Understanding hardiness zone maps

What zone are you in? No, I won't take "The Twilight Zone" for an answer. But I will take any number from 1 to 11. These numbers tell us what hardiness zone we live in. "Hardiness is defined as the ability of a plant to survive winter conditions in a given area." The quote is from The American Horticultural Society's A-Z Encyclopedia of Garden Plants, my gardening bible.

The U.S. Department of Agriculture developed the Plant Hardiness Zone Map so that gardeners, farmers, and anyone else interested would have something to use that would help them choose plants that are best suited for growing and thriving in a particular area. Zone maps (there are more than one) divide the United States into 11 hardiness zones based on average minimum temperatures.

AHS will soon update their hardiness zone map to include four new zones, 12 to 15 (subtropical and tropical regions).

To keep things simple, I don't use the "a" or "b" designations you'll see on some hardiness zone maps. If I want to try a plant that's rated to hardiness zone 6 and I live in zone 7a or 7b, I'll try that plant. If a plant is rated for hardiness zone 5a and I live in 5b, I won't let that prevent me from purchasing the plant. However, keep in mind that if you decide you're going to try a plant rated for hardiness zone 7 and you live in zone 5, you're pushing your luck.

Hardiness zone maps should be used as a guide to help us choose plants that do best in the areas we live, not in areas two or three zones either side of ours. Don't let hardiness zone maps confuse you; a little common sense goes a long way when selecting new plants for your landscape. After all, you don't want to pay good money for a gorgeous zone 8 tropical plant, and then watch it die trying to survive your zone 5 winter.

Another type of hardiness map you might want to refer to is a heat zone map. This map is divided into 12 heat zones according to the average number of "heat days," days that temperatures reach or exceed 86 degrees F. For example, heat zone 1 averages less than one heat day per year, and zone 12 averages more than 210 heat days.

I live in western Pennsylvania: hardiness zone 5 (-20 to –10 degrees F., lowest average winter temperatures that plants in zone 5 will survive), and heat zone 4 (14 to 30 – average number of days per year above 86 degrees F.) Be advised, neither heat nor hardiness zone maps take into consideration microclimates, local zones that might be warmer or cooler than the surrounding area. For instance, urban areas where brick, concrete, or asphalt absorb the sun's energy, heat up, and reradiate the heat to the ambient air. A microclimate might exist near bodies of water where air is cooled more readily. Our church is constructing a new walled, open-air courtyard where the added protection will allow us to plant as if it were zone 6 instead of 5. My neighbor's house has a drier vent just above ground level where for several years a zone 8-11 Moonflower (Convolvulaceae) over wintered, grew again in spring and bloomed. These areas are all microclimates ranging in size from a few feet across to many yards, and even larger. Some microclimates in my neck of the woods can provide enough heat to allow plants that are rated to zone 6 or 7 to live happily in zone 5.  

In conclusion, use the heat zone and hardiness zone maps to help you choose the appropriate plant for your climate. Don't let the "a" and "b" hardiness zone designations confuse you, use them to help you understand what temperate zone you are most likely to use when you purchase plants. If you know of a microclimate in your landscape, use it to grow plants you might not otherwise grow. And most importantly, pay attention to what your established plants are telling you and use this information, along with the hardiness and heat zone maps, to help you make educated decisions regarding which plants to use in your garden and landscape. 

HGTV's Gardening Zone Finder
National Gardening Association's USDA Hardiness Zone Finder
The United States National Arboretum Plant Hardiness Zone Map
Online version of AHS Plant Heat-Zone Map (State by State)

Topics: hardiness , zone , maps , understanding