3/27/2018 - By Harrell's

Many questions concerning possible winter-kill to turfgrass always arise this time of the year.

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.