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The USDA’s gardening zones shifted.
This map shows you what’s changed in vivid detail
By Daniel Wood, Connie Hanzhang Jin, Brent Jones and Jeff Brady
Recently, the USDA updated its plant hardiness map for the first time in 11 years.
If you’re a gardener — and everybody can be a gardener, even on a balcony or a stoop — this is a big deal!
The updated map opens up new possibilities for home gardeners, but there are limits. Let’s explore how the map has changed and what this means for your garden.
Enter your city and state:
Back then, coldest winter temperature was somewhere degrees Fahrenheit on average.
Now, the lowest winter temperature is degrees Fahrenheit on average.
The new 30-year minimum temperature average was 3.3 degrees F warmer than the previous average, which spanned 1976 to 2005.
Change in lowest winter temperature
-8
-6
-4
-2
0
+2
+4
+6
+8° F
About this data
The 2012 USDA hardiness zones were calculated using the average lowest winter temperature for the observation period of 1976-2005. The new zones are calculated using the years 1991-2020. These two observation windows overlap. Colors show the difference between the two 30-year averages for each place on the map.
Most of the changes across the country are due to the warming climate.
Winters are warming at a faster pace than other seasons, according to Deke Arndt, director of the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information.
At the same time, an increase in the amount and quality of data collected at weather stations across the country helped to improve the overall accuracy of temperature readings in recent years.
What does your hardiness zone tell you?
Some people might think their hardiness zone tells them which plants they can grow. In reality, it’s a little more complicated.
Your zone measurement is an average of the coldest yearly temperature in your area over the past 30 years.
Though these temperature estimates differ slightly from the data the USDA used to create the zone map, we’re using them to illustrate how zones are calculated.
However, the temperatures we’re showing here are based on estimates, and in this case they do not align with the more accurate, granular data that the USDA used. The USDA map classifies this area as .
This measurement, which predicts an area’s coldest temperatures, is only useful for plants that have to survive the winter.
They’re called perennials: You plant them once and they come back after each winter if they’re given the right environment to survive. Think things like trees, shrubs and woody plants.
Windmill palm
zones 7-11
Hydrangea
zones 3-9
Azalea
zones 6-9
Lavender
zones 5-9
Windmill palm
zones 7-11
Hydrangea
zones 3-9
Azalea
zones 6-9
Lavender
zones 5-9
Windmill palm
zones 7-11
Hydrangea
zones 3-9
Azalea
zones 6-9
Lavender
zones 5-9
And for predicting winter plant survival, knowing an area’s hardiness zone is a big help to gardeners, says Todd Rounsaville, a horticulturist with the USDA who was involved with creating the new map. He explains that the hardiness zone “is really one of the best predictors of winter survival and plant survival in general in the landscape.”
He advises gardeners to use the map as one very important tool of many in their risk assessment toolbox.
“Because the USDA map has really become the industry standard for rating things, it’s pretty rare that you will not see a zone rating on a plant, either on the tag or on a website,” he says.
Knowing what your average coldest temperature is helps rightsize your expectations about what might grow in your area. Live in Chicago’s Zone 6a? You can be assured that no citrus plants will survive your winter. Instead, try an apple tree. The apple tree is that kid you grew up with who wore shorts all winter. It needs the cold temperatures to set fruit.
Live in Miami’s Zone 11a? No apples for you. Instead, grow dragon fruit!
What does your hardiness zone not tell you?
On its own, your hardiness zone can’t tell you exactly what to grow in your area.
For example, parts of these three areas — Juneau, Alaska; Boston, Mass.; and Santa Fe, N.M. — are all in USDA’s Zone 7a.
Juneau, Alaska
Boston, Mass.
Santa Fe, N.M.
Juneau, Alaska
Boston, Mass.
Santa Fe, N.M.
Juneau,
Alaska
Boston,
Mass.
Santa Fe,
N.M.
“We know intuitively that the same plants can’t grow in these places,” Rounsaville says.
While Juneau may have relatively temperate winters, it also is extremely wet, averaging over 80 inches of snow a year. Santa Fe, on the other hand, is extremely dry, with much hotter summer temperatures than Juneau. Boston has both temperate winters and summers. It gets a good amount of rain but not nearly enough to sustain Juneau’s rainforest plants. It gets plenty of heat but is colder and wetter in the winter, making it inhospitable for desert dwellers, like cactuses and other succulents.
But all three cities rarely get below zero degrees each winter, so they are classified as the same zone.
So when you hear that your zone has changed, here are some things to keep in mind:
1 The hardiness map says nothing about your extreme lowest temperature
Just because your average lowest winter temperature has changed, doesn’t mean the temperature will never dip below your hardiness zone.
For example, the average coldest night of the year in St. Louis, Mo., tends to be around 2º F, meaning that it’s in Zone 7a. Because St. Louis has warmed, it moved up from its previous zone rating of 6b.
But notice that this is an average of the coldest temperature St. Louis gets each winter.
In the past 30 years, the temperature dropped below Zone 7a in at least 11 different years.
In 2014, the temperature dipped three half zones below St. Louis’ hardiness zone, to -10º F. Brrrr!
Many common plants that are hardy down to Zone 7, like rosemary, canna lilies or agave, would suffer significant damage or death from those temperatures, especially during a long cold snap.
2 The hardiness map says nothing about the frequency of extreme cold weather
Your poor plants have to stay outside all winter, so the duration and frequency of cold weather matters for plant survival.
“If you’re naked and you run through a freezer, it’s not going to kill you,” says Andrew Bunting, vice president of horticulture at the Pennsylvania Horticultural Society. “If you run into the freezer and have to stay there for an extended period of time, it’s probably going to kill you.”
If extreme, out-of-zone weather occurs during a quick cold snap, steps can be taken to protect your plants with temporary blankets or other shelters. Pots can be brought inside.
But if the extreme lows persist, tender plants will struggle to survive. Your hardiness zone does not take any of this into account.
3 The hardiness map can’t tell you if your plants will survive the summer
Summer temperature extremes matter a great deal but are not reflected in the USDA hardiness map.
Let’s look again at Juneau and Santa Fe, much of which are in Zone 7a. Juneau’s all-time high temperature was 90º F in 1975. Summer days in Santa Fe routinely reach the 90s. Some shade- and cool-weather-loving plants like ferns and hostas will thrive in Juneau but struggle mightily in a place like Santa Fe. Likewise, a cactus accustomed to high temperatures would struggle to thrive in the cooler summer temperatures of Juneau, to say nothing of the overwhelming rainfall.
Because of this tricky problem, there have been attempts to create a corresponding map that helps gardeners know which plants might survive summer in their area.
In 1997, the American Horticultural Society released a heat zone map that measured the average number of times per year that the temperature of an area exceeds 86º F.
American Horticultural Society
But this map didn’t become well known among gardeners. On a recent visit to a plant nursery outside Washington, D.C., nearly every plant tag had a USDA hardiness zone, but only one, out of the several dozen checked, had the AHS heat zone listed.
Above 86º F, plants from cooler climates rapidly become stressed.
Because of these complexities, more plant survival factors should be included in the 2023 map, says Tony Avent, who runs Juniper Level Botanic Garden and Plant Delights Nursery in Raleigh, N.C.
“If [these metrics] had been factored in, that would have given you a much more applicable map,” says Avent, who was a member of the committee that put together the 2012 version of the map.
“And that’s the part that’s a little disappointing.”
But including more plant survival factors in the USDA hardiness map runs the risk of creating an overly complicated map and muddying its intended use, Rounsaville says.
“In a perfect world, we could infinitely break down where plants will grow well, but that’s very hard to do and produce a map that is, you know, coherent but at a local resolution,” Rounsaville says.
Since the USDA plant hardiness zone can’t tell you everything about how a plant will fare in your garden, it’s a good idea to turn to local plant experts for guidance. Local nurseries and botanical gardens can be great resources for in-depth knowledge of the area and recent warming or cooling trends.
New plant varieties are constantly being bred with improvements such as increased hardiness, bloom count, bloom length or color combinations.
Some nursery owners like Avent enjoy experimenting with these plants. He and his team grow many varieties of plants — both typical and unconventional — to figure out which plants they can bring to market in Raleigh.
“We live to kill plants,” Avent says. He estimates that they’ve killed over 50,000 plant varieties in his career. Every one they kill, they record in a database.
If my zone changed, can I plant new things now?
Maybe, and maybe you already did! It’s possible you or your neighbors may have already noticed some of these climatic changes and have been experimenting with plant varieties that were once unusual for your area.
Keep in mind that the new USDA map is backward looking; it represents changes that have already taken place over the past 30 years.
In the 7a–7b Philadelphia suburbs, Bunting notes two perennials that he has noticed surviving Philadelphia winters in recent years.
“It used to be [that] if you had a camellia, it was in a little courtyard with lots of protection, maybe even wrapped [in protective cloth] for the winter.” But now, “It’s perfectly hardy. Same with figs. People used to wrap figs. You don’t have to do that anymore.”
Of course, your mileage may vary. As Bunting notes, where you plant a perennial in your yard — whether sheltered or in the open — matters. Some areas get southern exposure and lots of sun, others are behind a house, or under a tree. Every yard has many distinct microclimates, and learning how to harness these subtle differences in your yard can help you plant more ambitious varieties with more confidence.
“Gardeners know that if they’re near paved surfaces or brick and mortar structures, that there’s a lot of radiant heat that those absorb during the day,” Rounsaville says. “And they can really push hardiness zones through the winter to help with plant survival.”
Aside from local nurseries and botanic gardens, cooperative extension services can be a great place to find local gardening advice. The extension services are part of a national network of local experts who provide advice on everything from agriculture to gardening.
NPR reached out to services in over 30 areas across the country, and many told us about changes they’ve seen in what they can and can’t plant over the past 15 years.
Julie Weisenhorn, extension horticulture educator and associate extension professor, University of Minnesota
With that, you have what you need to start a garden. Big or small.
Happy planting!