The latest USDA plant hardiness zones reflect our warming climate. Read more to learn how gardeners still must pick plants that can handle our coldest nights of the year.
By Amy Halsted
When reading the November 20, 2023, news of updated plant hardiness zones, my heart leapt at the thought of Zone 5 lavender (Hidcote, Munstead, Twickle Purple, and more).
As northern backyard habitat and pollinator gardeners, we’ve stretched our season, knowing that echinacea, rudbeckia and many other plants still bloom long after they once did. Tithonia now lasts into October—provided there isn’t a one-night frost. After the monarchs have begun their journey southward, in my backyard, the birds and bees continue collecting pollen, the bees often a little Tithonia-tipsy as a result.
Our instincts to the lengthening season point to climate change: in fact, the new plant hardiness map—updated after more than a decade—has shifted more than half the continental United States a full half zone. The 2023 map is roughly 2.5 degrees Fahrenheit warmer than the 2012 map.
Is the new map based solely on the effects of climate change? The answer is mixed. If you are a backyard gardener in Florida, you must adapt tomatoes to a variety that can handle higher temperatures to avoid burning. If you are a habitat gardener in the Mahoosucs, you must continue to consider the coldest single night each year. In early February 2023, the temperature in Bethel was -17 degrees Fahrenheit, with the dew point at -24 degrees Fahrenheit and wind gusts to 26 mph with a wind chill value of about -44.2 degrees Fahrenheit.
Two key contributors to the new map offer their definitions of how the single night winter chill determines the other 364 days of hardiness planning. Chris Daly, director of the PRISM Climate Group at Oregon State University, who jointly developed the map with USDA, is hesitant to explicitly attribute the specific changes from the 2012 map to the 2023 map to climate change because of the volatility of the key statistic they used to create it. They were mapping "the coldest night of the year, each year, over the past 30 years and it's a highly variable figure,” said Daly.
In an "A Way to Garden" article regarding the new hardiness map, USDA horticulturist, research scientist and chairman of the technical review committee for the new map, explains, “The Plant Hardiness Statistic is a single data point per year, and that is the coldest temperature recorded on the coldest night. And then the map is generated by taking that data point and averaging it over 30 years.”
According to the USDA, the hardiness map is a general guide for growing perennials. It also does not guarantee that a 5b plant will withstand other environmental factors. “In addition to hardiness zones, wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can greatly affect the survival of plants. Warm season heat and moisture balance are particularly important in this regard. The way plants are placed in the landscape, how they are planted, and their size and health can also influence their survival,” writes the USDA in its “How to Use the Maps” section of the site.
Click the USDA link below and enter your zip code to get a closer look at microclimates in your backyard (within ½ mile of all locations). Our regional range is now between 3b and 5a. A note that each zone represents a five-degree Fahrenheit increment. You can also review historical references; and, on the left-hand side, click the bottom icon for map layers. It is as interactive as you’d like.
In my garden, it seems wise to continue the annual autumn-applied mulch volcano around the butterfly bush (Buddleja) and to stick with Zone 4 Munstead lavender.