funnystrange.com

National Arbor Day Foundation updates hardiness zones.

The hardiness zone map divides the country into zones based on average low temperatures. Plants are often sold by zone; for instance "hardy to zone 8" means that a plant will survive the winter in zones 8,9 or 10, but should be protected or treated as an annual in zone 7. Of course, this is important information for gardeners but not the only important information. Average high temperatures aren't considered in the zone map, nor is humidity, rainfall, etc etc. I gather that a zone in the West is very different from the same zone in the East, in terms of what plants can be grown and how to care for them.

I'm not any kind of expert on hardiness zones but as a novice gardener, I've been reading a lot about it in the past year and a half, as I try to figure out what to plant in my own garden. Since I started gardening I've found the zone map puzzling, because it didn't match my experience at all. Here in Durham we were in zone 7b, which means the average low temp could get down to 5°F. Well I've been keeping track for two winters now, and that's way off. Last year the temperature never got below 15°, which would put us in zone 8b. So far this year it hasn't dipped below 20°. I realize that the map shows average lows, and the lows might be higher or lower in any particular year. But my (admittedly rather vague) memory is that we haven't seen temperatures below 10° in a long long time here. I would call a 5° low in Durham a freak occurance, not an average.

So I wasn't surprised to see that the new map puts us firmly in zone 8. A full zone makes a big difference for selecting plants. For instance, there are plants (like dahlias and tuberose) that can stay in the ground in zone 8, but have to be dug up and stored over winter in zone 7. I hesitated to plant them because that sounded like too much work. But everyone I know leaves them in the ground every year, and never has a problem. I even know a woman who grows a huge bay leaf tree in her backyard. Granted, it's a nice protected spot between the house and a low wall. But still, that should be impossible in zone 7. Her bay leaf grows way taller than the protective wall and the branches never die back.

Since this map was developed by the Arbor Day Foundation rather than the USDA, I'm not sure what impact it will have on plant sellers, if any. Some plant catalogs ship live plants based on the customers zone, and I bet they'll probably stick to the old map. But at least I can read plant labels with less uncertainty, knowing that my suspicions were correct and we're really in zone 8.

I should add that while this change makes things easier for me personally, it's reflective of some not very good news on a larger scale. In other words, the dreaded "we all know it's just junk science and isn't really happening, right? right?" global warming. The Irascible Gardener has a good essay on the larger implications of so much of the country getting so much warmer in such a short time.