Determining Plant Hardiness in Your Garden: The New USDA Map & Other Resources

More Data, More Specifics

The new map uses not only recent temperature data, but data from more weather stations, more sophisticated mapping technology, and a more detailed scale than was available or utilized to create the 2012 Map, and all of this allows for more accurate zone classifications. According to the USDA.gov website, “The 2023 map incorporates data from 13,412 weather stations compared to the 7,983 that were used for the 2012 map.” Additionally, there are more resources—such as a “Tips for Growers” section. When you enter your zip code, the information provided includes your current zone, your previous zone, and the number by which your average low temperature has changed. The number is accompanied by a ‘+’ or ‘-,’ or the results will indicate ‘No change.’ This good information helps you know just how much variation has taken place in your hardiness zone, whether you are on the cusp of your former zone, and/or how much warming has taken place even if it did not shift you into a different zone. Another new feature with the updated map is the ability to zoom in all the way to street level. This is not only lots of fun, it illustrates beautifully the existence of variations in hardiness zones that may occur even in the same neighborhood. You may be classified as one zone and just down the street your neighbor may be classified in another. You can explore the new map at: https://planthardiness.ars.usda.gov/.

On the left (or on the top if you are on a mobile device) is a view of the United States separated into hardiness zones using the 2012 Map (1976-2005 data), and, right (or bottom if you are on a mobile device), you can see how zones in many parts of the country have shifted on the 2023 Map (1991-2020 data). Click to enlarge.

It is important to note that this lowest, or coldest, temperature reading is not the be all and end all in predicting plant survivability; rather it is meant to be a guide. Rounsaville characterizes the map as a “risk management tool,” it’s merely one data point, one factor to be considered, for whether a plant will survive winter’s coldest days. This average number is just that, an average. It may tell us about the average coldest temperature over the past 30 years, but it is not the actual coldest recorded temperature during the season, nor any indication of what that might be. Most of us in colder climates are familiar with seasonal variations with some winters that are “mild” and others that may be particularly frosty. In colder years—deep freeze years—the lowest temperature could be much lower than the USDA average, and in those circumstances, even hardy plants may struggle to survive, especially if temperatures remain low for an extended period.

Another point to consider is that being freshly reclassified into a warmer zone may reflect an average temperature increase of only 1°F. And for some of you, your average yearly coldest temperature may have increased by 3 or 4 degrees, but you are still within the same range of your half-zone, so you did not see a reclassification. Here at the farm in Morris, CT (the northwest part of the state), we were formerly classified in Zone 5b and are now classified a half-zone warmer, in Zone 6a. This is the result of  our average lowest temperature rising by 4°F. How will that impact which plants we grow and trial in our display gardens? With the USDA’s confirmation that we are experiencing slightly warmer winters, we look forward to trialing an array of plants that formerly were not hardy in our zone.

Note our locations—The Nursery & Farm, near Litchfield, and our shipping facility in Torrington. All are squarely in Zone 6a on the USDA’s revised hardiness map, but there can be significant variability in conditions even at the same site, and this impacts which plants we choose for particular locations.

---

These weather and site conditions contribute to the creation of what are called “microclimates” or “micro-zones,” something the USDA Hardiness Map could not possibly take into consideration. You as a gardener will learn to recognize them and take them into account as you choose plants for borders and beds. It is quite possible that one area of your yard, say a spot that is south-facing, is a different microclimate than a north-facing backyard. A plant may safely overwinter in the front, but that same plant may not survive in the colder back-of-the-house location.

Resources for Gardeners in the South & West

Although we utilize the PHZM information to assist in predicting winter survivability of plants, states in the South and West have issues that impact summer growing conditions, such as high humidity or dry air. Plants that may be happy in the Northeast all summer may not do well in more extreme heat conditions if they are not provided with shade during the sunniest, hottest parts of the afternoon in the South or West. In some southwest desert regions, dry air causes extreme temperature shifts with cold temperatures during the night and morning followed by hot temperatures for the rest of the day. If you look at the USDA Hardiness Zone Map, you will see that our nursery has the same hardiness zone designation as many areas of the Southwest, however, the two climates could not be more different. To help guide gardeners in Pacific coast and some Western states, the Sunset Climate Zones map utilizes both the average lowest winter temperatures that inform the USDA Map and also considers the climate factors noted above. This non-USDA map is an additional resource that can help guide gardeners in these particular areas of the country.

Also, some plants need a period of cold in the winter, something known as a chilling period. Blueberries are a perfect example, thriving in Connecticut and other parts of the Northeast, but not all varieties can be planted in regions beyond Zone 7 in the South or Zone 8 in the West because they wouldn’t get the period of sustained cold they require, nor would they survive the summer heat. It is wise to be mindful of these South and West hardiness numbers when gardening in these areas of the country. That said, the detailed nature of the updated Hardiness Zone Map will reduce the need for these designations and enable us to phase out this system in the future.

Identifying Your Zone & Half-Zone

In case you haven’t noticed, we provide you with your zone information on the back cover of the catalog you receive in the mail. This has historically been shown as a whole number not specifying a/b half-zones. We are happy to announce that starting with our March 2024 catalog, you will see your specific ‘a’ or ‘b’ half-zone noted. We hope this makes it easier for you to choose plants from the catalog. Additionally, on our website, there is a link near the top of the home page (see below) to help you identify your plant hardiness zone. This is based upon your zip code and the new PHZM updates. However, for the time being, it will still be provided to you as a whole number.

Click on the FIND HARDINESS ZONE link on our webpage to open the Zip code search box:

On the product pages on our website and in our catalogs, our plant hardiness zones will also remain as whole numbers for the foreseeable future (or at least until our database makes it feasible to incorporate half-zones). The good news is half-zones typically do not have a significant impact on plant selection. Climate variables are far more important in determining whether a plant will survive in your microclimate. For example, if you are in Zone 6a and are looking at a plant that is hardy to temperatures in Zone 6b, which is slightly warmer, we may only indicate that the plant is hardy to Zone 6. This is where observation of your own yard is helpful.  A spot in your garden that is warmer in winter than other areas might be the right place to “push the zone,” and try a plant that might not survive in elsewhere in your garden.

You & Your Garden