A Warm Welcome to the New USDA Hardiness Zone Map

Jeb Fields, Abdi, Damon

The Louisiana Agriculture magazine logo stands against a white background.

Jeb Fields and Damon Abdi

At the close of 2023, the U.S. Department of Agriculture released an updated Plant Hardiness Zone Map — the first time it has been updated since 2012. This map is based on 30-year averages of the lowest annual winter temperatures in specific locations with data collected from 13,412 weather stations — nearly double as many as the 7,983 used for the last version. The addition of these weather stations provides more robust data, including more information from stations located in remote areas. This assists in providing greater data resolution (i.e., increased data density) across many areas. For example, Alaska received a major increase in resolution as the updated map has a 0.25 square mile resolution compared to the previous 6.25 square miles — a nearly 25-time increase.

As anyone reading this likely already knows, hardiness zones are talked about quite often in horticulture. Hardiness maps are used to determine the suitability of various crops to survive in different regions. The USDA hardiness map indicates the average lowest temperature for an area, as opposed to the Heat Zone Map put out by the American Horticultural Society, which represents the average number of days above 86 degrees Fahrenheit annually. While the zone numbers are indeed very similar, many students mistake the meaning of the two maps. No industry relies on hardiness zone for decision-making more than ornamental horticulture, with literally hundreds to thousands of different taxa in our trade that span a spectrum of different hardiness requirements.

There are 13 hardiness zones (zones 1 through 13, with higher numbers indicating warmer temperatures) represented in the U.S., with each zone split into two sub-zones (i.e., A or B). A hardiness zone represents 10 F, while a half zone represents 5 F. Many of our Louisiana residents may not notice a difference in this new map; however, the gradual shifts are reflected in the half-step increases for north Louisiana, which moves up to 8b, and the southernmost portions of the state, which are moving up to 10a way down by Port Sulphur. Similar shifts were observed for many areas throughout the state, including in Hammond, which increased from 8B to 9A.

This trend is not localized to Louisiana. Approximately half of the U.S. remained in the same hardiness zone while the other half saw an adjustment, typically moving up a half zone warmer. The map confirmed what many have physically felt with each successive summer, as heat records have been set in several cities throughout Louisiana and the country at large. Armed with the knowledge that our communities are indeed getting warmer, the new hardiness zone map can be used to make more informed decisions in our plant selection.

The importance of understanding what the lowest annual temperature is in a particular area is essential to selecting our perennial plant material. Perennial plants go dormant over winter, where cold temperatures are necessary to spur the flower blooms that are cherished in our landscapes. However, if temperatures get too cold, those plants may not reawaken for the next season (a gentler way to describe their mortality in the landscape). Selecting plants that can reliably be revitalized following winter’s chill is necessary to the long-term success of many of our herbaceous perennials, shrubs and trees. On the other end of the spectrum, understanding that our average temperatures are becoming warmer can be a guide to selecting plant material that can handle temperature extremes during summertime.

What does this mean for our green industry stakeholders? Landscape contractors and nursery growers can work together to devise a palette of plants that can beat the heat and shoulder the colder temperatures. Many plants that have reliably grown may be less hardy in the not-too-distant future, as the increasing temperatures may prove too much to take for these selections; conversely, plant taxa from climates warmer than Louisiana (sometimes this is hard to imagine) may become competitive in the market as the increase in average warmer temperatures may signal suitability for these plants here in Louisiana.

While all feel the heat, feeling the pressure of a warming climate can be avoided by modifying practices. Through selecting plants that are site-appropriate for the current and future climate, this can be used as an opportunity to utilize new plants in the landscapes and shift away from plants that just can’t take the heat anymore. As weather patterns will continue to become more erratic, a range of resources, such as the USDA Hardiness Zone Map, can be leveraged to keep Louisiana’s green industry blooming.

Maps and map resources are available for download at the USDA Plant Hardiness website: https://planthardiness.ars.usda.gov/pages/map-downloads.

Jeb Fields and Damon Abdi are assistant professors at the LSU AgCenter Hammond Research Station. They support the nursery and landscape industry across the state of Louisiana.

A panoramic shot captures a lush garden.

Hammond, home of the LSU AgCenter Hammond Research Station, increased from 8B to 9A on the U.S. Department of Agriculture Plant Hardiness Zone Map, indicating a warmer lowest average temperature than the previous map, released in 2012. Photo by Ashley Hickman

A map shows the lowest average temperature of several regions of Louisiana over a decade.

At the end of 2023, the U.S. Department of Agriculture released an updated Plant Hardiness Zone Map based on 30-year averages of the lowest annual winter temperatures collected from 13,412 weather stations, an increase from the 2012 map. Many areas of Louisiana saw a change in the map, which charts the lowest average temperature of an area. Map courtesy of the USDA

A map shows the lowest average temperature of several regions of Louisiana over a decade.

Above: The U.S. Department of Agriculture map from 2012. At the end of 2023, the U.S. Department of Agriculture released an updated Plant Hardiness Zone Map based on 30-year averages of the lowest annual winter temperatures collected from 13,412 weather stations, an increase from the 2012 map. Map courtesy of the USDA

5/27/2024 9:09:58 PM
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