The new hardiness map put out by the USDA is not going to help you at all if you’ve been gardening in the same spot for any length of time with your eyes open and your mind engaged. Let’s face it: common sense and experience will trump aggregated data every time. It doesn’t matter what color you are on the map, you already know what your winter highs and lows are, and–even more importantly–you know when your first and last frosts are, where the frost pockets are and where the wind blows all winter long. You know if your soil is clay or sand or plant nirvana. All that is far more important than the average winter low interpolated from the nearest recorded data points. Let’s take a look at those words I put in italics.
Average Temperatures Don’t Kill Plants
According to the USDA’s website, the low temperature for each zone “does not represent the coldest it has ever been or ever will be in an area, but it simply is the average of lowest winter temperatures for a given location for this time period.” What does average mean? I like this definition: “a single value (as a mean, mode, or median) that summarizes or represents the general significance of a set of unequal values.” Unequal values is the key concept here. When you are talking about the average of winter lows, that means that some values were lower than the average. One winter with significantly lower temperatures than is typical can kill an awful lot of plants. Before I invested a bunch of money in a plant (like a tree) that I want to live through a lot of winters, I’d want to know how often temperatures dip below the “average.”
Just as important as the average low–and not even covered by the map–is the date of your last spring frost. For annual plants, which include almost all vegetables, how cold it gets in winter is irrelevant. It’s how soon it warms up in spring that counts. Lately I’ve been telling people that I have a zone 6 winter with a zone 4 growing season. I haven’t seen too many subzero winter nights, but frost still comes in June, and many of those plants that can tolerate a zone 6 winter can’t tolerate a zone 4 spring. Sure, I would stick a zone 6 plant in the ground if someone gave me one for free, but I’m not gambling my own money on one. That’s my hard-earned local experience talking.
Sophisticated Guessing is Still Guessing
The USDA says “a very sophisticated algorithm was used to interpolate low-temperature values between actual weather reporting stations.” Interpolation is just a fancy word for guessing. Okay, educated guessing. Specifically, educated guessing using “a unique knowledge-based system that uses point measurements of precipitation, temperature, and other climatic factors to produce continuous, digital grid estimates of monthly, yearly, and event-based climatic parameters. Continuously updated, this unique analytical tool incorporates point data, a digital elevation model, and expert knowledge of complex climatic extremes, including rain shadows, coastal effects, and temperature inversions.” Granted, it’s more accurate than uneducated guessing, but depending on how far away you are from the “point” where measurements are taken, and how much your geography diverges from that around you, it might not be as good as the guess as you could make yourself, knowing your own terroir as you do.
So go ahead and have your fun, and tell your friends whether you’ve gone up or down a zone–or stayed the same. The map wasn’t made to tell you what you already know, but to tell strangers what they need to know about some place far away. Strangers–you know–the guy who moved in next door from halfway across the country who knows it gets colder here, but doesn’t really know. Strangers, like the mail order nursery in North Carolina that needs to know when it is safe to ship your plants. And, sadly, probably the big box store in your own county. (Buyer beware!)