Bluebells (Mertensia virginica) bloom in the spring and thrive in hardiness zones 4a to 9a. Image © Melissa Sandoval

If you’ve searched the internet for shrubs, perennials or shade plants, no doubt you’ve run across the term “hardiness zones.” Not every website tells you what these are or how to use them, though. Knowing which zone you live in and understanding how hardiness zones work is essential for gardening and landscaping your property.

For example, Melissa wanted to buy her mom a flower for Mother’s Day that she could enjoy for years to come. Melissa bought her mom a beautiful “Hershey Red” azalea, which Mom happily planted in her yard. Melissa’s mom lived in western Ohio, however, which is in hardiness zone 6a, while “Hershey Red” azaleas thrive in hardiness zones 6b through 8b. The “Hershey Red” died the first winter. Melissa did her homework the next time and bought her mom a gorgeous “Rosy Lights” azalea (zones 4a through 8a) which still blooms in Mom’s yard every year.

How to Find Your Zone

The USDA divides the US into 13 hardiness zones, which they further divide into “a” and “b” areas. The different growing zone numbers represent the average annual extreme minimum temperature (AAEMT) for the area, which is the lowest you could expect the temperature to drop on the coldest winter day. Each zone number covers a ten-degree range. For example, zone 1 in Alaska has an AAEMT of -60 to -50, while zone 13 in Puerto Rico has an AAEMT of 60 to 70. The USDA uses temperature data from 1976 to 2005 to determine hardiness zones. They post an interactive map of the zones on their website, where you can click on your state to see the map of its hardiness zones, or enter your zip code to find out which zone you live in.

What the Zones Mean

The USDA zones are a guideline for choosing any type of perennial plant for your property, from flowers to shade trees. Most nurseries include the hardiness zone number on the information sheet for the plants they sell, along with the plants’ needs for soil type, sunlight and water. A local nursery probably won’t sell you a tree or shrub that wouldn’t survive in your climate, but not knowing your zone could be a costly and frustrating mistake if you order online or through a catalog.

The National Gardening Association says that the USDA hardiness zone map is a good tool for the eastern US. They point out that the map has limitations, though, such as not accounting for the effects of insulating snow cover, freeze-thaw cycles or water drainage patterns during winter months. They also warn that the hardiness zone map isn’t as effective for the western US, where differences in ocean currents and elevation can have large impacts on plant hardiness.

What will you be planting this year?

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