Learning how to read the USDA Plant Hardiness Zone Map is a very important skill to develop when it comes to gardening. The purpose of this map was to help gardeners to select plant material that was winter hardy in their area.
Prior to the first creation of this map in 1927, this gardening knowledge was haphazard and passed on from gardener to gardener. But Harvard’s Arnold Arboretum would change all that. It was again updated in 1938. This beginning map only contained 8 zones and consisted of survival rates of trees and shrubs. The 1938 version took 40 years of data from the National Weather Service and utilized this data to create the map. Both versions of the USDA Plant Hardiness Zone map were published in manuals more geared toward professionals.
In 1960, the USDA Plant Hardiness Zone Map was published. The information gathered allowed a standard to be set, which was 10-degree Fahrenheit. This 10 degrees Fahrenheit separated each zone. Eventually, this replaced the Arnold Arboretum map.
Then, in 1990, the map was revamped again. This data came from thousands of weather stations across the United States. Since the data was more detailed, the map could include subdivisions in each zone or the “a” and “b” one sees for each zone.
In 2012, the map got an upgrade with help from the Prism Climate Group at Oregon State. This group utilized advanced mapping techniques to incorporate new data.
Again, the map was updated in 2023. This map is based on data from 1991 to 2020 and incorporates information from more weather stations and PRISM’s group advanced mapping technology algorithms. This is the current USDA Plant Hardiness Zone that is used presently.
Now you may be wondering at this point as to why the history lesson. This information is so easily available online and in plant/seed catalogues. The point of the USDA Plant Hardiness Zone Map history is to understand how it was created and why. Prior to 1927, this gardening information was hit-or-miss at best and was shared from one gardener to another through experience. If you wanted to try something unorthodox, well, you simply gave it a shot and hoped for the best. But the creation of the first USDA Plant Hardiness Zone Map changed that by using data collected over many years and plant knowledge to inform gardeners about their local area and plant survival.
Today, USDA Plant Hardiness Zone Map can be found online, on seed packets, plant labels and in your favorite seed catalogue. The “zones” simply tell you how low the temperature can get in your area for your selected plants to survive.
If you decide to use the online version, you can take the guesswork out of trying to read the map by just typing in your zip code and like magic your USDA Plant Hardiness Zone will appear. But this is not the end of the story. Yes, this map will give you general information about your area, but nothing replaces really understanding your garden space, regardless of what or where it may be. Each garden space has microclimates built into it. These microclimates can be caused by the lay of the land, surrounding buildings and even hardscape that is around your garden. These elements will affect your garden in ways that the USDA Plant Hardiness Zone map cannot measure.
So, what is a gardener to do? Step one is to gather your information from the USDA Plant Hardiness Zone Map. Next, get out into your garden and learn more about your gardening space. This information is invaluable when it comes to maximizing what you can grow and keeping your garden area thriving. Finally, plan, not just plan in your head but map out your garden space. Note the shady areas, the areas that are warmer and areas where dew settles in the valleys of your land. All this information combined will give you a strong background by which you can build upon as your gardening journey continues, and you reap the rewards of successful gardening.
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