News alert! You may not be aware that the USDA updated the Plant Hardiness Zone Map on November 15, 2023. This changes a lot of things that we gardeners are going to face in 2024 and onward. Gardeners of all skill levels use the plant hardiness map. It determines when to plant, what will grow best in certain areas, and when the first frosts are likely to arrive. A newly updated plant hardiness zone map is highly significant. A lot of gardeners are going to have to alter their garden to compensate for the hotter weather on this planet we call home.

Here’s what you need to know.

What is a Plant Hardiness Zone Map?

A Plant Hardiness Zone Map (PHZM, the USDA acronym) provides information about average annual minimal temperatures in specific areas. This means that hardiness zone maps focus on how cold it gets in different areas and averages those data points into buckets. Those buckets provide the different hardiness zones.

13 separate zones make up the USDA Plant Hardiness Zone Map. These categories further break down into half-zones, ‘a’ and ‘b’. The half-zones allow for delineations within zones that exist due to local land or water features (lakes, elevations, etc.) that can affect temperatures within a designated zone. The legends of all the USDA Plant Hardiness Zone Maps explain these zones in further detail.

To give you an idea, the zones go from Zone 1a (-60 to -55 degrees Fahrenheit / -51.1 to -48.3 degrees Celsius) to Zone 13b (65 to 70 degrees Fahrenheit / 18.3 to 21.1 degrees Celsius). This means that a gardener in zone 13b, will only occasionally see temperatures colder than 65 degrees Fahrenheit / 18.3 degrees Celsius.

When did the USDA Last Update the Plant Hardiness Zones Map?

The USDA last updated the Plant Hardiness Map in January of 2012. So, it’s not unusual to have an updated Hardiness Map. But it has been 11 years since the last update. The USDA’s Agricultural Research Service unveiled the new map on November 15^th, 2023 in collaboration with Oregon State University’s PRISM Climate Group.

[See Oregon State University’s announcement: New plant hardiness map, used by gardeners nationwide and based on OSU climate data, unveiled]

What is Different Between the 2012 and 2023 Hardiness Maps?

According to the USDA itself, the updated hardiness map, “When compared to the 2012 map, the 2023 version reveals that about half of the country shifted to the next warmer half zone, and the other half of the country remained in the same half zone” (Agricultural Research Service, U.S. Department of Agriculture, Nov. 15, 2023).

You can see the impact of this change for yourself in the image comparison below:

The 2023 Plant Hardiness Zone Map is comprised of data over a 30 year period: from 1991 to 2020. Whereas the 2012 map used the 30 year period of data from 1976 to 2005. The two maps aren’t exactly parallel however. Improvements in technology and additional data available from 2005 to 2020 means that some of the hardiness zones shifted between the two maps unrelated to temperature shifts.

Even so, in its map creation explanation page, the USDA states, “The new PHZM is generally about one quarter-zone warmer than reported in the 2012 PHZM throughout much of the United States….” (Information About How the Map Was Created, USDA.)

When Has the Hardiness Zone Changed Before?

Prior to 2012, the USDA published a Plant Hardiness Zone Map in 1990. Given the technology and imaging capabilities of the era, the map looks much different to the way it looks now. The Geographic Imaging Systems (GIS) technology that the USDA uses today for their 2012 and 2023 maps didn’t exist in 1990.

the 1990 map also lacked plant hardiness zones 12 and 13. These hotter zones were added to the 2012 and 2013 map to give more of a delineation to the tropical regions.

This 1990 hardiness zone map utilized data from weather data from 1974–1986. Which means the 1990 map only used 12 years of data to extrapolate results. However, the 2012 and 2023 hardiness zone maps doubled that data set. The later maps use a longer timeframe of 30 years of data to draw conclusions about weather pattern changes. As any data analyst (like myself) will tell you, the more data available, the greater the interpretation of results.

Why is it Important to Know Your Hardiness Zone?

Hardiness zone maps, like the ones produced by the USDA and other organizations worldwide, are important to gardeners in a lot of ways. Since not all plants can survive in all conditions, it’s important to understand the average temperatures for your area. That way, you’ll have a good basis for which plants produce well in your particular growing zone.

The large shift in growing zone for about half the United States means that many gardeners should plan to adjust their expectations for their gardening in 2024 and beyond. Plants that didn’t grow well in particular zone due to the cold, may now be able to flourish in areas where they couldn’t before. Likewise, plants that did exceedingly well in certain cold areas may now be at risk of scalding in the hotter months of the year. Because of the rising temperatures, gardeners may wish to plant cold weather plants like kale and broccoli later in the year for their autumn crop. Temperature sensitive plants, like tomatoes, may need to be watered more often, or provided with a sunscreen to prevent scalding in the hotter temperatures.

How Do You Find Your Hardiness Zone?

Gardeners can find their USDA Plant Hardiness Zone easily through a simple Google search. But for those more curious about the areas surrounding them, the USDA offers an interactive map at https://planthardiness.ars.usda.gov/. There, Gardeners can insert their zip code into the search field and the interactive map will zoom into that growing zone.

Other Growing Zone Classifications

The USDA Plant Hardiness Zone Map is not the only classification of growing zones. As stated above, the USDA calculates hardiness zones based on the average cold temperatures. Other growing zones look at the data differently.

Climate Zones

Most of us learned about Climate Zones in elementary school when studying topics like the rainforest. Unlike hardiness zones, climate zones are based off of degrees of latitude from the equator rather than by temperatures of any particular region. Generally speaking, these categories appear as:

• Tropical Zone (0°–23.5° Latitude)
• Subtropical Zone (23.5°–40° Latitude)
• Temperate Zone (40°–60° Latitude)
• Cold Zone (60°–90° Latitude)

Don’t confuse Climate Zones with the more scientific Hardiness Zones or the below Heat Zones. Those measurements utilize both local landscape variations and historical temperature data models.

Heat Zones

Head Zones could be considered the exact opposite of Hardiness Zones. Rather than looking at the average cold temperatures, Heat Zones look at an area’s average hot temperatures. Heat Zones are relatively new to the landscape of climate analytics. The Heat Zone Map was created in 1997 by the American Horticultural Society President Dr. H. Marc Cathey.

Similar to the USDA Plant Hardiness Zone Map, the AHS Plant Heat Zone Map is made up of 12 different zones. Unlike looking at average temperature of an area, the heat zone map divides its zones by the number of “heat days” experienced. A “heat day” is a day that the temperature climbs to over 86 degrees Fahrenheit. At 86 degrees F, it’s common for plants to stop producing or even growing until the weather cools (This is one reason some tomatoes stay green in the heat of summer).

On the AHS Plant Heat Zone Map, A Heat Zone of 1 has no heat days while a Heat Zone of 12 can have as many as 210 or more heat days. Gardeners looking at their area’s Heat Zone get an idea of when they need to protect their more vulnerable plants.

Be Prepared

Now armed with the latest and greatest data on the changing world, you’re ready to plan your gardens for the future. Nothing in nature is static, so it’s good that we gardeners have the latest information at our fingertips to best plan for our gardens to thrive.