Ask the Placer County Master Gardeners: Choose plants that are in your zone

By: Elaine Applebaum, Placer County Master Gardener
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Question: Catalogs and some nurseries list “zones” in their description of plants. What are they talking about? Answer: Athletes strive to be “in the zone” and Caltrans asks us to “Slow for the Cone Zone,” but gardeners need to know their USDA or Sunset zone to select plants that will thrive in their climate. The United States Department of Agriculture has mapped the country into 11 hardiness zones based on the average minimum winter temperature. This helps gardeners and farmers determine which plants can survive through the local winter conditions. The lower the zone number, the lower the average annual minimum temperature. Western Placer County is in USDA zone 9, with an average annual minimum temperature range of 20 to 30 degrees fahrenheit. A plant listed with a USDA zone of 9 or below will be able to tolerate the average winter lows of our area. Compare this to Tahoe City, which is in USDA Zone 5, with average annual lows between -10 and -20 degrees. Plants rated for zones 6 through 11 would die from the cold temperatures of the Tahoe basin. But winter low temperatures tell less than half the story. A much more useful guide for California gardeners are the zones set out in the Sunset Western Garden book. Sunset breaks the western U.S. into 24 zones based on minimum and maximum temperatures, rainfall, humidity, ocean influence, elevation and length of growing season. Placer County is comprised of four Sunset zones: Roseville is in the marine-influenced zone 14, Lincoln to Auburn is in Sunset zone 9 defined as a thermal belt with high summer temperatures and dense winter fog, Meadow Vista to about Blue Canyon is considered zone 7 with hot summers and defined colder winters, points east are in zone 1A with cold winters and a mild, very short growing season. Zones listed in mail-order catalogs are generally USDA hardiness zones; local nursery signage usually lists Sunset zones. If you’re not sure which is being used, ask. Regardless of the official zone listing, gardeners should also take into consideration their local micro-climate, or conditions on their individual property. For instance, someone on a south-facing slope in zone 7 might be able to get away with growing zone 9 plants. Now that you know how to use zone listings when selecting plants, you and your garden should be in a comfort zone! If you have gardening questions, call the Placer County Master Gardener hotline at (530) 889-7388.