For decades, gardeners and growers have relied on the USDA’s Plant Hardiness Zone map to decide what perennials, shrubs and trees are best to grow in any specific region of the country.
The information is so vital for understanding what plants will or won't thrive in your area, that we include this information every time we write about or recommend plants to help readers select those that are most likely to survive winters in their individual gardens.
But, for the first time since 2012, a new plant hardiness map is now available. Released in November 2023, the updated map was developed by Oregon State University’s PRISM Climate Group and the USDA’s Agricultural Research Service.
The map has 13 zones and is based on 30-year averages for the lowest annual winter temperatures in specific regions.
“The map is divided into 10-degree zones and 5-degree half zones,” says Christopher Daly, PhD, professor, and director of PRISM Climate Group, College of Engineering, Oregon State University. “The new map is about 2.5 degrees warmer than the previous map on average, which puts about half the country into the next warmer half zone, while half stayed put.”
Here’s what the new USDA Hardiness Zone Map means for you as a gardener:
Why Is There a New USDA Hardiness Zone Map?
The new map uses more sources. It incorporates data from 1991 to 2020 from 13,412 weather stations, compared to the 7,983 weather stations that were used to create the 2012 edition map.
The vast amount of new data helped scientists devise the updated map. “Some changes could have been caused by increased station data, or 68 percent more stations than before, or improvements in modeling techniques between the two maps,” says Daly. “These tend to be isolated, rather than regional in nature.”
Enhanced data gathering techniques also played a part in revising the zones. “We improved our ability to identify pools of cold air that lie in valley bottoms, increased the amount of information at high elevations, and improved how we modeled proximity to the coastlines of the oceans and Great Lakes,” says Daly.
Does the New Map Change What I Can Grow?
There's a good chance it will.
“With half the country shifting to the next warmer half-zone, such as from zone 7a to 7b, for example, this could mean that those areas that shifted could potentially grow plants that are slightly less cold hardy,” says John Keller, vice president of production planning at Monrovia.
However, growers won’t need to change the hardiness zones listed on plant tags. “The hardiness zone systems itself hasn’t changed, only the geographic location it is applied to has changed,” says Natalie Carmolli, public relations with Proven Winners Color Choice Shrubs.
“So our USDA Hardiness Zone 5 plant, for example, is still hardy to USDA zone 5. It’s just that USDA zone 5 looks different now.”
Areas were more likely to shift to a new zone if already on the warmer edge of one, and less likely to shift if solidly in a zone or on the colder edge. “This doesn’t mean that one person gets to try new plants and the other does not. It’s just the luck of the draw in terms of where the zone boundaries happen to be,” says Daly.
How to Use the New Hardiness Zone Map
You can find your USDA Hardiness Zone on the updated interactive map, here.
Remember that hardiness zones can help you buy the right plants for your region. But it’s never a guarantee that a plant will do well for you. Other factors such as drought, humidity, disease, and pest pressure can affect whether or not a plant thrives in your garden.
It’s also important to understand that these zone changes don’t mean you can suddenly start growing tropical plants outdoors year-round in cold climates or tulips in south Florida. “Along with the USDA hardiness zones, gardeners need to use their own past knowledge of how weather has affected their plantings and adjust accordingly,” says Carmolli.
Essentially, the new map means you can experiment a little if your region has shifted into a new zone. “Based on these changes, I would not go out and pull up all of the stuff in my garden and buy new plants,” says Daly. “But I might start thinking about trying out some plants rated to the next warmer zone in the warmer parts of my garden, such as areas protected from the wind or a nice, sunny, south-facing wall.”
