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.