Most of us know not to plant watermelons in the mountains or aspen trees at the beach. But when it comes to the countless plants available 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 preferred dry conditions or a tropical environment. Or if the flower you saw in Boston could 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 standard by which people could determine which plants were most likely to thrive where.
Boston skyline as viewed from the Arnold Arboretum
Originally composed of eight ‘climatic’ zones, the Arnold map was based on a survey of plants and their ability to survive in different geographic parts of the country. But, the sectors were ill-defined and the map eventually fell out of favor.
The 1927 Arnold Map/Courtesy Arnold Arboretum Archives
Then in 1960, the United States Department of Agriculture (USDA) published its own standard; the USDA Plant Hardiness Zone Map. Instead of defining regions by physical characteristics, it divided them by average annual minimum temperatures.
This map established a uniform system of zones based on 10° Fahrenheit ranges.
The 1960 USDA Plant Hardiness Zone Map
In 1990, the USDA updated the map to reflect a growing body of meteorological data. While retaining the 10° Fahrenheit system, it further broke things down into 5° F zones labeled (a) and (b).
The 1990 USDA Plant Hardiness Zone Map
THE ZONES SHIFTED OVER TIME
In 2012, the USDA published yet another map. This new interactive map comprised data collected from weather stations over a 30-year period. It shifted hardiness zone boundaries one 5° F zone warmer than the previous maps. And it added two more tropical/subtropical zones.
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. As an example, the state of Maryland where I live is located in zones 6 and 7. But my town is located in zone 7a.
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. 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. And the highest zones 12 and 13 (added in 2012), comprise Hawaii and Puerto Rico.
THINGS ARE HEATING UP
One drawback of the current map is that it doesn’t include anything about average maximum temperatures, which gardeners agree are beginning to impact plants. In fact, a growing body of evidence suggests 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 depicting average annual minimum temperatures, it divides regions by average maximum temperatures. This new map is slowly being adopted by nurseries to code plants’ heat and cold tolerance ranges to better reflect the climate of each area.