Bert McCarty and Grady Miller, Clemson University and North Carolina State University

Many questions concerning possible winter-kill to bermudagrass have come across our desks with the recent cold snap. The most common are: (1) can I spray a wetting agent to help remove surface moisture? (2) can I spray a film forming spray to coat the turfgrasses to minimize cold damage? but the most common is, (3) Can I “ice” my greens during this cold weather and protect them in much the same way citrus growers do in Florida? Since little if any research or experience is available with these questions, less discuss what makes the most agronomic and plant physiological sense.

What Is Winter (Low Temperature) Kill? Low temperature or cold weather damage to plants is a collective term used to describe several forms of injury. Death by cold is the consequence of damage to cell membranes and the breakdown of metabolism, primarily that involving nucleic acids, proteins, and energy to the cell. The major types of low-temperature injury are caused by:

Crown Hydration. When turfgrass plants remain in or under constant moisture following a thaw, crowns of the plant can absorb (imbibe) high levels of water. If temperatures then decrease below freezing, ice crystals form within the plant cell walls (or intracellularly), rupturing them and, thereby, killing tissues. Intracellular freezing injury is common in spring when plants become de-acclimated, especially in low, poorly drained areas. Ice crystals can also form “extracellularly,” or in intercellular spaces between protoplasts and the cell wall. As ice crystals form, the vapor pressure becomes lower than in the protoplasts; thus, water is drawn from within the cells. The protoplasts then shrink in size and the concentration of dissolved substances, such as salt ions and organic acids, increase and become toxic via osmotic (or moisture stress) damage. If temperatures remain low for a long period of time, cells may become dehydrated to a point where injury or death occurs. Plants surviving winter are more successfully able to tolerate ice formation between cells (extracellularly). Extracellular freezing is most common during dry, cold weather on exposed sites, such as low mowed, sand sites such as golf greens or along hillsides exposed to winds. This occurs on turfgrass plants that are unprotected from wind and have limited soil moisture due to either drought conditions or low temperatures (frozen soil moisture). When temperatures rise above 32°F (0°C), turfgrass leaves lose water to the atmosphere due to pressure gradient created from the low humidity conditions. Consequently, turfgrasses dry out, causing desiccation of plant tissues and important plant proteins.

Direct Low-Temperature Exposure. When turfgrass plants are exposed directly to a rapid decrease in temperature below 23°F (5°C), the injury can be lethal, resulting in significant stand loss. Turfgrass leaves initially appear water soaked, turn whitish-brown in color, and then turn dark brown. Low-temperature exposure causes plants to lose control of membrane function, resulting in solute leakage and, eventually, a water-soaked appearance. Damaged turfgrass tends to mat over the soil surface and emit a distinct putrid (rotten) odor.

Insufficiently hardened turfgrass plants have lush growth (or high moisture content) becoming more susceptible to direct low-temperature injury. Hydrated, non-hardy cells are more freeze susceptible than dehydrated, acclimated cells. Turfgrass plants become “hardened” by being exposed to gradual decreases in temperature and frosts prior to a drastic, sudden drop in temperature or a hard freeze. This hardening process (often referred to as osmotic adjustment) involves an environmentally induced 10 to 25 percent reduction in plant tissue water content and an accumulation of carbohydrate reserves. This reduces ice formation inside cells, preserving cell walls. Although intercellular water freezes at 14 to 23°F (-10 to -5°C), concentrated solutes from osmotic adjustment may serve like antifreeze agents, lowering plant cell freezing to -4 to -40°F (-20 to -40°C), depending on the type and amount of solutes present.

Desiccation. Desiccation is caused by winds in combination with low relative humidity. This desiccation or “drying out” occurs on turfgrass plants that are unprotected from wind and have limited soil moisture due to either drought conditions or low temperatures (frozen soil moisture). When temperatures rise above 32°F (0°C), turfgrass leaves lose water to the atmosphere due to pressure gradient created from the low humidity conditions. Consequently, turfgrass crown, nodes, and roots dry out, causing desiccation of these plant tissues and important plant proteins. Plant injury from desiccation can result in significant losses of turfgrass. It is important to maintain adequate soil moisture during these conditions, especially in spring when turfgrass roots are developing, to prevent desiccation from occurring. If the area is not continuously covered by snow or ice, artificial covers are often placed to help protect the turf from desiccation.

Traffic.  Vehicular and/or foot traffic that occurs on frozen or slush-covered turfgrasscan play a significant role in low-temperature turfgrass injury. Frozen turfgrass crowns are easily damaged from traffic. Traffic should be minimized or diverted from frozen turfgrass until the soil and turfgrass plants have completely thawed. Lightly syringing frozen turfgrass areas prior to allowing traffic will aid in reducing traffic injury. Highly trafficked areas are also prone to high soil compaction. Generally, compacted soils reduce turfgrass rooting and overall plant health, resulting in weak turf that is more susceptible to low-temperature injury.

Parameters Influencing Turfgrass Susceptibility to Low-Temperature Stress Several parameters influence the susceptibility of warm-season turfgrasses to low-temperature stress; however, only a few are controllable by turfgrass managers. Protecting turfgrass crowns from low-temperatures is essential for survival as leaves, roots and lateral shoots regenerate from these. Temperature, freeze and thaw frequency, and freeze and thaw rate are factors outside a turfgrass manager’s control. Conversely, several parameters within the control of turfgrass managers include (1) degree of shading, (2) drainage, (3) fertility, (4) irrigation, (5) mowing, (6) use of covers, (7) soil cultivation, (8) thatch level, (9) traffic, and (10) turfgrass selection.

Shade. Exposure of turfgrass plants to sunlight is critical for several reasons, such as food production (photosynthesis) and hardening. Prolonged shade reduces carbohydrate levels in turfgrass plants, produces weaker plants, and suppresses soil temperatures. Shaded areas also remain colder for prolonged periods due to minimal sunlight exposure, and tend to maintain dew coverage longer than exposed, sunny areas, which can promote diseases. In addition, competition from tree roots for nutrients and water also weakens turfgrass plants. Bright sunlight exposure also signals plants to harden off during fall. Turfgrass plants are ‘hardened’ by being exposed to light frosts prior to a heavy freeze. Northern or northeastern facing slopes receive less (or little) direct sunlight in the winter months and are more susceptible to low-temperature stress.

Drainage. Poor surface and/or subsurface drainage can result in direct low-temperature damage or injury to the crowns of turfgrass plants. The level of water or moisture content within turfgrass crowns is positively correlated to the degree of low-temperature injury. The higher the moisture content of turfgrass crowns, the higher the susceptibility of low-temperature damage. Turfgrass plants growing in areas with poor drainage (surface or subsurface), high compaction, or excessive irrigation are at a greater risk of low-temperature injury.

Irrigation. Water is the primary requirement for turfgrass growth and survival. Irrigation should be utilized to “condition” the turfgrass. Several methods exist on scheduling irrigation; however, irrigation should be managed according to the exact amount of water needed at any given time or any given condition. Just as excessive moisture may increase low-temperature damage or injury, the lack of moisture can be equally damaging. A certain amount of water or moisture is needed for crown tissues to survive low-temperature stress. If a moisture deficiency occurs, crown tissues become desiccated; thus, have less buffering capacity against temperature swings. Maintaining soil moisture at or slightly below field capacity and preventing standing water helps keep crown tissue properly hydrated.

Traffic.Traffic (either vehicular or foot) can be very damaging to frozen turfgrass crowns, leading to the eventual death of plants. In addition, traffic leads to soil compaction that reduces soil oxygen levels (porosity) and water infiltration and percolation rates.

Turfgrass Selection.Turfgrass species and varieties or cultivars within a particular species have varying degrees of low-temperature tolerance (Table 1). Generally, cultivars possessing inherently better low-temperature tolerance exhibit a deeper growing, denser rhizome network which, in turn, are better insulated. These then are less exposed to low-temperature damage or injury.

Table 1.Relative low temperature tolerances of selected bermudagrass from laboratory studies (Anderson, Taliaferro, and Martin 2002).

Turf Covers. Turf covers may help protect turfgrass crowns from direct low winter temperatures and desiccation. When present, snow is probably the best insulator and should remain as long as possible. Artificial covers which use an air layer to insulate the turf from extreme moisture and temperature conditions are next best, followed by straw and other organic sources. Strategies vary on the use of covers. A starting point is to use them continuously when the temperature drops below 25 F and are combined with desiccating winds. In the absence of wind, if temperatures are expected to suddenly drop below 23 F, covers should be considered.

Icing Greens. The concept of icing greens during low temperatures is to provide a thin ice cover which would hopefully keep the soil temperatures around 32 F. This method is often performed with citrus groves during moderate freezes.

In citrus, the temperature has to be between 28 and 32 F and for no more than 4-hours for this to effectively work. Lower temperatures or longer durations tend to reduce its effectiveness. Also, the water is constantly added (irrigated) lightly about every hour so the grower takes advantage of the slight heat released when water molecule turn into ice crystal.

As explained earlier, the major damage to turfgrass is when too little or too much moisture is in the plant during cold temperatures. Icing greens would probably work best for a one-time freeze. However, since this is rarely the case, icing would melt during the day, saturating the soil, causing plant crowns to excessively hydrate (imbibe) and when the freeze reoccurred that night, intracellular ice formation would possibly occur, cause cellular damage and eventual crown death.

Diagnosing Low-Temperature Injury

Superintendents should anticipate certain areas of turfgrass which may be more susceptible to low-temperature injury than other areas. Cultural factors contributing to low-temperature injury and increased susceptibility include poor drainage include soil compaction, excessive thatch, reduced light intensity (shade), north or northeastern facing slopes, areas prone to winds such as high spots and west facing slopes, excessive fall nitrogen fertilization, and low mowing heights.

If any of the aforementioned conditions are present, or have occurred, and low-temperature injury is suspected, utilize the following methods to determine the severity of the potential damage.

1. Collect turfgrass plugs using a cup cutter from suspected low-temperature damage areas.
2. Place or plant these plugs in a suitable container of native soil with drainage holes.
3. Place the containers in a greenhouse, or in a room beneath a heat lamp or grow light, or as a last resort, in a southern facing window.
4. Keep the turfgrass plugs adequately watered.
5. Note that turfgrass should initiate growth and greening within 7 to 10 days.
6. Assess the amount of greening after plugs have been grown for two to three weeks. Suspected areas with less than 50 percent greening should be considered extensively damaged from low-temperature exposure and will probably require renovation. Lesser damaged areas may recover with proper management practices and extra attention.
7. Repeat the above sampling procedure on a 14- to 21-day interval through the periods of potential cold weather injury.

Another method to ascertain plant damage is to inspect the crowns of the plants. If they remain white and flaccid, damage has been avoided. However, if they appear brown, black, and collapsed, damage has occurred.