Plant cold hardiness is usually described by zones, and the most widely accepted system is the USDA Plant Hardiness Zone Map (available online at www.usna.usda.gov/hardzone/ushzmap.html). The USDA system divides North America into zones with average winter lows within a 10 degree increment. Most of the Salt Lake Valley is USDA Zone 5, where the lowest winter temperatures drop between -10 and -20 degrees Fahrenheit.
Zone 6 and 7 plants may survive many mild winters with minimal winter damage. To play it safe, especially when purchasing larger and more expensive trees and shrubs, choose those that are hardy to -20 degrees. Many newly-available cultivars are more hardy or tender than their label claims, but the only way to know if something will survive in your own yard is to try it.
Plants' location within a landscape can promote or compromise their cold tolerance. Intuitively, people assume that the south side of buildings and walls offers more protection. However, many plants actually suffer less winter damage while growing on the north side of buildings and walls.
Southern exposures may help marginally-hardy plants, but south-side temperature swings during a mild and sunny winter week may lure acclimated plants out of dormancy. When true winter returns the following week, plant cells freshly filled with sap may freeze and burst.
Evergreens, especially broadleaf evergreens such as laurels, are susceptible to freeze-thaw damage and drying. They survive best with afternoon shade through winter. Wind tunnels and low spots challenge plants with dessication and late spring freezes.
Snow helps plants survive winter by insulating their roots, crowns and lower branches. Upright junipers and other evergreens should be shaken immediately after snowfall to prevent branch bending and deformity. While snow cover is usually beneficial, it may hide voles that often chew around the bases of shrubs and young trees. Plants girdled this way usually die, although some shrubs may sprout new limbs in spring. Keep the ground bare around tree trunks and shrub bases during winter so that voles are less likely to girdle plants.
Most trees and shrubs need moist soil throughout winter. Evergreens, especially, may need irrigation during a dry winter. If more than 4 weeks of mild, sunny, dry weather passes during winter, check soil moisture and water if necessary.
Canyon winds or urban wind tunnels can dessicate plants quickly. A burlap bag fitted over two tall stakes may be just enough wind break to help young and tender plants survive windy sites.
Native Utah species like penstemons, cacti and alpine plants prefer dry soil through winter. Raised beds filled with native soil amended with sharp sand, vermiculite or pumice will promote the excellent drainage most native plants prefer.
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* MAGGIE WOLF is an assistant professor for Utah State University Extension in Salt Lake County. E-mail her at maggiew@ext.usu.edu.
