What The New USDA Plant Zone Hardiness Map Means For Your Garden
Are your winters getting warmer? The new USDA Plant Hardiness Zone Map suggests they might be, but some growers aren’t so sure.
The United States Department of Agriculture updated its Plant Hardiness Zone Map for the first time in a decade, revealing significant shifts that some scientists are connecting with global warming. Other growers report that their winters aren’t getting warmer, but are actually more unpredictable than ever before—which could still be a symptom of climate change.
The USDA discourages anyone from concluding that these changes are indicative of climate change, but media outlets like the Associated Press and the New York Times are making the jump, publishing articles titled, “New hardiness zone map will help US gardeners keep pace with climate change,” and “What Plants Will Survive in Your Garden? This Map Plans for a Warmer U.S.,” respectively.
Keep reading to understand what caused these changes and what this new information might mean for your garden.
What is the USDA Plant Hardiness Zone Map?
What are hardiness zones?
Hardiness zones—or growing zones—are geographic regions categorized by average lowest winter temperatures. Since 1960, the USDA has been compiling weather data into the Plant Hardiness Zone Map (PHZM) and using that information to divide North America into zones based on one data point: the average lowest winter temperature for a given area.
The coldest region in North America described by the 2023 USDA PHZM zones is frigid Zone 1a in Interior Alaska, which has an average minimum temp of -60 degrees Fahrenheit. The warmest region is Zone 13b in balmy Puerto Rico, where winter temps seldom dip below 65 degrees Fahrenheit. The contiguous United States spans from Zone 3a in the North to Zone 11a in the southernmost regions.
Why are hardiness zones important?
The USDA produces and updates the PHZM regularly to ensure that gardeners, farmers, and researchers have a tool that describes which plants will grow in different areas of the country. A recent press release introducing the new USDA PHZM explains that “USDA’s Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location.”
Growers and plant enthusiasts reference hardiness zones when buying plants and seeds to gauge the suitability of perennial plants for a specific climate. These zones are also connected to average frost dates, which affects the timing for planting and transplanting annuals.
Why was the USDA map updated?
The first USDA PHZM map, published in 1960, featured ten different zones, with a ten-degree difference between each zone. The 1990 version of the map included a warmer Zone 11 and further divided Zones 2–10 into mini-zones with a difference of five degrees, marked a and b. The PHZM was eventually expanded in 2012 to include Zones 12 and 13 to account for areas with even warmer average winter temperatures.
Temperature is a significant factor influencing plant growth, and it is always continually evolving. To ensure accuracy, the team behind the PHZM collected 30 years of weather data, calculating the average to map the 13 zones. Recognizing the ongoing nature of this process, the USDA acknowledges the need for continuous data collection and future revisions for improved precision.
What does the 2023 USDA PHZM leave out?
Sunlight, water, soil type, and nutrients are equally important to plant health, but these factors are harder to measure since they may vary drastically within the same region. While it’s worth understanding hardiness zones, they are not the single determining factor to the success of any crop.
Hardiness zones may work well for gardeners in the East and Midwest, but growers in the West realized that they needed another system better suited to the unique weather patterns reflective of mountain ranges and large bodies of water. So, the University of California and Sunset Magazine produced a new zoning system they named “Climate Zones.”
What’s new about the 2023 USDA PHZM map
The map maintains the interactive Geographic Information System (GIS) feature introduced in 2012, allowing users to search for specific areas by zip code. Like the 2012 map, the updated map is available online for free, aiming to be accessible to growers and researchers alike.
There are a couple of notable differences and improvements that come with the 2023 USDA PHZM:
Access to more data
Compared to the 2012 version, the 2023 PHZM incorporates advancements in data collection, utilizing nearly twice as many weather stations—13,412 in total.
More attention to detail
Increased data allows for greater accuracy in hardiness zones than ever before. Researchers and mapmakers were able to distinguish microclimates that were previously lost to the hardiness zone of the surrounding area.
Because of buildings, pavement, and industry, cities tend to be slightly warmer than rural areas. Similarly, communities near unfrozen bodies of water stay warmer than more inland areas, and valleys are warmer than mountaintops. The new USDA PHZM shows these small differences at a scale not possible before.
Noteworthy improvements also include increased detail to Alaska—as much as 96% in some remote areas.
Resources for Growers
One exciting new addition to the 2023 map is a “Tips for Growers” library featuring several gardening-related YouTube videos produced by the USDA. Video topics include using the maps effectively, weed management, soil health, and general gardening tips and offer valuable insights for both hobbyist gardeners and plant breeders.
Who is affected by the 2023 USDA PHZM?
The wealth of data and technological improvements in the 2023 map have redefined hardiness zones, but feedback about the new map is as varied as the new hardiness zones themselves.
Some gardeners anticipate opportunities to grow a wider variety of plants, but others remain skeptical. The new map has ignited discussions in gardening forums, with some questioning its reliability. Many growers express concerns about fluctuating weather patterns and suggest that cold snaps can still jeopardize plant health.
The impact, however, extends beyond home gardeners; farmers, plant nurseries, seed companies, landscaping businesses, and even scientists are affected by this new information. The USDSA specifically mentions that researchers often incorporate the PHZM into their models, such as those predicting the spread of exotic weeds and insects.
Is the 2023 USDA map evidence of climate change?
The ARS refrains from directly linking the PHZM updates to climate change, citing the variable nature of extreme minimum temperatures and the sophisticated mapping methods employed. While the USDA acknowledges the warming trend across the 1990, 2012, and 2023 maps, they caution against immediate associations with global climate change.
Whether or not your hardiness zone shifted, it might be helpful to think of the new PHZM as a suggestion, but not the rule book. Take the new normal with a grain of salt, and while we don’t want you to be deterred from trying something new, don’t put all your eggs in one basket, if you catch my meaning. While the map offers guidance, don’t forget about the other factors that influence plant growth. After all, your own experience in your garden is the best indicator of what you can grow in your area.