USDA releases an updated plant hardiness zone map

Perhaps only avid gardeners will find the idea of an updated U.S. Department of Agriculture climate map spine-tingling.

The last map was updated in 1990. The changes are based on average annual extreme minimum temperatures from 1976-2005. They are not based on the lowest temperature that has ever occurred. Researchers have divided the map into 10-degree Fahrenheit zones.

This is the first time the map is available as an interactive geographic information system-based map for anyone with a high-speed Internet connection and as static images for those with slower Internet access. You can simply type in your ZIP code and find your hardiness zone. The USDA is not printing posters at this time, but you can download maps by state, region or for the nation.

OK, I admit I was all excited and typed in my ZIP code - 89704 - and up popped Zone 7a: 0 to 5 degrees F. My balloon burst. I was hoping for a zone that actually reflects my experience with the lows in Washoe Valley, which have been in the -20s two or three times (maybe more) during the last 23 years. If I plant trees that only will tolerate 0 to 5 degree minimums, I probably will lose those trees every 10 years or so. I had to remember that the map is based on average minimum temperatures. Warm temperatures such as we are having this year really skew an average!

I also checked Carson City's 89701, 89703 and Douglas County's 89410 and 89423 ZIP codes, which came up Zone 6b: -5 to 0 degrees F. I was surprised to find Carson City listed colder than my house. Carson City ZIP code 89706 is zoned 7a, same as Washoe Valley yet still warmer than 89701. Following my weather observations for my house and looking at the zone breakdown, I would only buy plants for zones 4b (-25 to 20 degrees) or possibly 5a (-20 to 15 degrees).

The map is not as failsafe as I had hoped. However to do the new map justice, authors do state "there might still be microclimates that are too small to show up on the map."

My property is probably one of those unidentifiable microclimates. This map is a good tool if you remember that the plant hardiness zones are just guides and many factors besides temperature contribute to plant hardiness including light, soil moisture, duration of exposure to cold and humidity.

The new map is available online at http://planthardiness.ars.usda.gov/PHZMWeb/.

• JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at skellyj@ unce.unr.edu or 775-887-2252.