The dead basil in my container garden is a testament to the fact that certain plants are not cold tolerant. Many tender plants in our gardens are tropical in origin and do not have strategies for tolerating temperatures below 32 degrees. When the water inside the cells of these tender plants freezes and expands, it damages cell walls and causes cell death.
Plants that are able to tolerate hard frost or much lower temperatures have adapted to freezing temperatures in several ways. Woody plants that originate in northern climes with very cold winter temperatures have developed a type of “supercooling,” where the formation of ice in cells is suppressed. However, even hardy trees with supercooling have their limit. This limit is about minus-40 degrees, the point when ice forms within their cells. The very hardiest of plants, those that can survive below minus-40 degrees, use a different method to avoid cell death. They move water from their cells to between the cell walls, lowering the freezing point of the rest of the cell contents.
The recent cold spell was hard on us, but what about our garden plants? The good news is that the drop to our severely cold temperatures was gradual, plus it was late enough in the fall that hardy plants were probably fairly well acclimatized and able to tolerate the low temperatures. The physiological changes that help hardy plants survive cold weather are prompted by shorter day lengths in late summer and fall months along with gradually cooling temperatures in fall and early winter.
Some local gardeners may have assumed that our recently past mild winters would continue, and felt safe planting plants not rated as completely hardy for our region. Plants that were not rated as hardy for our region, or only marginally hardy, may have sustained some injury from the recent cold weather, but we will not know that until next spring and summer.
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If you go to the USDA hardiness zone map at planthardiness.ars.usda.gov and type in your ZIP code, Tri-Citians will find that we are in Zone 7a (0 to 5 degrees). To be safe, I recommend only planting plants that are rated as hardy to Zone 6b (0 to minus-5 degrees). Of course, gardeners typically like to “push the envelope” and see if they can get away with growing less hardy plants. That is OK, if they understand that they could lose those plants during a severely cold winter.
While the snow that accompanied the cold weather may have been seemed like a curse when shoveling the driveway, it may have been a blessing to some of our plants, such as perennial flowers. Most perennial flowers die back to the ground and regrow from their crowns and roots the next spring. Snow provides insulation for plants. Having the snow blanketing the ground during our cold spell may have helped save the more tender perennials in our gardens.
Now that milder temperatures have returned, we will have to wait and see if our plants sustained any damage from the cold weather. Only time will tell.
For more information on the causes of cold temperature injury to woody plants, go to: https://research.libraries.wsu.edu/xmlui/bitstream/handle/2376/6041/FS196E.pdf?sequence=1&isAllowed=y or look for WSU publication FS196E “Environmental Injury: Cold Temperature Injury of Landscape Woody Ornamentals.”
Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.