What are the AHS heat and USDA cold zone maps?

Gardeners can use the USDA (United States Department of Agriculture) cold hardiness zone map in conjunction with the AHS (American Horticultural Society) heat zone map to determine which plants can survive and thrive outdoors based on average low and high temperatures in their area.  Knowing what the USDA and AHS maps are and how to use them can help you choose the best plants for your location.

USDA Plant Hardiness Zone Map

The USDA hardiness zone map was first developed in the 1920’s but it wasn’t until 1960 that the USDA published the first hardiness zone map.  Created by the U.S. National Arboretum, the current map divides the United States (including Alaska and Hawaii), Canada and Mexico into 11 zones of average annual minimum temperatures based on records from the 1970’s to the 1980’s.  Following an upgrade to the map in the early 1990’s, nine of the zones were subdivided into a and b rankings.

To use the cold hardiness map, which you can find here, locate where you live.  To enlarge the view of the map, click on your area or click on the state name beneath the map.  The color bands on the map correspond to a legend where you can find the number of your zone.  For example, look at the northern most portion of Texas and you can see the west (left) top is light yellow, which is zone 6a, and the east (right) top is dark yellow, which is zone 6b.  To the left of the color legend is the range of average annual minimum temperatures in F degrees.

When ordering plants or seeds from a catalog or the internet, you can use the USDA hardiness zone map to check to see if the plant can survive the winter in your area.  Your local plant nursery will make plants available to you that most generally are cold hardy to the climate zone in which you live.  If the nursery is offering plants outside your zone, like some varieties of roses, the cold hardiness zone should be noted on the plant’s tag.   If you choose to grow a plant that is not cold hardy in your area and want to keep the plant in your landscape, it may be possible to dig the plant up and shelter it in a warmer location during the winter.  This technique of winterizing plants is typically done only for ornamental flowers like roses or geraniums.

AHS (American Horticultural Society) Heat Zones

Just as winter cold can affect plants, so can summer heat.  Heat can cause blooms to wilt prematurely and leaves to droop.  Sustained periods of heat greater than the plant can tolerate will eventually lead to the death of the plant.

The American Horticultural Society created a 12 zone map of the U.S. depicting the average number of days within each zone where the temperatures are over 86 degrees F.  “Heat days” is what the AHS calls those over 86 degree F days; and AHS adds that during periods of heat days, plants “begin suffering physiological damage from heat.”

Open the heat hardiness map, which is located here in PDF format.  Enlarge the map if needed to pinpoint your location.  Use the map color of your area to compare with the colored legend of numbers 1 through 12 to determine your heat zone number.  Using northern most Texas as an example, you can see the west (left) top is light green, which is zone 7, and the east (right) top is yellow, which is zone 8.  To the left of the color legend bar, you can see an informational range of days when the temperature exceeds 86 degrees F.

Not all plants catalogs, plant websites and plant tags include AHS rankings yet.  Those catalogs, websites and tags that do provide this information can help you make a more guided choice for plants that can thrive in your specific climate zone.  When included, heat zone ranges and cold hardiness zone ranges may appear together with the heat zone ranges on a ascending scale (like “cold zones 6 to 9,” or simply “6-9”) followed by the heat zone ranges in descending scale (like “heat zones 7 to 4,” or simple “7-4”).  The plant tag or description may read as “6-9, 7-4.”

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