Photograph 1. A common scenario for freeze injury is a warming (thawing) in late winter where the turf, in this case Poa annua, loses its cold hardiness, followed by water from precipitation or a poorly drained site where water stands followed by a rapid freezing.
Scientists have identified and studied the roll of specific plant genes in freeze tolerance. A group of genes called cold-response (COR) genes apparently plays an important role. The activation of these genes requires a period of low but nonfreezing temperatures (32 to 50 degrees F (0 to 10 degrees C). The activation of these genes is then associated with the hardening or freeze tolerance of the plants. A possible reason why plants in effect die when exposed to freezing temperatures without a hardening period is due to the lack of COR gene activation. Interestingly, light in addition to low but nonfreezing temperatures is needed for gene activation, which may explain partially why we see turf in shade more susceptible to freeze injury (Danneberger, 2006).
Although turfgrasses undergo cold acclimation, freeze injury is a potential problem on cool season turfgrasses like annual bluegrass (Poa annua) and perennial ryegrass (Lolium perenne) Freeze injury and conversely tolerance is due in large part to how the turfgrass plant reacts to cell dehydration. As temperatures drop below freezing water within the plant freezes intercellularly (between cells) causing a decrease in water potential outside the cell. The cell begins to move out of the cell toward the ice crystals in the intercellular spaces and subsequently freezes. Thus, dehydration occurs within the cell. The colder the temperatures the more water travels down the gradient toward the frozen water. At 14 F (-10C), 90% of the osmotically active cellular water will move out of the cell into intercellular spaces (Thomashow, 1998).
As water leaves the cell, the plasma membrane contract and pull away from the cell wall. With the arrival of warm temperatures the ice present intercellularly melts and the water flows back into the cell where hydration takes place. If no damage has occurred to the plasma membrane (ex. punctured, ruptured) then the cell is alive and well. However, if the cell rehydrates and damage has occurred to the plasma membrane cell death is eminent. The most common type of freeze injury occurs at relatively high freezing temperatures 24 to 28 F (-2 to -4 C) during late winter/early spring. This type of freeze injury is sometimes described as "expansion-induced lysis" because it occurs during freeze/thaw cycles. In this freeze/thaw scenario, the plant loses its cold hardiness through warming temperatures which leads to the expansion of the plasma membrane.
Turf managers have some control of increasing the likelihood of winter survival by:
Raising the mowing height on warm season turfgrasses during the fall. This will provide more some protection to the growing point during freezing temperatures.
Provide drainage for removal of water from excessively wet areas. During freeze/thaw cycles the presence of excessive moisture can enhance freeze injury.
Reduce thatch. A significant thatch layer results in the plant's growing point to lose contact with the soil as it elevates into the thatch layer. This will expose the plant more readily to freezing temperatures.
Potassium fertilization. On warm season turfgrasses potassium is often applied for increasing the chances of winter survival. Potassium is an ion that helps lower the osmotic potential of the cell decreasing water the potential for water flow from the cell.
Reduce the likelihood of excessive growth going into the winter. Overstimulation of growth promotes succulent high water content cells that are more likely to encounter freeze injury.
Reduce shading. Although not fully researched, a degree of correlation has occurred with freeze injury and degree of shading. Shading may contribute to increased freeze injury due to plant cells tend to be 1) more succulent in shade and have larger intercellular spaces, 2) lower carbohydrate levels, which may influence water potential, as well as the energy requirements of the turf and 3) shaded areas tend to be wetter, which may contribute to the severity of freeze/thaw cycles in late winter.
Danneberger, T.K. 2006. Another brick in the winter fortress. Golfdom 62(11):38.
Thomashow, M.F. 1998. Role of cold-responsive genes in plant freezing tolerance. Plant Physiology 118:1-8.