The newest version of the USDA’s Plant Hardiness Zone Map makes official what many have been observing in their own back yards over the past few years. (Graphic by Oregon State University)

During gardening’s winter off-season, gardeners stay active looking ahead to spring, eagerly perusing seed catalogs, making notes, and placing orders.
Of all the vital pre-planting particulars armchair growers doublecheck first, the USDA’s Plant Hardiness Zone Map (PHZM) is among the most crucial.
This year, though, it’s advised that even seasoned gardeners revisit the map, as it was updated in 2023.
The newest version, found at www.planthardiness.ars.usda.gov, makes official what many have been observing in their own back yards over the past few years.
“The map is based on the average annual extreme minimum winter temperature, displayed as 10-degree F zones and 5-degree F half zones,” according to the USDA’s Agricultural Research Service.
The original idea for a hardiness map came from Alfred Rehder, a Harvard University Arnold Arboretum taxonomist, in 1927. In 1960, the USDA initiated its map.
In 1990 the overall map included Hawaii, Alaska, Canada, and Mexico for the first time. Then, in 2012, frost-free zones 12 and 13, Hawaii and Puerto Rico, were added.
Since it’s 1960 inception by USDA, the color-gradiated map has served as an invaluable general guideline to help determine where perennial plants have the best chance to survive, based on average lowest temperatures (but not the lowest ever).
Mikaela Boley, Talbot County’s Principal Agent Associate for Home Horticulture and Master Gardener Coordinator, wants to help local gardeners get familiar with the changes, and what they mean.
“This release of the updated Plant Hardiness Zone Map is an indicator of several things,” Boley said. “One, we can visually see what many of us have been experiencing, which is a warmer climate than we’ve had before. I think 50% of the U.S. has changed hardiness zones, and nearly all of that is for the warmer. For the Shore, this means many counties have switched to zone 8a, and the remainder are in zone 7b. This is a half-zone increase from the previous map.”
Horticulturist Miri Talabec, Extension’s Home and Garden Information Center Coordinator, concurred.
“The 2023 USDA hardiness zone map is available online and appears to differ from the prior edition by an increase of about half a zone (5 degrees F) for many areas of Maryland (as well as other states). For example, areas which previously were a zone 7b may now be an 8a. This is not new per se, as data is compiled for multiple years to create averages to base temperature zones on, but reflects an expected warming trend overall, due to both climate change and (for some areas) more urbanization,” she explained.
“The first thing found at the page linked above includes an explanation of how the map was created, and you can look up zone designations by ZIP code,” Talabec added.
This current update, like it’s most immediate predecessor in 2012, is Geographic Information System-based and has been developed specifically for interactive internet use.
The ZIP code zone locater is one such feature.
Using digital GIS technology enabled the 2012 and 2023 maps to provide a higher level of resolution, showing smaller areas of zone delineations than in maps made in 1990 and before.
“For example, cities tend to hold more heat because they encompass large areas of concrete and blacktop, so a city or town might be assigned to a zone warmer than the surrounding countryside. Higher elevations tend to be colder than surrounding lower areas, so the top of a mountain might be an area of cooler zones. A location near a large body of unfrozen water, especially downwind from prevailing breeze from that water, might provide milder winter weather and be in a warmer zone,” according to the ARS explanation.
The 2012 and now 2023 maps also include two new zones, 12 and 13, designated for areas with “annual extreme minimum temperatures above 50 degrees and 60 degrees, respectively, appearing only on the maps for Hawaii and Puerto Rico,” ARS adds.
But the information pertaining to these “frost-free zones” can be useful for those caring for tropical and semitropical plants used as house or patio plants in many other places.
It also offers knowledge to help gardeners decide when to bring tropical plants indoors from a deck or patio as the temperature cools.
On the 2023 PHZM USDA pages, along with a bounty of detailed information concerning the map itself, growers are also reminded that, while important, additional information besides hardiness also factors into a plant’s ultimate success.
Lisa Pearson, writing for Harvard University’s Arnold Arboretum in July 2023, agreed: “Ultimately, guidance from any hardiness map should be considered in concert with information about the plant in question and the grower’s local conditions.
In a November 1940 article in the Bulletin of Popular information Donald Wyman alludes to this necessity.
“Hardiness of plants is an indeterminable quantity,’ he wrote, ‘based not only on a plant’s resistance to minimum temperatures, but also on its resistance to maximum temperatures, and other factors such as lack of water, exposure, soil conditions, length of growing season, etc.
It would be impossible to prepare a map depicting all these factors, though several might be included on a complex one. However, since a map based on the average annual minimum temperatures agrees in many instances with the known limits of hardiness of certain plants, these data were adopted as the basis for hardiness zones.’”
For questions about the USDA PHZM Zone Map 2023 and the mapping process, contact the USDA Agricultural Research Service at phzminfo@usda.gov.