Now that Halloween has passed and our clocks have fallen back, the idea of the impending winter season becomes more of a reality with each passing day. Although the summer period often ranks at the top in terms of turf-related challenges in these parts, a primary limiting factor that dictates turfgrass adaptation is low temperature. Most perennial warm-season (C4) grasses are easily killed by lower temperatures, and are therefore much better utilized in warmer locations. Cool-season (C3) grasses are well-suited for culture in the Northeast because they have an inherent ability to survive the winter conditions typical of this region.
There are two basic mechanisms of turfgrass injury by low temperature, or what is often called direct low temperature kill:
Intracellular freezing occurs when the temperature dips to the point at which ice crystals form in the fluid within plant cells. The sharp crystals rupture the cell membrane resulting in leakage and eventual death of the cell. When this phenomenon occurs within the turfgrass crown (the main growing point of the plant) death of the entire plant is likely.
Extracellular freezing involves ice crystal formation outside of plant cells during low temperature exposure. The ice crystals create a negative potential gradient that draws water out of cells and leads to desiccation, cell collapse and death of tissue.
In response to shorter days and cooler temperatures in the fall, grass plants gradually shift resources away from growth and towards preparation for winter survival. During this acclimation to decreasing temperature (hardening), the ability of plants to reduce crown hydration and accumulate solutes such as carbohydrates, soluble proteins and amino acids helps to protect plants from direct low temperature kill. The period after which shoot growth ceases but the turf remains green is especially critical for acclimation. The bulk of the carbohydrates produced by photosynthesis during this phase are used by each plant to protect vital cells from freezing. A gradual decline in temperature over a period of 3 or 4 weeks is preferable during the last stage of the hardening process. Although air temperatures often fluctuate (sometimes considerably, as they have this season) soil temperatures are buffered and therefore the descent is typically more measured.
Even among well adapted cool-season grasses, there is significant variability in low temperature tolerance and overall winter hardiness at both the species and especially the cultivar level. Genetics, furthermore, are only part of the equation, as a whole series of environmental, management and plant factors will play a role in the actual risk of cold injury any given year. In the end, much depends on the quality of acclimation and also the timing and pace of de-acclimation come spring. A lot of frustration originates from the fact that many aspects of the process are beyond our control. Research is ongoing to increase our understanding of low temperature hardiness in C3 grasses.
Unfortunately, direct low temperature kill isn't the only winter menace in town. Keep an eye out for other potentially damaging agents in the ‘winter injury complex’:
Frost damage – Frost can in some cases be lethal to seedlings, but it is less common that frost alone causes lasting damage to mature turfgrass plants. A notable exception is when traffic occurs on frosted turf. The compression drives the frost crystals into cells, rapidly killing tissue. Even in the event that crowns are not directly affected, frost injury can destroy frozen shoots and leave crowns more exposed and vulnerable to other stresses.
Desiccation is caused by excessive drying of plants from dry air and winter winds. Moisture is driven from vital tissues and is not replaced as a result of dormancy and frozen soils, resulting in plants that do not recover when growth resumes in the spring. Desiccation is especially common during ‘open’ winters or due to other factors that leave turf exposed for lengths of time.
Low temperature diseases, particularly snow molds, which are fungi that can grow and thrive despite cold temperatures. Often exacerbated by available moisture, higher N levels going into winter, and extended snow cover.
Ice damage – injury caused by lack of sufficient gas exchange when ice cover is present on turf for long periods (typically > 60 days). Can be widespread under the wrong mix of weather conditions.
Mechanical damage and heaving – Physical surface disruption from either natural frost heaving or snow removal operations. Can necessitate extensive springtime repairs.
Critter damage – Voles are the most common culprit. Winters with lasting snow cover provide voles with protection from predators, frequently resulting in damage to turf areas and also woody plantings.
Salt damage – De-icing salts that contact turf areas can impact plant growth in several ways: by affecting the ability of plants to able to absorb water from the soil, by alteration of physical soil structure, by competition with desirable nutrients in terms of root uptake, or simply by direct toxicity from some critical level of undesirable salts.
Submitted by: Jason Lanier