Cold Hardiness

Climate models show a warming of winter’s coldest temperatures by 2050, spelling changes for the future of the USDA Hardiness Zone map and new potentials for agriculture. The USDA uses multi-year averages of annual coldest temperature to assign hardiness zones that are used in horticulture to identify where plants can survive over winter.

Parker_Figure330 years of minimum temperature data from 20 global climate models were used to assess how these extreme annual cold temperatures may change under climate change projections and what these temperature changes might mean for horticulture. The study shows that annual coldest temperatures are projected to warm at a greater rate than average winter minimum temperatures, resulting in a widespread shift in cold hardiness zones across the US. Still, it is important to note that these temperature changes are based on 30-year averages and do not suggest that colder extremes will not occur.

Fifth generation farmer Conni Mahoney of Gallatin Grown in Manhattan, Montana used the USDA zones to help select crop varietals when she and her husband began farming 8 acres on her family’s Gallatin Valley ranch. “If the climate did change and it changed our zones, it would definitely impact our farm,” Mahoney said, noting that not only could shifting hardiness zones influence their varietal choices, but warming winter temperatures could also impact pest and weed management.

Warming of the annual coldest temperatures may not only change varietal choices, but could allow for an expansion of areas suitable for the cultivation of crops such as almonds and oranges into new geographic locations.

Current (left column) and future (right column) locations with non-mortal overwinter temperatures for oranges (top), kiwi (middle), and almonds (bottom).

Current (left column) and future (right column) locations with non-mortal overwinter temperatures for oranges (top), kiwi (middle), and almonds (bottom).

While ongoing work is seeking to address other factors (e.g. growing season, summer temperatures, and rainfall) that influence where crops may be cultivated in a future climate, increases in the coldest temperatures and shifting hardiness zones suggest that surviving the winter may no longer be the limiting factor.

For her part, Mahoney says that aside from opening up options for new varietals, “We’ll just have to wait and see what impact [changing hardiness zones] had and roll with the punches.”

For the paper published in Environmental Research Letters click here.

For the 2015 Northwest Climate Conference poster, click here.
For the 2015 REACCH Conference poster, click here.

Changes in hardiness zones for RCP 4.5 and RCP 8.5 over early, middle and end of 21st century time periods can be found here, along with other cool decision support tools created by Dr. Katherine Hegewisch and other folks in the Climatology Lab in collaboration with REACCH researchers.