There’s good news and bad news from the United States Department of Agriculture. The good news is that USDA is very close to publishing an update of its Plant Hardiness Zone Map, the basic tool by which gardeners can determine if a particular plant will flourish in the climate of any given area of the United States. The old map, published in 1990, is badly out of date; the North American climate has changed significantly since that time.

The bad news is that the USDA has designed its new map in a way that minimizes any recognition of recent climate change. Indeed, one could argue that the plan the USDA is following ensures that the new map will be outdated at the time of its completion.

Let me explain. The concept of the Plant Hardiness Zone Map is that it divides the United States (including Hawaii and Alaska) into 11 different geographical zones; the zone to which your area is assigned is based on the lowest temperature you experience in an average winter (in the current map this average was derived from 13 years of records—1974 through 1986—collected from weather stations all around the United States). If, for instance, your average winter low falls between -10° and 0°F, the map assigns you to Zone 6. If, on the other hand, your average winter low is between -20° and -10°F, the map includes your region in Zone 5 (the colder the zone, the lower the number). The nursery industry uses this map as a reference to inform gardeners about which plants will survive in their region. That’s why on most plant labels you’ll find some message such as “Zone 4,” which means that the plant in question will typically survive winters—is “hardy”—in USDA Zone 4. Summer heat can also be challenging to plants, and that’s why sometimes the labeling will list the hardiness as a range of zones, as in “Zones 4–8.” That indicates that the plant will survive winters as far north as Zone 4 and summers as far south as Zone 8.

As the North American climate pursues it current warming trend, the 1990 USDA map became less and less reliable as a guide to local climate, and so less reliable as a guide to plant hardiness. If you are located in the southern part of what the USDA still calls Zone 3, your winters may now be warm enough that you should actually be included in Zone 4. Plants that the USDA map says won’t survive a winter in your region may actually be fine there today. Similarly, many southern gardeners are finding that summers in their regions are significantly hotter, so that northern plants that formerly would grow in their regions no longer thrive there.

Six years ago the American Horticultural Society, under contract to the USDA, produced an updated map based on 16 years’ worth of weather records. However, the USDA rejected it, claiming that the software in which the map was written didn’t suit its needs. People involved in the negotiations, though, said that the USDA (part of a federal government that at the time was denying that global climate is changing as a result of human activities) was upset by the picture the new map presented of a warming North America.

Whatever the reason, the USDA decided to start from scratch and produce an updated map in-house. Its new map will be interactive so that gardeners can zoom in for a more detailed regional picture. This will be useful no doubt and, in drawing climate zone lines, the new software takes into account geographical features such as altitude and nearness to major bodies of water, things that can depress or increase temperatures locally. That’s good, too.

These bells and whistles, however, don’t compensate for what is an on-going refusal to acknowledge climate change. The committee in charge of designing the new USDA map chose to base the map on a 30-year run of weather records — twice the previous run — thus minimizing any evidence of recent warming, because the warmer winter temperatures of today will be averaged with decades of colder ones. The effect will be to produce averages, winter lows, that are colder on the whole than what gardeners are actually experiencing now. When I called the USDA to ask why the long data run, spokesperson Kim Kaplan insisted that the 30-year perspective produces averages more consistent with long-term climate. That’s true, from a scientific standpoint. But the problem as I see it is that the survival of garden plants depends on what they experience day by day. The plants don’t care about scientific consistency. The immediate reality is what governs them.

Fortunately, there’s an alternative to the USDA maps. When the American Horticultural Society map was rejected by the USDA, the National Arbor Day Foundation used a similar set of weather data and similar methods to produce a plant hardiness zone map of its own. This map uses the same system of average winter lows but bases it on 15 years of data to divide the country into 10 zones (the Arbor Day map doesn’t include Hawaii, which eliminates the USDA’s warmest zone, Zone 11). Because it gives the same zone numbers to the temperature ranges as the USDA maps—Zone 6 in the Arbor Day map still includes any place where the winter low falls between -10° and 0°F—the Arbor Day map works fine as a guide to understanding the hardiness information you’ll find on nursery labels. But the Arbor Day map is different from the USDA maps, old and new, in that it reflects the kind of winters American gardeners actually are experiencing now. I find it a far more accurate reference tool than the existing USDA map, and I don’t expect that to change when the USDA publishes its update.

The National Arbor Day Foundation has provided one more service in connection with its map. If you click here, you’ll find yourself connected with an animation in which the old 1990 USDA zones redraw themselves to suit more recent weather data (as in the Arbor Day map). Watching the boundaries of the zones creep north offers a vivid and sobering illustration of how our climate is changing.

Of course, categorizing plants by cold tolerance alone is a very crude way to judge their hardiness. There are a number of other tools, such as the American Horticultural Society’s Heat Zone Map, that are useful supplements to the Arbor Day or USDA maps. I’ll describe some of them in a future post.