It seems that winter has arrived. Not so much in terms of snowfall, but certainly with frigid nights, not-so-warm days, biting winds, and often dreary skies. We humans have various means of dealing with the cold. A wardrobe of warm weather garments, a toasty hearth, and perhaps the ability to avoid the cold altogether by moving inside to more suitable environs. Yet, the plants that we have chosen and tended so carefully in our landscapes enjoy no such privileges. How do they cope?
Let’s explore the cold hardiness of native plants.
Plants are genetically equipped for surviving the cold in their respective USDA Plant Hardiness Zones, which is a mix of Zone 5b and 6a in southeast Nebraska. In general terms, cold or winter hardiness is predicated upon the ability of a given plant to prevent the formation of ice crystals within individual cells. The damage we see in early fall after a frost or freeze, especially in tender annual plants such as tomatoes or impatiens that have their origins in much warmer climates, is the result of ice crystals that have formed within individual plant cells. Those crystals then puncture the cell walls, allowing the cell contents to leak out and resulting in the collapse of the cells and subsequent death of the plant.
Acclimation
Plants use environmental clues to prepare themselves upcoming seasons.
For cold hardy woody plants, the shortening days of late summer and early fall signal these plants to begin moving water out of vulnerable tissues and, in some cases, creating plant ‘antifreeze’. This effectively lowers the freezing point of water that still remains or must remain in plant tissues. This process is called acclimation. Subsequent lower temperatures during the day and evening incrementally continue the acclimation process, such that the plant eventually achieves the state of cold tolerance, allowing it to overwinter successfully – its cold hardiness rating. As winter turns to spring, the process reverses, and de-acclimation prepares the tree or shrub for the reinitiation of growth when spring arrives.
It is important to remember that both acclimation and de-acclimation are processes, not states.
Cold hardiness is not an absolute.
Plants use environmental clues during late summer/early fall and late winter/early spring to prepare themselves for the needs of the coming seasons – and gradual is better than all of a sudden. Significant, rapid changes in weather, be it warm-to-cold or cold-to-warm, and cold, dry winds can have adverse effects on plants that have not had an opportunity to acclimate to the cold weather. It is also possible that they had the misfortune of beginning the process of de-acclimation too early. We can see this in our landscape in various forms including injury to the trunks of young, thin-barked trees, twig or branch dieback, or the loss of early season blooms or foliage. While these injuries are often not fatal and the plant can call on reserves for recovery, these potential stressors underscore the importance of good landscape management practices such as consistent, deep watering, proper mulching, and possibly trunk protection to help your landscape be as resilient as possible during the extremes of winter in the Midwest.
