USDA Hardiness Zones are Hot and Cold

By Jeneen Wiche

USDA Plant Hardiness Zone Map by USDA-ARS and Oregon State University (OSU)
USDA Plant Hardiness Zone Map by USDA-ARS and Oregon State University (OSU)

Cold Hardiness

The USDA Hardiness Zone Map has long been a guideline for cold hardiness of plants; about every ten years it is revised in order to provide a bit more detail in our changing climate. The most recent map was revised in January of 2013 and is based on temperature information from 1976 through 2005.  Climate researchers collected temperatures from over 4,600 weather stations across the United States.  They take the average coldest temperature of a location to come up with an “average annual extreme temperature” to determine an area’s hardiness zone.  Digitally created maps have high resolution and can easily show the microclimates that typically pop up around cities.  This is the reason that some of us around Louisville are in Zone 7a (to the west of downtown averages 0° to 5°); 6a (Simpsonville where I live has an average extreme of -10° to 15°); and others are in Zone 6b (St. Matthews has a slight edge with an average extreme of 15° to 0°)

The USDA hardiness map has also added four more zones that designate areas that experience average annual minimum temperatures of 50°, 60°, 70° or 80° F, respectively.  Each Zone has a trendy color attached to it, too.  Our area is various shades of green. The American Horticultural Society’s makes it very easy for us to access our exact zone by plugging in your zip code here: An interactive GIS –based site will tell you if you are still the old Zone 6 (which is now 6a or 6b); or if you have graduated to the “warmer” Zone 7.  With that said don’t worry about changing anything you do in the garden.  If anything, take more risks!  For most of us we remain in Zone 6, those of us closer to the city may be included in Zone 7 due to a heat-island effect.

Heat Zones

Picture of Delphiniums (Spring by Wonderferret)
Picture of Delphiniums (Spring by Wonderferret)

In addition to mapping the Plant Hardiness Zones we have a map to help with how heat affects the health of our plants. To find this map as well as the cold hardiness map visit and scroll down to navigate to “Gardening Maps,” then select the map you want to see.  If you have ever tried to grow delphinium in the garden you know that it is not the cold hardiness that thwarts success, instead it is the heat and humidity that makes them melt by mid-June.

In 1997 The USDA’s Plant Heat Zone map was introduced.  The heat map divides the United States into 12 different heat zones.  They compiled temperature data from 1974 through 1995 from the National Climatic Data Center and established zones based on the average number of days a location experienced 86 ° Fahrenheit or above.  Heat Zone 1 has less the 1 day above 86° F and Zone 12 has 210 or more days at or above 86° F.  Why 86° degrees?  Well, at 86° F plants begin to show heat stress.  Leaves may wilt, roots may stop growing, flowers drop, bud initiation ceases or heat recovery overnight doesn’t occur because temperatures are too high.

In an effort to establish guidelines for heat tolerance the Plant Heat Zone map allows us to be better informed about the plants we buy and what realistic expectations about their summer performance should be.  Some nurseries are already including heat hardiness information but the usage on tags is primarily only seen with larger wholesale plant producers.

It seems sort of confusing and it may take a while to get used to but here’s how to decipher the four-code system for heat.  There are two sets of numbers, for example, (6-9, 9-1).  The first set of numbers recognizes that the plant is cold hardy in zones 6, 7, 8 and 9; the second set of numbers indicates that the plant has heat tolerance from zones 9 down to the coolest of zone 1.   Just be sure that zone 6 or zone 7 is included in the number spread of both cold and heat ranges and the plant is approved for both heat and cold hardiness in Kentuckiana!

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