Hardiness Har Har

In case you missed it, the USDA just issued their Updated Winter Hardiness Map, led by former PDN/JLBG staffer, Dr. Todd Rounsaville. While the results show an expected warming trend, the results are a bit concerning from the point of view of what plants people will be encouraged to plant. You can find the new map here.

The new map shows our location outside Raleigh, NC, as moving from Zone 7b to Zone 8a. Since the purpose of the map is to show people what they can grow in each zone, this is troubling. Using a 30 year data set, our winter low temperatures average out to 13.03 degrees F. That temperature is certainly Zone 8a, but when you look at the low temperatures experienced during that 30 year stretch, we experience six years where the temperatures were well below Zone 8a.

During that stretch, we have had:

4 Zone 9a winters, with a low of 20-25 F

7 Zone 8b winters, with a low of 15-20 F

13 Zone 8a winters, with a low of 10-15 F

3 Zone 7b winters, with a low of 5-10 F

2 Zone 7a winters, with a low of 0-5 F

1 Zone 6b winter, with a low of -5-0 F

Therefore on 6 occasions during the last 30 years, all of your Zone 8a plants would have died. Is this really acceptable? We think not.

Having been on the map committee in 2012, we requested that items like this and many other map deficiencies be addressed with the new map, which does not seem to have occurred. If the purpose of the map is simply to show how the low temperature averages have changed on average, then the map works fine, but it is actually used for so much more. I hope the USDA will come up with a plan to address these concerns.

6 thoughts on “Hardiness Har Har”

  1. This was my first thought as well upon seeing the new map: folks are going to be really upset when they lose expensive perennials twice per decade! Undoubtedly our climate is warming, but many parts of Central NC now zoned 8a experienced 7b temps just last winter, well into the Triangle area. Even more baffling is the inclusion of Greensboro in Zone 8a… wow!

  2. Like you, as an experienced gardener of many years (N.Texas), I hedge my bets with the hardiness zone map. I usually lower the hardiness zone by one or two numbers, just to be safe. If I do buy something new that pushes the limit, I give the plant extra protection and watch the weather reports. Thanks for helping us understand this new map.

  3. “Yes” to your comments. Last winter in Western North Carolina it was unusually mild, except that there was a precipitous drop to 2 degrees for two nights around Christmas, and then was very mild again, and I lost plants that had weathered minus 5 in previous years with no trouble. I imagine the plants had not gone fully dormant and then were too stressed to survive the remainder of the winter.

  4. I don’t think the average gardener – the average customer at garden centers where I’ve worked – pays any attention to hardiness zones. I think they find a nice plant, plant it in approximately the right amount of sun, and hope for the best.

    I think most experienced gardeners who do pay attention to hardiness zones also think about the temperatures themselves, not just the zone number. If I plant a normally safe zone 8 plant and notice the temperature is actually predicted to go down to 5 degrees – as happened in 2021 when I lived in Austin, TX, then listed as 8b and now 9a – I get out there and start putting frost blankets or even cardboard boxes over my most tender plants! I didn’t lose any plants that year, even my newly planted kumquat.

    So I am fine with the zone indicating the 50th percentile lowest temperature of the year, not the 30th percentile like they use for “average” frost dates (it did take me until this year to figure that number out).

    Looking at your figures above, it actually seems that both the 50th percentile and the 30th percentile lowest temperature fall in zone 8a. Do you know which percentile they are really using?

    1. I can’t speak to a typical average garden center customer, but I know that a majority of our customers care about choosing the right plant for their zone. The ones I chat with almost 100% look at the Zone number without paying any attention to the actual temperature. We constantly have customers insist that we increase the hardiness range because a plant came through one or two mild winters. When we pin them down on the actual temperature, it doesn’t remotely match their actual hardiness zone. In the case of the new map, it would have been correct 80% of the time. In my world, that’s a much too high error rate.

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