Now that the controversy of Scotts and the NWF has come to a more or less satisfactory close, let us turn to the newly released USDA zone map. It may not be as scorching a topic, but I think it has plenty of interest—especially since this map has been so long in the making.
I remember nurseryman Tony Avent talking about the new zone map when I interviewed him in 2007. Back then, it seemed like it was almost ready to announce. First Avent talked about the botched zone redo of 2003: "That map was not accurate. It got rid of half the zones [the a's and b's]. It was easier to use, but it was wrong. Chicago would have been zone 6." After that first map (which Avent says was created by a consultant the USDA sent off to make a map so he'd "stop bugging them"), the UDSA called together a new committee (of which Avent is a member), and decided on a 30-year average of temps rather than a 20-year average, which would have created too dramatic a shift. An early plan to have a, b, and c zone gradations rather than just a and b was put off for a future revision.
And now, more than five years after we had that conversation, the map he was discussing is finally out. What took so long? The process was a comedy of errors in many ways, according to Avent—read the whole story on his website—but basically they had to go back to the drawing board a couple of times. They’re happy with the results, despite the delays, because the new map is much more functional than previous versions, as well as incorporating the subtle a and be gradations. Their map has all the pretty colors we’ve come to expect, but this time it’s clickable; you can zoom in on your area to get a better look at the color gradation to figure out exactly where you fall, you can check by zip code, and there's more data available however you check.
There’s also already criticism. I have heard comments that the 30-year average was chosen to minimize the effects of global warming, and the USDA seems to have anticipated this with the following statement included in the map’s accompanying narrative:
Climate changes are usually based on trends in overall average temperatures recorded over 50-100 years. Because the USDA PHZM represents 30-year averages of what are essentially extreme weather events (the coldest temperature of the year), changes in zones are not reliable evidence of whether there has been global warming.
Others think the USDA doth protest too much. In a WaPo AP story, a Boston professor remarks, "“People who grow plants are well aware of the fact that temperatures have gotten more mild throughout the year, particularly in the winter time," and one of the creators of the 1990 map states "the latest version clearly shows warmer zones migrating north." Whether the map is reliable evidence of climate change or not, even its own narrow standard indicates that the coldest days are not as cold as they were. Which NOAA will officially confirm in July. It seems like the rabbit's out of the hat on this one.
FWIW, my zone did change. I have gone from 5b to 6a, which I checked by comparing this with the one on the National Gardening Association site—still the 1990 one. The Arbor Day map has me between 5-6. After a decade-plus of gardening, however, I generally choose plants based on what I think will happen to them on my property, often not even looking at the zone on the card. (When I do, it can be surprising; some nurseries have been selling zone 6 plants here for years.)
The thing is, there are so many things besides technical hardiness that have an impact on plant survival. I trust a good nursery (of which we have many) to sell what should survive in my locality. But I fret about too-damp soil, too much shade, root competition, and—of course— my own ineptitude. I deal out death with abandon, regardless of zone hardiness. Zone maps are not for the likes of us, really. They provide guidelines for gardeners just starting out, innocents who cherish hopes that the illiterate plant world pays any attention to rules, zones, and fine print.
I have to be honest. This confirmation that Buffalo is slightly balmier than was previously thought might lead to a few changes. I won't be planting palm trees, but maybe I won’t baby the macrophylla hydrangeas as much.
Have you played with this map yet? Has your garden moved, compliments of the USDA?