USDA maps plant hardiness zones

Readers are requesting information on plant hardiness zones. The United States Department of Agriculture Plant Hardiness Zones (http://www.usna.usda.gov/Hardzone/ushzmap.html) and the Sunset's Climate zones are the most common hardiness references for selecting plants.

The USDA hardiness zone system is based on average annual minimum temperatures and whether a plant will survive these lows. It divides the United States and southern Canada into 11 areas based on a 10-degree Fahrenheit difference in the average winter low temperature. These are all subdivided into 'a' and 'b' with 5-degree differentials. Sunset's zoning system incorporates more than the just the low temperature, including factors such as the length of the growing season, timing and amount of rainfall, winter lows, summer highs and humidity. This is essential information when a climate is as harsh as ours is. A plant may survive northern Nevada's winter low temperatures with ease, but not tolerate our lack of winter moisture, humidity or winter snow cover. We have dry windy winters, hot sunny summers and may have sudden cold snaps at any time of the year. As gardeners, we have to know whether a plant will survive all seasons, not just winter.

In addition to our changeable weather, there are also microclimates gardeners should consider within each zone where the temperatures may be colder or hotter. Where I live is colder than what the zones show. There are also microclimates within neighborhoods and around individual houses.

On the Web map of USDA plant hardiness zones, much of Carson City and surrounding areas are listed as zones 6b and some 6a. 6b ranges from -5 to 0 degrees Fahrenheit with zone 6a ranging from -10 to -5 degrees. Many gardeners, who know their winter lows drop lower than that, may want to look for plants that will work in a 5b or 5a zone. 5b lows range from -10 to -15 degrees. Minimum temperatures in 5a are -15 to -20 degrees. With my cold yard, I buy plants for a USDA 4b zone. 4b range is -25 to -20 degrees.

Looking in the 2001 Sunset Western Garden Book, Carson City is shown as a zone 2B. This zone has minimum temperatures averaging from 12 to 22 degrees with extremes in the -10 to -20 range. Sunset zone 2A has winter temperatures "that hover between 10 and 20 degrees Fahrenheit with drops between -20 to -30 degrees every few years."

Since trees are expensive and take a long time to establish, choose those that can survive the occasional -22 to -25 degrees.

- JoAnne Skelly is the Carson City / Storey County Extension Educator for University of Nevada Cooperative Extension.