Why plants survived the extreme cold

In this week's cold weather, temperatures were reported as low as 10 to 15 degrees. I noticed that some of the plants in the flowerbed near my house were not killed. Some look quite well. How can this be?

There are several considerations in determining why your plants faired so well. First, the reported temperature is probably not the temperature experienced at your house. There can be considerable variation in temperatures over a relatively small geographic area. This difference is increased by the location of the flowerbed near your house. Structures and walls absorb heat during the day and radiate that heat to nearby objects at night. They may also radiate heat from inside the house (leaking the heat you paid for). Because of this, the area of the flowerbed may be considerably warmer than an area several feet away. The location of the flowerbed relative to north and south may also have a great influence. North sides remain shaded and cold while south sides absorb the greatest amount of solar radiation to warm the soil and structure. This protects plants on that side from the lowest potential temperatures.

Another consideration is the type of plant. Some plants are killed quickly by freezing temperatures; others protect themselves by becoming dormant. Some plants just tolerate the cold until some limiting cold temperature is reached, then the plants die. This temperature limit will vary from one type of plant to another. Another consideration may also be how quickly the plant warmed after freezing. Some plants can freeze and thaw again if the thawing is slow. In this case, plants on the south side of a building may be at a disadvantage unless it remains cloudy.

As you know there are some plants that appear to be green and healthy even during very cold weather. These include junipers, pines, and other evergreens. Surprisingly, plants such as rosemary also remain green and active through the winter in much of New Mexico. In fact, winter is the season when they are most actively blooming. They are among the plants, however, that will do well until a limiting cold temperature is suddenly achieved, then they will die or exhibit damage.

Finally, plants benefit from gradually lowering temperatures. If a warm autumn season suddenly gives way to extreme cold, plants that could otherwise survive may die. The relatively gradual downward movement of temperatures this year has allowed many plants to acclimate and survive.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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