Remember last winter? As June warms and temperatures in the 90s are in the forecast, it may be hard to recall, but here in the Columbia Basin, it was dry with a few notable cold spells. That combination of cold and dry can be hard on plants, agricultural and ornamental.
Plants, both annuals and perennials, from wheat to lavender, alfalfa to ziziphus, vary in their ability to survive winter conditions. However, there are three factors that combined to make last winter a “hard winter.”
First, there were sub-zero temperatures. Below is a record of the temperature and precipitation for Moses Lake (http://www.wrh.noaa.gov/). You can see two periods, in early December and early February, when temperatures dropped below 0° F. These cold temperatures can overcome the freezing defenses of plants and kill their growing points, especially when combined with dry weather, another of last winter’s factors.
Moses Lake had a very dry November through January. During dry spells like this, the soil will dry out, even with cold temperatures. Dry soil can cool more quickly, and to lower temperatures, than wet soil. The dry soil, combined with cold-induced dessication (dry, cold winds), can also stress plants, and sometime kill them. The dry weather also provides no snow cover which, when present, can benefit plants in several ways. It acts as an insulator, keeping the plants warmer than if exposed. The daily variation of temperatures is also reduced under snow cover, and it keeps the soil wetter than bare soil.
The last stress-factor for plants last winter, not uncommon here, is the early warm-up followed by extreme cold. See the January spike to over 50° F, before the early February cold snap. Here, the time between the two events might have lessened any damage, but in general, these large swings in temperature, alternating between freezing and thawing, are hard on plants.
What can be done to limit winterkill? First, in dry Eastern Washington, make sure soils are not dry going into winter with a late irrigation. Although this can increase the risk of leaching if we get a wet winter, the risk can be reduced by limiting the amount of nitrogen fertilizer applied to only that needed for fall growth.
Residue cover (or a mulch in a garden) can also help. The cover acts like snow cover, providing insulation and moderating temperature swings, and reducing evaporation. Residue cover will also prevent wind erosion. Wind erosion can be a factor in winterkill by reducing the soil cover over the crown of wheat plants, or even expose the crowns. The exposed crown is then more at risk of dying from desiccation and cold damage.