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Plant hardiness zones
As gardeners and farmers across the country decide which plants to grow this spring, many look to plant hardiness zones for guidance.
The U.S. (including Alaska and Hawaii) is divided into 13 plant hardiness zones based on each location’s annual minimum temperature, averaged over 30-year normal periods. Each zone spans a 10°F range in average annual minimum temperature. For example:
- Cold-tolerant plants such as garlic and asparagus thrive in Zone 3 (-40 to -30°F), which runs across the upper Northeast, Midwest, and Plains.
- Plants adapted to mild winters and hot summers, such as watermelons and tangerines, thrive in Zone 8 (10 to 20°F), which runs through the Southeast and central Texas.
Maps from USDA and NOAA show bands of similar planting zones running roughly east to west across the U.S. (except in high mountains and along coasts).
A location’s annual minimum temperatures (on which planting zones are based) affect which plants and insects can survive over winter—and therefore partly determine the geographic range of wild, cultivated, and invasive species. The USDA also uses plant hardiness zones to set some crop insurance standards.
In addition to annual minimum temperature, other factors such as growing degree days, local rainfall, and soil type also affect which plants can thrive in a given area.
Planting zones shifting north across the U.S.
As our climate warms due to heat-trapping carbon pollution, planting zones have shifted north toward higher latitudes and elevations.
Planting zones are based on annual minimum normal temperatures, and winter warmed the most between the 1981-2010 and 1991-2020 normal periods.
Maps for the three most recent normal periods show plant hardiness zones shifting to the north as the coldest time of year warms.
As minimum temperatures warm and a location’s planting zone increases (e.g., from Zone 5 to Zone 6), certain plants and insects may expand into areas where they previously wouldn’t have survived the coldest winter temperatures.
While this can have benefits (e.g., expanding the range of high-value crops), it can also expand the range of harmful and/or invasive plant and insect species such as:
- Kudzu, a fast-growing invasive vine projected to continue to expand from the Southeast into the Mid-Atlantic, Midwest, and Northeast.
- Brown marmorated stink bug, an urban nuisance pest and agricultural pest in at least 15 states, whose geographic range is projected to grow partly due to warmer, wetter winters.
Coldest annual temperatures warming locally
Climate Central also assessed the long-term change in the coldest temperatures of the year—which determines which plants are likely to thrive—in 247 U.S. locations. Summary statistics below are based on 242 locations (see Methodology).
This analysis averaged each location’s annual minimum temperatures over a rolling 30-year period, from 1951-1980 to the present (1993-2022), and found:
- Average annual minimum temperatures warmed in 231 (95%) of 242 locations.
- Compared to a 1951-1980 baseline, the average annual minimum temperatures during the most recent 30-year period (1993-2022) were 3.1°F warmer on average across all 242 U.S. locations.
- 121 (50%) of the 242 locations experienced an increase in their plant hardiness zone (e.g., from Zone 5 to Zone 6) at some point over the period of analysis.
- The locations that experienced the largest increase in average annual minimum temperatures between the first (1951-1980) and last (1993-2022) 30-year periods were: Reno, Nev. (9.3°F); Anchorage, Alaska (8.7°F); Las Vegas, Nev. (8.6°F); Las Cruces, Texas (8°F); Juneau, Alaska (7.8°F).
- The largest differences between lowest and highest average annual minimum temperatures observed over the period of analysis were in:
- Boise, Idaho warmed 10.8°F (from -5°F to 5.8°F), equivalent to a shift from Zone 6 to Zone 7
- Bowling Green, Ky. warmed 9.6°F (from -4.2°F to 5.4°F), equivalent to a shift from Zone 6 to Zone 7
- Nashville, Tenn. warmed 9.5°F (from -0.3°F to 9.2°F), equivalent to a shift from Zone 6 to Zone 7
Growing in a warming world
As planting zones shift toward higher latitudes and elevations, it affects which plants can grow successfully in different parts of the country.
If carbon pollution continues, these shifts could continue or even accelerate—forcing farmers and gardeners to adapt by selecting plants adapted to a wider and warmer range of temperatures.
- Research from the USDA projects dramatic northward shifts in future plant hardiness zones with continued carbon pollution.
- Another study found that, if carbon pollution continues, annual minimum temperatures could shift north at a rate of about 13 miles per decade—with the northern Great Plains and the Midwest expected to see the fastest shifts.
- Although this would mean expanded ranges for high-value crops such as almonds, oranges, and kiwis, it could also bring expanded ranges for weeds and pests.
This story was orginally published by climatecentral.org.
Editor’s note: Find an updated plant hardiness map from the U.S. Department of Agriculture at planthardiness.ars.usda.gov