Climate Corner: A crash course on plant hardiness zones

Aug 3, 2024

Danesha Seth Carley

editorial@newsandsentinel.com

Many people are familiar with the term “growing zone” as it is often included on plant labels that help gardeners decide which plants might thrive in their home gardens. A plant tag may say “Zones 3-6,” which means the plant can grow anywhere in the indicated zones. Zones are identified based on the average annual extreme minimum winter temperature displayed in 10-degree F zones ranging from zone 1 (coldest) to zone 13 (warmest). Each zone is further broken into “a” and “b,” with the “b” section being the warmer of the two sections. Every 12 years the United States Department of Agriculture (USDA) issues a new Plant Hardiness Zone Map (PHZM). PHZMs serve as general guides for growing perennial plants, and are based on the average lowest temperatures, not the lowest ever experienced in that area. The first U.S. hardiness zone map was published in 1927 with 8 hardiness zones.

There have been several versions of this map since then, including an expansion from 8 to 13 zones. The most recent PHZM was just released in May 2024 and is based on 30-year average cold temperature data through 2020 (which has changed quite a bit since the late 1920s). The most recent map differs from the original in more ways than simply the number of zones.

If you don’t follow planting recommendation shifts closely, let me catch you up. When I was born in the mid-1970s, the PHZ for my hometown in the Mid-Ohio Valley was 5. Then when the map was updated in 2012, our zone shifted to 6a. Now, in 2024, my hometown sits firmly in hardiness zone 6b. On the surface, this change doesn’t seem terribly significant, but it does represent a 10-degree F change from 2012 to 2024, and an increase of over 15-20-degree F across 47 years. More than the average lowest temperature has changed. The MOV is getting fewer hard freezes, and more extreme climate events – like heavy rainstorms, record high temperatures, and long, hot dry spells – are becoming more common.

A Word of Encouragement for Plant Lovers

While these fluctuations in weather and bloom times are undeniably due to a shift in climate, not all climate change events need be approached with fear and loathing.

For example, have you ever seen banana plants in West Virginia? I know, it sounds crazy since bananas are typically grown in places such as Hawaii and Florida; locations with hardiness zones of 9-13. Some varieties can grow in the 8a and 7b zones. I have had success growing a robust banana colony in my yard in Raleigh, N.C. (previously PHZ 7b, now PHZ 8a). In fact, I see them all over the upper south. Online experts say that banana “trees” (Note: bananas are not really a type of tree, believe it or not, it is the world’s largest herb) can grow just fine in USDA plant hardiness zones 7a/7b. But, thanks to a shifting climate, gardeners with a fondness for plants that aren’t “supposed” to grow well in their zone may have the opportunity to grow a special plant in a “shoulder zone.” A zone 7 plant may survive in zone 6 when carefully protected over the winter.

A great example of this is that banana plant from my yard. My dad fell in love with that banana plant and despite my warning that they were not likely to survive in West Virginia, he dug up a small shoot and took it back to plant in his yard in Ritchie County. The PHZ map tells us that it likely gets too cold for my dad’s banana plant in zone 6, but he has painstakingly cared for it by carefully cutting back the foliage in the autumn and heavily mulching the remaining material to protect it over the winter. His zone 7b banana is still doing great, and although it does not produce fruit, it is a tall, lush, eye-catching plant.

PHZMs are an important guide for those gardeners who are averse to losing their more sensitive plants. Passionate and careful gardeners can usually squeeze out an extra zone in either direction for certain very special plants. With the new PHZM, you can now think about planting crepe myrtles in your yard in the Mid-Ohio Valley, or if you’re lucky, maybe even try your hand at growing your very own banana non-tree.

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Dr. Danesha Seth Carley is a West Virginia native and professor at North Carolina State University’s College of Agriculture and Life Sciences, department of horticulture.