Pick up any garden catalog and you’ll see a map of plant hardiness zones that are supposed to tell gardeners what they can and cannot reasonably expect to grow in their climate. The zones are based on an average of each year’s coldest recorded temperature for that specific area over a period of a few decades. Each zone represents a 10-degree block of temperatures on a scale, and each zone is split into two sections labeled A and B.
In 2012, the USDA released an interactive online map to replace the map that had been published in 1990. The new map caused a bit of a stir, since it showed a pretty good shift in the zones that indicate that the country isn’t as cold as it used to be. While the map doesn’t take into account the highest temperatures (only the lowest), so while it doesn’t show evidence that our country is warmer, it does show that our country isn’t as cold as it used to be.
For example, I was surprised to find on the 2012 map that West Virginia had added an additional zone — zone 7a, in parts of Kanawha County along the river and between Elkview and Sissonville. Back in 1990 it had been listed as 6b.
This means that our average coldest temperature was now between zero and 5 degrees Fahrenheit and not minus-5 to zero degrees — a big difference.
The zones in the U.S. range from 1a to 13b. In zone 1a, the coldest average temperature is minus-60 to minus-55 (brrrr!), which is found in the northern reaches of Alaska. In zone 13b, the average coldest annual temperature is 65 to 70 degrees. You’ll only find zone 13b on the island of Puerto Rico (its lowest zone is 11b). Hawaii is a close second — it ranges from 9a to 13a.
What does this mean for gardeners? It means that plants in those areas are less likely to be damaged by freezing.
Now, we do have years (like last year) that are the exception to the norm. But many gardeners in the Kanawha valley have taken advantage of this climate, whether they knew it or not. Giant crepe myrtles can be seen throughout the area and other plants that wouldn’t grow as well in other areas. I certainly like to push boundaries in what I can grow.
While I say you shouldn’t run out and buy a bunch of zone 7 plants just because you can, you also shouldn’t be afraid to try them if you can. Those crepe myrtles that grow large in the city can also easily freeze outside of the city if the cultivar you select isn’t especially hardy (I know, because I get lots of calls about them after a hard winter).
There are some crops that require cold in order to produce, so shifting zones can affect what can grow in that way as well. For example, certain onions (called short-day onions) typically grow best in zone 6 or lower while long-day onions are better suited for zone 7.
If you haven’t checked out your zone for a while, you may want to take a look. All of the zones have shifted northward, and if you were on or near a line between zones you could have shifted. Zone 5, which used to reach along the eastern section of the state well into the south, has now contracted to just the most mountainous regions of the state.
The new online map will also let you zoom in to your specific area, and therefore is much more accurate. You can find small pockets of zone where before we just had big lines across a map.