The New USDA Hardiness Map and Cold Climate Gardening

– Posted in: FAQ

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!)

About the Author

Kathy Purdy is a colchicum evangelist, converting unsuspecting gardeners into colchicophiles. She would be delighted to speak to your group about colchicums or other gardening topics. Kathy’s been writing since 4th grade, gardening since high school, and blogging since 2002.

When dealing with frost it is always best to be paranoid. In the spring never think it is too late for one more frost to come. And in the fall never think it too early.

~Rundy in Frost

Comments on this entry are closed.

Todd@BigBlogOfGardening February 27, 2012, 9:56 am

Great post! Yes, we veteran gardeners are well aware of what will work and what won’t in our landscape. I’m officially 6b, but I can easily overwinter Zone 7 plants on the south side of my property.

Scott Supak February 5, 2012, 4:51 pm

What the map does is reflect the reality of global warming, something the Bush administration refused to do.

Danny Pizdetz February 5, 2012, 3:41 pm

I’m still a beginner, so I do use the hardiness maps. But I also watch the weather carefully for frost issues in spring and fall. For what it’s worth, if you want to find out your plant hardiness zone via zip code, you can do it at the USDAs website: I live in Kansas City and we’ve been moved from 5 to 6a.

Kathy Purdy February 5, 2012, 5:17 pm

Danny, I talk about using the hardiness map in the post after this one. You can actually get a lot more specific than zip code, if you wish to. Thanks for taking the time to comment.

Frances February 4, 2012, 11:04 am

Buyer beware, for sure! Our area did not change, and yes, I know better than any averages or educated guesses about the climate and dates in my own garden. The climate is even different in the front yard down by the street than it is at the top of the slope along the back property line. One year will be warm, like this one, last year was terribly cold and snowy. Plants need to be able to tolerate both kinds of years to live here. Do go with the free plants no matter the zones, that is good thinking!

Carol February 4, 2012, 11:01 am

Good info. The big box stores in my county have buyers in the south who ship frost tender plants in far sooner than they should be planted. Buyer beware, and educated.

Gail February 3, 2012, 9:41 pm

Kathy, You’re so right, we know what will survive our garden temperature highs and lows. When I look at a plant label I pay attention to ph preference, drainage needs and humidity tolerance. Those three are often more important than other factors in a Middle Tennessee garden. gail

Cindy, MCOK February 3, 2012, 9:23 pm

Kathy, good points! I’ve found that hardiness zones, at least on plant labels, are more useful as a guide to what can’t take our extremes of summer heat. If a plant can take winter temperatures below 0, it’s a safe bet that plant won’t be too happy with days on end of 100+ degrees!

Leslie February 3, 2012, 9:31 pm

I found the “zone 6 winter with zone 4 growing season ” fascinating. Of course that is important! And it never occurred to me that some gardeners have to think of all these variables. Excellent information Kathy!