Last year was the warmest on record, and climatologists updated the U.S. Department of Agriculture’s hardiness zone map for the first time in more than a decade.
Hardiness zones inform gardeners on what plants could best survive the winter; maps are based on the average of the lowest annual winter temperature.
The update uses a 30-year average between 1991-2020, compared to the average of 1976-2005 for the 2012 map. The new map also had nearly twice the number of weather stations collecting data compared to the previous version.
About half the country was reclassified into warmer growing zones, including much of southern Idaho. Idaho has eight zones, ranging between (-35)-to-(-30) degrees Fahrenheit, to zero-to-five degrees Fahrenheit. Zones are in five-degree increments.
“You have to remember that word average in there,” said University of Idaho Twin Falls Extension Horticulture Director Andy West. He created a reference chart and guide for general spring planting dates for growers in Idaho.
Boise’s zone (7a) didn’t change this time compared to the USDA’s last map in 2012, but a lot of Idaho did. The Treasure Valley west of Caldwell, and Kuna east to Mountain Home, for example, shifted up one five-degree zone. Hailey shifted two zones warmer.
West says within these zones, garden-altering microclimates are to be expected.
“I live outside of Twin Falls. One of my master gardeners lives in Twin Falls. His garden froze three weeks before mine did, because he sits in an ‘urban sink,’” West explained.
Another example: West said watermelon or cantaloupe grow best in Hagerman because of heat reflecting off the walls of the Snake River Canyon.
“You can create microclimates in your area and make your spot warmer if you want to grow something different,” West said. “By planting certain plant material in a certain spot or putting in a fence, you can actually modify your climate and actually make it benefit you better.”
But you have to know your plants, too. Nectarines, for example, might grow well in zone five but the timing of localized frosts are likely to negatively impact fruit production.
West said if you want to maximize your garden’s output: know your microclimate, befriend your local green thumb and use knowledge available from local ag extension programs.
