2023 USDA Plant Hardiness Map Updates
finally. Finally, there is an up-to-date official government source that gardeners across the land can blame for any failed planting efforts.
Well, maybe not for all the failed plantings, but at least there’s another arrow in the gardener’s quiver to help us all be more successful in our efforts.
After many years of analyzing massive amounts of meteorological data, consulting weather rocks on the front lawn of the USDA building in Beltsville, Maryland, and dusting off the government’s official Ouija board, our friends at USDA have released a long-awaited update. For cold hardiness zone map.
If you are not familiar the mapIt is a graphical representation of the United States divided into 26 regions based on the average annual minimum temperature. For decades, this map has been used by gardeners to identify plants that might have a chance and selections that don’t stand a chance in any given patch of American dirt. It’s become an industry standard that makes gardening feel a little less like you’re staring down the barrel of Dirty Harry’s 44 magnum – “Are you feeling lucky, Punk?”
What is the USDA Cold Hardiness Zone Map?
A bit of history. Since the dawn of time, gardeners have asked a simple question: “Will these plants grow here?” For the time being, perhaps the most common answer was: “I don’t know. Try it…” Then, back in the 1920s, a group of pioneers at Harvard’s Arnold Arboretum decided to create a map that would at least help answer the winter survival part of this age-old question. They plotted areas across the United States to give gardeners an idea of how cold it generally gets during a typical winter. Over time, gardeners and the garden plant industry have adopted these regions almost as part of the plant’s name. “Does anyone have any zone 6 camellias for sale?” For example
After several iterations, USDA scientists revised, updated, and streamlined the effort with the first USDA Cold Hardiness Zone map published in 1960. This map, now frequently revised and presented in ever-increasing detail, has become the generally accepted standard for By home gardeners. and horticultural professionals across the United States.
How many zones are there on the USDA cold hardiness map?
As of the latest release, the November 2023 map divides the United States into 13 cold hardiness zones at 10°F intervals, with each zone divided into “A” (the warmer half) and “B” (the cooler half). Sub-regions. The coldest parts of Alaska represent Zone 1A where average annual minimum temperatures range between -60 and -55 degrees Fahrenheit. (Yes, that’s almost the North Pole.)
Revising the map for 2023, my Louisville garden is in Zone 7A with an average low temperature of 0-5 degrees Fahrenheit. The warmest zone, 13b, covers areas where the average annual minimum temperature does not usually fall below 65 degrees Fahrenheit. Sign me up for this!
Close inspection of the wonderfully detailed 2023 Hardiness Zone Map provides further information. Due to Louisville’s urban heat island effects, my home garden in zone 7a is in a different cold hardiness zone than the U-Dale Botanical Gardens (6b), just a 20-minute drive down the road. You can check your area’s rating by going to planthardiness.ars.usda.gov/ and entering your zip code.
What does a cold hardness map measure?
This is the thing that most people miss about the cold hardness map. Regions represent He means Annual minimum temperature, not minimum temperature. So, in my current 7a garden in Kentucky (it was 6b in the last version of the map) there’s nothing that says my plants won’t experience the occasional -3, -5, or -8, as was the case in the late 2022 polar blast that destroyed many plants in All over the city. It just means that on average, this won’t happen very often. Remember, the record low temperature in Louisville, Kentucky reached -20 degrees Celsius!
So, as growers, how do we use the cold hardiness zone map? cautiously! Averages can be misleading.
I grew up in an area of New York City that the current map conveniently classifies as Zone 7A — like my backyard in Louisville, Kentucky. However, the low temperatures recorded in New York are about 10 degrees higher than the temperatures recorded in Louisville. How could this be if both cities are in the same cold hardiness zone?
New York City is located on the edge of the Atlantic Ocean, a massive heat-buffering effect that is absent from Central America. Here in Kentucky, all it takes is a mosquito to sneeze and the jet stream falls off Indianapolis and we’re all in trouble. Coastal areas tend to have less temperature fluctuations due to the moderating effect of the huge volume of water present in the oceans. The Ohio River doesn’t have enough water to make much of a difference.
Every plant has a temperature below which it has little chance of survival. Each plant also has a temperature range that results in partial damage but not complete death. And every plant has a temperature above which everything is well and good. But of course, all of these temperature points depend on the plant being in optimal health, growing under ideal conditions, and providing test temperatures at normal times. Color outside those lines and all you can do is wait and see what appears on the other side.
If you’re a gardener who doesn’t like surprises, it’s best to be conservative in your plant choices and not push hardiness zones. If you live in zone 6b and grow plants classified as zone 6, you will probably be fine 90% of the time. But if you’re a regionalist when it comes to choosing plants for your garden, “You have to ask yourself, ‘Do I feel lucky? Well, do you, punk?’
Paul Cappello is executive director of Yewdell Botanical Gardens, 6220 Old LaGrange Road, yewdellgardens.org.