Q.  What are microclimates and why are they important in gardening?

Gardeners throughout the U.S. should be familiar with the USDA plant hardiness zones which are based on minimum winter temperatures. Most nursery stock and seeds sold in the U.S. are labeled with the USDA hardiness zone range for that plant. Since the USDA system is based only on winter minimum temperatures, its usefulness is limited. If you want to know your USDA zone, see https://planthardiness.ars.usda.gov/. 

Sunset plant hardiness zones apply to the western U.S. and are based on winter minimum and summer maximum temperatures, length of growing season, humidity, and seasonal rainfall. This system better reflects the diversity of California’s growing climates. If you want to know your Sunset hardiness zone, see https://www.sunsetwesterngardencollection.com/climate-zones/.

You’ll notice that the Sunset map is quite detailed, but even within these smaller, more precise zones there can be considerable variability in growing conditions. This is especially important if your zone is borderline acceptable for growing certain things. For instance, citrus trees are frost-sensitive to varying degrees. Kumquat and Satsuma mandarin are two varieties that are relatively cold-tolerant. Lime and citron (Buddha’s hand) are the least cold-tolerant varieties. In general, if the temperature drops to 29F for more than 30 minutes, any citrus will likely show some damage.

If your winter low temperatures fall to just below freezing only for short periods, you still may be able to grow citrus or other frost-sensitive plants. Locations near a south- or west-facing wall can benefit from radiated heat, especially if the wall is made of concrete block or stucco. Other protected locations include areas near a sidewalk or concrete patio, at the top of a berm or embankment, or near a pool, pond, or other body of water. Smaller plants can be protected by planting under eaves or near larger plants or boulders. New plantings that may be sensitive before getting established can benefit by placing a “nurse rock” nearby. Young trees can be especially sensitive to frost simply because they are closer to the ground. Covering with burlap or stringing Christmas lights can give a baby tree just enough protection to survive the winter.

On the other hand, if you want to grow plants that need a cooler environment, you can locate them away from walls, fences, or other heat sinks (objects that hold onto heat and radiate it for a long time). Many fruit trees need a minimum number of winter chill hours and will decline and die within a few years if this requirement is not met. Locating an apple or pear tree at the bottom of a slope may give it enough chill hours to thrive.

Sometimes a plant that has been frost-damaged may appear to be a lost cause. Don’t remove any frost-damaged plants until mid-spring. Occasionally a tree may be defoliated (lose all its leaves) but the trunk and root system are still alive. Wait a bit and you may find that your tree is still alive.

Los Angeles County

[email protected]; 626-586-1988; http://celosangeles.ucanr.edu/UC_Master_Gardener_Program/

Orange County

[email protected]; http://mgorange.ucanr.edu/

Riverside County

[email protected]; 951-955-0170; https://ucanr.edu/sites/RiversideMG/

San Bernardino County

[email protected]; 909-387-2182; http://mgsb.ucanr.edu

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