Guide to the New USDA Plant Hardiness Zone Map

If you live in Fargo, North Dakota, you probably know better than to plant a rosemary bush in your yard because of the region’s cold winter temperatures. But for those living in Albuquerque, New Mexico’s high desert landscape, it’s fair game. But how do you know for sure what is safe to plant in your area? Enter the U.S. Department of Agriculture’s (USDA) Plant Hardiness Zone Map, which helps gardeners and growers select the best plants to grow in their own region.

In November 2023, the map’s planting zones were updated for the first time since 2012. Read on to learn more about the USDA plant hardiness zones and what the recent changes mean for your garden.

See more: 10 Flowers That Bloom in Winter

What Is the USDA Plant Hardiness Zone Map?

The USDA Plant Hardiness Zone Map is a nifty tool that gardeners and growers can use to select the best perennial plants that will thrive outside year-round in their region. It’s important to consult this map so that your garden will survive the coldest temperatures in your area.

History of the USDA Plant Hardiness Zone Map

While the USDA came out with its first planting zones map in 1960, mapping growing zones in the United States goes back a few more decades. Harvard University’s Arnold Arboretum compiled the first U.S. planting zones map in 1927 led by Alfred Rehder, a German-American botanical taxonomist and dendrologist. This first map included eight growing zones. In 1938, another Arnold Arboretum scientist published a new map, which used 40 years of data starting in 1895. The Arnold Arboretum map was updated in 1951, 1967 and 1971.

When the USDA published its first map in 1960 — based on data from 450 weather stations —it used different criteria than Arnold Arboretum, which resulted in conflicting maps. It wasn’t until 1990 that the USDA map became the country’s standard planting zones map.

See more: 11 Things You Can Grow in Your Winter Garden

How Does the USDA Determine Hardiness Zones? 

The map uses average annual extreme winter temperatures to determine planting zones and half-zones. Data scientists from the USDA’s Agricultural Research Service (ARS) and Oregon State University’s (OSU) PRISM Climate Group collaborated to create the new map, which utilized data from 13,412 weather stations across the country. The team used this information to determine the coldest night of the year each year over the last three decades. Then, they plugged this information into the map.

USDA growing zones are measured in 10-degree increments, while half-zones come in 5-degree increments (with the exception of the coldest half-zone, zone 1a, which covers a 15-degree range). For instance, half-zone 1a includes some areas of more northern states like South Dakota, Iowa, Wyoming and Idaho. Here, the average annual extreme winter temperatures fall between a blistering -60 to -55 degrees Fahrenheit. On the other end of the spectrum, half-zone 13b’s average extreme winter temperatures are much balmier at 65 to 70 degrees Fahrenheit. This half-zone covers parts of the Commonwealth of Puerto Rico and the state of Hawaii.

Although it may seem that more northern locations should lie in the coldest USDA zones, that is not necessarily true. Some parts of Nebraska are chillier than some parts of North Dakota, for instance. And some states have a lot of variation in zones depending on their unique geography and weather.

How to Use the USDA Plant Hardiness Zone Map

To use the map, simply type your ZIP code into the search bar. A line of text indicating your zone will appear. Once you know your zone, you need to determine the hardiness of the plant you’d like to grow. For instance, a simple web search reveals that rosemary is generally hardy in zones 8 and 9. That means people living in places like central Texas and much of the South are safe putting it in the ground. However, people living in colder zones should not plant this shrub outside — at best, the bush would endure winter damage and poor growth, while at worst, it would die.

You can also view and download state, regional and national growing zones maps on the Map Downloads page. Here, you can also print poster-size national maps, maps without some of the extras like A/B zone divisions, and maps from past years.

What About Annual Plants?

It is important to refer to the USDA zones map when planting perennials, which are meant to endure winter. For annual plants, you can depend on other information like the length of your growing season and the typical dates of your first and last frost.

What Has Changed in 2023? 

Overall, things are looking warmer across the country. The 2023 map is about 2.5 degrees warmer than the 2012 map. This means that about half of the country has shifted into a new, warmer half-zone. For instance, one region in central Arkansas moved from zone 7b to zone 8a. For people living there and in other regions that have warmed, this could present an opportunity to grow new types of flowers, fruits, vegetables and other plants.

The zone shift could be connected to rising global temperatures, but the USDA said that it might have more to do with other factors. For example, compared to 2012, scientists are now using more sophisticated computers and information from nearly twice as many weather stations.

See more: What Is an Extension Office? And How It Can Help Your Garden

What Should I Do if My Planting Zone Changed?

Don’t start ripping your plants out of the ground or going hog wild with new plants just yet. According to the USDA, what has been thriving in your garden will likely continue to do well. The agency adds that hardiness zones are based on the average annual extreme minimum temperature over a 30-year period and not necessarily the lowest temperature that has occurred or might occur.

Plus, you may just live in a microclimate that is too small to show up on the map. Microclimates can be small heat islands caused by concrete or cool spots caused by small hills and valleys. Your individual garden may even have its own microclimate! That’s why it’s important to consult the USDA zones map but also to trust your own gardening experience.