What are the horticultural implications of the USDA's updated Plant Hardiness Zone Map for U.S. growers?

According to new data recently unveiled by the U.S. Dept. of Agriculture (USDA) Plant Hardiness Zones are slowly shifting across the United States. This is big news for gardeners as they spend the winter months preparing for spring planting — but, researchers say, is not a huge cause for alarm.

"If your hardiness zone has changed in this edition of the USDA Plant Hardiness Zone Map (PHZM), it does not mean you should start removing plants from your garden or change what you are growing. What has thrived in your yard will most likely continue to thrive."

— USDA Press Release

“Horticulturists have long recognized that the accurate prediction of winter injury is a key component of the effective cultivation of long-lived woody and herbaceous perennial plants in many climates,” explains Daly (2023). “Winter injury can limit long-term plant survival and vigor and can reduce production of valuable horticultural products, including flowers, foliage, fruit, and seeds.”

As the USDA continues to monitor these changes over time, they are also altering its horticultural recommendations for gardeners in different regions to adapt to changing weather trends.

And, while this data predominately caters to gardeners and hobbyists, it also holds a deep significance in the world of agriculture — since “the USDA’s Risk Management Agency refers to the map’s plant hardiness zone designations to set certain crop insurance standards.”

Here are some of the highlights of the new study:

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What are Plant Hardiness Zones?

According to the USDA, the Plant Hardiness Zones Map was created using a large swath of data collected over the last 30-year period from hundreds of weather stations across the country (mostly operated by the National Weather Service) in order to extrapolate the average extreme minimum and maximum temperatures of each micro-climate in that time period (not the lowest temperature ever recorded).

Once the new map was devised from the data set, it was reviewed and approved by a panel of horticulturists, botanists, and climatologists before its release.

Measuring Changes in “Weather” — Not “Climate”

Researchers did track a slight rise in the national average low temperature, which increased by 2.3 degrees in the 30-year span of the data set. However, they argue — due to the size and span of the data collected — it is “not reliable evidence of whether there has been global warming.”

Rather, this data set, which encompasses a span of 30 years (instead of a 50- to 100-year span), is meant to predict changes in “weather” within each micro-climate rather than changes in “climate,” itself.

So what can the zone changes and addition of hotter, sub-tropical zones be attributed to? According to the scientists, their more sophisticated interpolation technique, greater physiographic detail, and more comprehensive station data were the main causes of zonal changes in complex terrain, especially in the western United States.

“If your hardiness zone has changed in this edition of the USDA Plant Hardiness Zone Map (PHZM), it does not mean you should start removing plants from your garden or change what you are growing,” the USDA said in a press release. “What has thrived in your yard will most likely continue to thrive.”

New Zones in Hawai’i & Puerto Rico

There are a few new designations on the map, Zones 12 and 13 (located in Hawai’i and Puerto Rico), which represent frost-free, sub-tropical areas that see average annual extreme minimum temperatures above 50 degrees and 60 degrees F, respectively.

“The two new zones will provide a way to share information about differences in cold sensitivity of tropical ornamental plants and could help gardeners decide when to bring tropical plants indoors from a deck or patio as the temperature cools,” said the USDA.

Distinguishing Elevation & Urban Proximity in Alaska

Also, compared with previous iterations of this map, the new version is specifically optimized for online usage and includes many algorithmic changes that increase the detail and accuracy to account for things like elevation changes within a specific zone as well as heat changes based on proximity to larger, urban areas (where temperatures trend warmer).

One of the areas of the map that saw the biggest changes and increased accuracy was the mountainous areas of Alaska where higher elevations have a very large impact on the growing season.

Farming — Image of male farmer digging in the garden

The Many Applications (and Implications) of the 2023 Plant Hardiness Zones Map

Since the PHZM is accessible via an interactive website, it facilitates a wide range of horticultural applications, especially when attempting to grow a plant that is not native to your region.

“Hardiness zones in this map are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future,” the USDA said in a press release. “Gardeners should keep that in mind when selecting plants, especially if they choose to “push” their hardiness zone by growing plants not rated for their zone,” said the USDA. “In addition, although this edition of the USDA PHZM is drawn in the most detailed scale (1/2 mile square) to date, there could still be micro-climates that are too small to show up on the map.”

In addition to Plant Hardiness Zones, the USDA also recommends a number of factors to consider when deciding what plants to grow in your area:

Many other environmental factors, in addition to hardiness zones, contribute to the success or failure of plants. Wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can greatly affect the survival of plants. The way plants are placed in the landscape, how they are planted, and their size and health might also influence their survival.

Light: To thrive, plants need to be planted where they will receive the proper amount of light. For example, plants that require partial shade that are at the limits of hardiness in your area might be injured by too much sun during the winter because it might cause rapid changes in the plant’s internal temperature.

Soil moisture: Plants have different requirements for soil moisture, and this might vary seasonally. Plants that might otherwise be hardy in your zone might be injured if soil moisture is too dry in late autumn and they enter dormancy while suffering moisture stress.

Temperature: Plants grow best within a range of optimal temperatures, both cold and hot. That range may be wide for some varieties and species but narrow for others.

Duration of exposure to cold: Many plants that can survive a short period of exposure to cold may not tolerate longer periods of cold weather.

Humidity: High relative humidity limits cold damage by reducing moisture loss from leaves, branches, and buds. Cold injury can be more severe if the humidity is low, especially for evergreens.

U.S. Dept. of Agriculture

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Researchers also highlighted a wide range of uses for the 2023 Plant Hardiness Zones Map in agribusiness:

For use by governmental agencies for risk management and development of recommended plant lists;
For horticultural firms to schedule plant shipments;
For commercial interests that market products seasonally;
For vineyard site evaluations in the Pacific Northwest;
or, for use by botanists and horticulturalists as a data layer in conjunction with moisture-balance data to predict the survival of Yugoslavian woody plants in South Dakota.