Turf MD: Cold hardiness

Karl Danneberger

At this time of the year, worrying about your turf takes a back seat to the holiday season. This is especially true with cool-season turfgrasses but also with warm-season grasses. Growth has slowed or stopped, and looking across a winter golf course, it is easy to think the turfgrasses have gone on “holiday,” too.

Since early fall, both cool- and warm-season turfgrasses have used this time to prepare for winter by switching physiologically to be more cold hardy. December and January is the time that turfgrasses are in their most cold-hardened state.

In general terms, cold hardiness occurs when plant cells, primarily those associated with growth, start to dehydrate. Water moves from the intracellular area within the cell to areas outside or between cells. The loss of cell water is described as desiccation. The plant cells remain in this desiccated state for the remainder of the winter. Once late winter or early spring arrives, cold hardiness is lost as cells hydrate and growth begins.

Cold hardiness is gained through an acclimation process. For example, in studies where wheat is grown at warm temperatures (80 degrees F) and then exposed to cold temperatures (23 degrees F), the plants die. However, when those wheat plants growing at 80 degrees F are exposed to cool, nonfreezing temperatures (< 50 degrees F), they survive at -4 degrees F.

Acclimation is the ability of plants to become adapted to the gradual reduction in temperatures. Turfgrasses exposed to cool temperatures (< 50 degrees F) for a relatively short time (three to seven days) initiate cold acclimation.

Plants that are growing in sunny conditions appear to acclimate to cold better than shaded plants. Frequently, golf course turf that suffers freeze injury is found in low-lying areas that are shaded. Factors besides reduction in light could affect the cold tolerance of turfgrasses like reduction in carbohydrate levels due to reduction in photosynthesis or colder temperatures associated with winter shaded areas.

At a molecular level, plants going through acclimation activate cold regulating genes, which encode for proteins that potentially provide the cold tolerance in plants. These genes are induced by temperature and light. Although temperature is a critical factor in acclimation and cold hardiness, recognizing the importance of light in the acclimation process is often overlooked or forgotten.

The most common freeze injury is expansion-induced lysis, often referred to as freeze/thaw injury that occurs in late winter/early spring when temperatures are cool (25 to 28 degrees F) and moisture is present. The turf is beginning to break hardiness as the cells in the crown area begin to rehydrate. Wet conditions are present, and often, the soil is saturated. Rain arrives, and a rapid drop in temperature often causes ice to form. Once temperatures rise again, the turf may initially appear healthy, but it rapidly dies within a few days. Cell rehydration followed by rapid freezing around the growing point has resulted in cell death.

At cooler temperatures (14 to 25 degrees F), plants can undergo differential membrane dehydration that can result in changes in cellular membrane fluidity and structure. As temperatures drop into this 14 to 25 degrees F range, the fatty acid makeup can shift to more saturated fatty acids. Saturated fatty acids are more “rigid” at colder temperatures than unsaturated. With breeding efforts of warm-season turfgrasses like seashore paspalum, looking at variety differences in the fatty acid composition at low temperatures may be a mechanism for screening for cold-tolerant varieties.

The final level of freeze injury occurs below 14 degrees F. Under these conditions, severe dehydration occurs. This kind of freeze injury most likely occurs at higher elevations.

Achieving cold hardiness is an annual occurrence in turfgrasses during the holiday season. Injury, however, can occur 1) if moisture levels remain high in and around the plant or 2) if severe dehydration/desiccation occurs at low temperatures. Even though winter may appear to be the slow part of the season, continue to monitor the turf. If extreme enough, cold temperatures cause turf stress.