Source: Maps are from USDA. I added the line marking the boundary between hardiness zones 5 and 6.
… and nobody cared.
Which is a good thing. I think. On balance.
On the one hand, it’s good that they released it. That’s my take on it, knowing the controversial history of the USDA hardiness zone map.
On the face of it, the red lines on the map above simply mark a data-defined boundary. Below that line is the area where winter temperatures should be expected to stay above -10F. That’s based on the 30 years of local weather data, prior to the map date. As the U.S. winter nighttime temperatures have warmed, those lines are moving north about 5 miles per year, in Missouri. And, as I understand it, at roughly that rate, averaged across the entire U.S.
Back to the here-and-now, if you look at the illustration above and immediately say, hey, what happened to the circa-2002 map? Why did they skip a decade? Then you get an interesting story.
The answer is, Republican administration. The Bush Jr. administration just somehow couldn’t quite seem to get around to allowing the public to see the updated version of that map. The widely-held presumption is that they withheld the information precisely because it showed what I’ve highlighted above: the USDA hardiness zones are migrating north. That’s easily-grasped evidence of the early impact of global warming on the U.S. And so that information was suppressed.
(This, despite the nonsensical CYA language that the USDA insists on including in the footnotes to the description of the map methodology. They seem to say that “climate change” requires 50 years of data, and since each individual map only covers 30 years, you can’t infer that this is the impact of climate change. Despite the fact that the underlying span of data across the full set of maps is now more than 50 years.)
On the other hand, I think those changes ought to get more press coverage. This isn’t natural variation. This is a clear and understandable signal of global warming’s initial effects. And as slow as these changes are, relative to a human lifetime, there’s nothing on the horizon to suggest that they are going to stop any time soon. Five miles a year doesn’t sound like much, until you realize that the U.S. is only 1000 mile north to south, and that things will move a lot faster once global warming really gets rolling. And that it’s fairly hard to grow corn and wheat in a sagebrush and cactus desert.
So, even though I’m still in Zone 7, I think this deserves more press than it has gotten. And I think that the Bush-administration suppression of the circa-2002 map needs to be remembered, right alongside the temperature data.
What are we talking about?
Source: USDA. I removed some details from the map (e.g., degrees C scale) to make it clearer. Thus, I must say that: a) the map is not the official USDA Plant Hardiness Zone Map, and (b) the USDA-ARS and OSU logos are eliminated. If you want to see the full official map, follow the link.
The map above shows the coldest wintertime temperatures in each year, averaged across 30 years of data. The 2023 map literally uses weather data from 1991 to 2020.
The map provides guidance as to what perennial plants can usually be expected to survive the winter, unprotected, in each location.
That’s guidance, not certainty. As the owner of a lime tree, I am acutely aware that citrus trees will typically die back to the ground if they go below about 28F. Plausibly, you need to live somewhere near Zone 10 or higher (e.g., Florida) before you can expect your citrus trees to survive reliably, out-of-doors, unprotected. Even so, the occasional freeze will hit Florida, so significant frost damage to Florida citrus groves seems to occur every few decades or so (reference).
More generally, if you ever buy a perennial plant from an on-line nursery, they’ll let you know the hardiness zones in which the plant is expected to survive. Or they’ll give you information such as “hardy down to 0F”, and leave it up to you to know what USDA hardiness zone you live in.
It’s not hard to get your hands on the underlying data from which these maps were created, for example, via NOAA. I’ve plotted the annual wintertime lows before, for the weather station at Dulles Airport. Here’s 60 years of wintertime lows, as recorded at Dulles.
The obvious upward trend that you see above is pretty much the norm for most of the U.S. So it’s no surprise that the revised USDA map shows those plant hardiness zones creeping northward.
In fact, my location (Vienna VA) graduated from Zone 7A (expected annual low of 0F to 5F) to Zone 7B (5F to 10F). I was firmly in the middle of 7A, now I’m barely at the edge of 7B. That’s reasonably consistent with the increase in wintertime minimums shown in the Dulles data above.
Footnote: Hardiness zone creep exaggerates average warming
One final footnote is that, due to the nature of C02-driven global warming, the northward creep of the hardiness zones exaggerates average warming.
The reason for this is simple: The largest impact of global warming is on nighttime temperatures. (E.g., via Scientific American). And on winter temperatures (E.g., via Axios). By inference, the biggest impact of all should be on nighttime winter temperatures. And, typically, the annual low temperature in an area is set during the course of some winter night.
If nothing else, knowing this is a quick way to dismiss denialist arguments that, somehow, the observed warming on earth is due to changes in the sun. (That, despite direct satellite measurement of solar irradiance, dating back to the 1970s, showing no such thing.) The fact is, the warming is more pronounced at night, and in the winter, both times of limited sunshine. Heuristically, if enhanced atmospheric C02 is a blanket, that blanket matters more when it’s cold and dark.
Conclusion
The real lesson here isn’t the map, per se. Anyone who cared to analyze the publicly-available weather data — as I did above — would already have a strong expectation that the official USDA climate zones would continue to move northward, in this most recent update of the USDA map.
Really, the big lesson here is the missing circa 2002 map. There was a time when Republicans so thoroughly insisted in keeping their heads in the sand, on global warming, that they found excuses not to update this map.
Has that changed? Are Republicans on board now, with the idea that global warming is real? I doubt it, but there’s no way to know. The last two iterations are both dated to periods with Democrats in control of the administrative branch of government. So, as to whether or not a Republican administration would allow this to be updated on a once-a-decade schedule, I guess we just won’t know until we see it. Or not.
