgrass to endure unsuitable environmental conditions until suitable environmental conditions return. Late-season nitrogen applications induce turfgrass shoot growth that can deplete the plant’s energy reserves, which leads to poor winter survival. Applying fertilizer earlier in the growing season allows the turfgrass to take up and use the applied nutrients to develop healthier, more resilient plants prior to winter.
• Physical injury: Frozen conditions occur when temperatures remain at 32 degrees Fahrenheit. Frost occurs when the temperature goes below the dew point and water vapor condenses to form ice on the turfgrass blades. Frost will often melt and evaporate as daytime temperatures increase. However, it is important to prevent foot and vehicular traffic across frozen or frosted turfgrass areas. Traffic compresses ice crystals to physically injure the turfgrass. Areas that have been trafficked during frozen or frosted periods can show footprints or tire tracks when the turfgrass exits dormancy and begins to grow.

Turfgrass cold tolerance

Warm-season turfgrass species do not have the same cold tolerance as cool-season turfgrass species. The cold tolerance description in the table is relative based on areas that have suitable environmental conditions for growing warm-season turfgrass species as perennial plants. Zoysiagrass has the highest cold tolerance, followed by bermudagrass. Centipedegrass is typically more cold tolerant than St. Augustinegrass. Differences in cold tolerance can exist between cultivars of a species. Please consult your local extension agent if you have further questions concerning winter injury. As a general rule of thumb, warm-season turfgrasses that have rhizomes tend to have greater cold tolerance.

Tips to reduce winter stress

• Plant the proper warm-season turfgrass species and cultivar.
• Maintain proper soil moisture to prevent winter desiccation.
• Have good drainage to prevent excessive soil moisture or ponding of water.
• Submit and follow soil test recommendations for a healthy turfgrass.
• Do not apply late-season nitrogen.
• Prevent foot and vehicular traffic across turfgrass that is frozen or frosted.

Download here: Louisiana Home Lawn Series: Winter Injury of Warm-Season Turfgrasses 3624-JJJ