JoAnne Skelly: USDA planting zones updated

These warmer days have me hankering for gardening. Although I received a small hydroponic unit as a Christmas present, I don’t think a 7-inch by 14-inch lighted box is going to satisfy my yearning for spring and being outside with my plants. I decided the least I could do was write about gardening!

My friend Paul recently pointed out that the USDA updated its Plant Hardiness Zone Map in November. This is the first update since 2012, and supposedly is more accurate with greater detail. It can be found at: https://planthardiness.ars.usda.gov/.

The site not only has the map, but also “Tips for Growers” where you can find information on research programs that might interest a gardener or a breeder. According to USDA “approximately 80 million American gardeners and growers represent the most frequent users” of the map.

“The 2023 map is based on 30-year averages of the lowest annual winter temperatures at specific locations. It is divided into 10-degree Fahrenheit zones and then further divided into 5-degree Fahrenheit half-zones.

The recent improvements are based on data collected from 13,412 weather stations versus the 7,983 used in 2012. The 2023 map still has 13 zones for the United States and its territories. Each zone has an A and B subzone.

One of the more interesting findings for the new map is that around half the country shifted to the next warmer half zone. This indicates a warming trend of 0-5 degrees Fahrenheit for those areas that shifted up. However, some areas warmed that amount but didn’t move up a half zone.

The map is based on averaging annual extreme minimum temperature for a given location over a 30-year time span. This can vary quite a bit based on local weather patterns. The USDA points out that these updates do not necessarily reflect climate change because of the variability of the extreme temperatures locally.

Global climate change is based on “trends in overall average temperatures recorded over long time periods.” In addition, better mapping methods and more data from weather stations have improved the accuracy of the data interpretation.

For zip code 89701 in Carson City, the zone is still 7a (0-5 degrees Fahrenheit), but with a plus 2 degrees temperature change. For 89706, also Carson City, the zone went from 7a to 7b (5-10 degrees Fahrenheit) with no change in temperature.

89410 in Gardnerville went from a 7a to a 7b with plus 2 degrees temperature change. Tahoe City warmed by 3 degrees but is supposedly still zone 7a. I find this confusing. How can Tahoe City be in the same zone as Carson City? Explore the website for yourself.

JoAnne Skelly is Associate Professor & Extension Educator Emerita at University of Nevada Cooperative Extension. Email skellyj@unr.edu