In The Zone: The USDA Plant Hardiness Map Explained

The 1967 Arnold Map/Image courtesy of Arnold Arboretum, Harvard University.

Most of us know not to plant watermelons in the mountains or aspen trees at the beach. But, when it comes to the myriad plants available to gardeners and landscapers at the nursery, things can get murky. That’s when a handy tool called the USDA Plant Hardiness Zone Map can make all the difference. Not only can it tell you what plants will survive where, but it can also ensure a year’s worth of success in the garden.

HOW THE HARDINESS MAP CAME TO BE

Imagine not knowing if a plant is better suited to a dry or tropical environment. Or if the flower you saw in Boston can survive back in Washington, DC. In the early 1900s, people were concerned about these things, too. So in 1927, researchers at Harvard’s Arnold Arboretum decided to create a system of plant hardiness zones. 

Boston skyline as viewed from the Arnold Arboretum

Originally composed of 8 ‘climatic’ zones, the Arnold map was based on a survey of plants and their ability to survive in different regions of the country. The downside of this approach, however, was that the lines separating the regions were very approximate.

1927 Arnold Plant Hardiness Zone Map

The 1927 Arnold Map/Courtesy Arnold Arboretum Archives

Over the following decades, the map was updated to include two more zones until eventually falling out of favor.

Meanwhile in 1960, the United States Department of Agriculture (USDA) published what would later become the standard for gardeners and nurserymen throughout North America. Entitled the USDA Plant Hardiness Zone Map, it divided geographic areas by average annual minimum temperatures. And in contrast to the Arnold map, it established a uniform system of zones based on 10° Fahrenheit ranges.

1960 USDA Plant Hardiness Zone Map

The 1960 USDA Plant Hardiness Zone Map

In 1990, the USDA updated the map to include a growing body of weather data. The plan retained the 10° Fahrenheit system. And it further broke things down into 5° F zones labeled (a) and (b). 1990 USDA Plant Hardiness Zone Map

The 1990 USDA Plant Hardiness Zone Map

THE ZONES SHIFTED OVER TIME

In 2012, the USDA published yet another map. The new interactive map used data collected from weather stations during a 30-year period. In general, it shifted hardiness zone boundaries one 5° F zone warmer than the previous maps. It also added two more tropical/subtropical zones.

2012 USDA Plant Hardiness Zone Map

The 2012 USDA Plant Hardiness Zone Map

Developed specifically for the internet, the new map includes a ZIP Code zone finder as well as static maps for each state. This is the map we use today.

HOW THE PLANT HARDINESS MAP WORKS

Today’s digital map has a total of 13 zones, each representing a 10° F difference in annual minimum winter temperatures. Each zone is further subdivided into 5° F increments, with (a) representing the lower temperature range and (b) representing the higher.

To check out what zone you’re in, click on the map. For example, the state of Maryland where I live is located in zones 6 and 7. But my town is located in zone 7a. Most plants and seeds today come with a label listing their plant hardiness zone.

Maryland section of the USDA Plant Hardiness Zone Map

The Maryland page of the USDA Plant Hardiness Zone Map

In the United States, the warmer zones have the higher numbers and the colder ones have the lower. For instance, zones 9, 10 and 11 encompass the deep southern half of the country and the western coastal margins. The middle zones 6, 7, and 8 cover the middle portion of the mainland, and the colder zones 2-5 include the northern part of the central interior. The coldest zone of all, zone 1, occurs in the northern part of Alaska. 

(The highest zones 12 and 13, added in 2012, include Hawaii and Puerto Rico.)

THINGS ARE HEATING UP

Still, one problem with the map is that it doesn’t show anything about average maximum temperatures which gardeners agree are beginning to affect plants. A growing body of evidence says that plants that can survive the winter in their zone may not be able to handle the heat of the summer.

To address these concerns, the American Horticultural Society (AHS) has developed a Heat Zone Map. It’s similar to the Plant Hardiness Zone Map, but instead of average annual minimum temperatures, it divides regions by average maximum temperature. This new map is slowly being adopted by nurseries to code plants’ heat and cold tolerance range to better reflect the climate of each area. 

 

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