With the recent extreme winter cold that gripped much of the Intermountain West, Texas and beyond several weeks ago, this topic is on the front burner of gardening conversation. While most of the country uses the USDA winter cold hardiness numbers as a measure of cold tolerance, this system should only be used as a very general guide line. (The same goes for the Sunset Magazine cold hardiness rating system.) There are many factors that affect plants and their ability to withstand cold other than just the temperature.
Cold is most often fatal to plants when it comes suddenly. When the fall is long and warm, plants don’t harden off adequately to protect themselves from a sudden November storm that drops the temperatures into the single digits. These storms kill a lot of plants.
How long the cold lasts will also determine the extent of its damaging effects. When temperatures hover below zero for days at a time, the cold is able to penetrate plant tissues to the extent that stems and crowns are killed or badly damaged. When compared to prolonged cold, a quick dip into the freezing zone just before sunrise will cause little damage.
How we treat our plants can affect cold hardiness as well. Late summer/Fall applications of chemical fertilizers will force late season growth that is readily damaged by cold temperatures. Late season watering will often cause a loss of winter hardiness, especially if fall temperatures are warm. The plants just keep growing without adequately preparing for winter. This is especially true of succulent plants like cold hardy cacti, hardy Iceplants (Delosperma and other South African succulents) and Century plant (Agave).
Many perennial plants are most cold hardy when their stems are left on the plant over the winter. Fall mums, Hummingbird mints (Agastache), hummingbird trumpet (Zauschneria), Texas Sages (Salvia greggii and hybrids) are just a few examples where curbing your desire to cut back and neaten up the garden in fall is best ignored. Wait until mid-spring.
Cold hardiness comes with maturity. The California Redbud is an excellent example of a woody plant that gets more cold hardy as it matures. By providing some degree of winter protect for the first couple of winters, many plants such as this Redbud gain cold hardiness as their roots and crowns increase in size and depth of penetration into the soil. This is true of many small woody perennials like Agastache, Salvia greggii and others.
Late winter and early spring moisture is also a killer. This is especially true when seasonal temperature fluctuate above and below freezing, causing the snow to freeze and thaw. In areas that often experience rain one week and snow the next, this same problem of freezing and thawing will cause plants to lose their roots and crowns to rot. It’s always best when it stays cold and the snow stays frozen.
As a general rule of thumb, be conservative when it comes to choosing trees and shrubs that will be a cornerstone of your landscape. Make sure they have a cold hardiness rating that matches or is cold hardier than your areas winter lows. You can experiment and push the boundaries of hardiness with perennial plants, grasses and groundcovers. They are more easily replaced and grow quickly to their mature size.