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.

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.”