How can USDA zones help you? Whether you’re an experienced or a novice gardener, you’ve likely seen a flower, shrub or tree at some time that you’d love to grow in your own garden. While experimentation is a good way to gain experience in gardening, there are times when experimentation just won’t work. That’s why consulting the United States Department of Agriculture (USDA) Plant Hardiness Zone Map is an excellent first step to help you determine what kind of plants will work in your garden.
The USDA Zones for Plants
The USDA plant hardiness zones designate on the average minimum temperature at thousands of locations across the United States. The temperatures are calculated using data from the most recent 30-year climate period. Consequently, hardiness zones can change as climate warms, as it has recently, or cools.
The plant hardiness zones are divided into ten-degree Fahrenheit increments and range from Puerto Rico to Alaska. The temperatures range from low minimums of 65 to 70 degrees in Puerto Rico, designated as Zone 13, to low minimums of -50 to -60 in Alaska and northern Canada, labeled as Zone 1. Each zone is further divided into two five-degree increments. Plants that can tolerate a low minimum temperature of 0 to 5 degrees Fahrenheit can be expected to thrive in Zone 7a while plants that need minimum temperatures no lower than 5 to 10 degrees are recommended for Zone 7b.
How the USDA Zones Were Developed
Plant hardiness zones were first identified during the early decades of the 20th century, but they weren’t officially published by the USDA until 1960. At that time, an early version of the USDA Plant Hardiness Zone map with which gardeners today are familiar was developed by Henry Skinner, the director of the Arnold Arboretum at Harvard University in Cambridge, Massachusetts. The plant hardiness zone map was updated in 1965 to include temperature data that wasn’t part of the 1960 Plant Hardiness Zone map.
In 1990 the map of the USDA zones underwent an extensive revision thanks to work done by H. Mark Cathey, the director of the National Arboretum. The zone hardiness map was revised due to changes horticulturists and gardeners had noticed in the climate that had resulted in the loss of plants frequently used between 1940 and 1960.
The new 1990 zones map included Mexico and Canada. Alaska, Hawaii and Puerto Rico are included in the most recently updated plant hardiness zones map published in 2002 and 2012. The 2002 and 2012 maps indicate that plant hardiness zones appear to be moving north. The addition of two new climate zones was a feature of the 2012 USDA zones map.
Recent Changes to the USDA Zone Maps
The 2012 USDA zone maps now used by horticulturists and gardeners feature a computer-based geographic information system (GIS) that more easily captures and displays data related to the Earth’s surface. The zones map is also designed for use with the internet, which gives users the kind of detailed information in the map that they’ve not been able to access before.
Using 30-year climate data, rather than the 12-year data set that was part of the 1990 map revision, has offered the benefit of better incorporating yearly fluctuations of temperature in a specific location. The more accurate overall result will give you a more accurate picture of the kind of weather you can expect in your garden.
Things the USDA Zone Maps Don’t Tell Gardeners
While USDA plant hardiness zone maps are excellent tools for helping you as a home gardener determine whether the plants you want to use can tolerate the cold temperatures at your location, there are a number of things the map doesn’t report. For example, nowhere in the USDA zones map is any indication of the record maximum temperatures seen in specific locations over ten or thirty years. This information is just as important as the low-temperature data.
It’s important to know the actual average low temperatures reached your location, but there are nuances that the zones map can’t include. Knowing whether the average low temperatures occur only every few years, or can be expected yearly, is just as important. Also, there’s no way to indicate whether low temperatures can be expected for several days or even weeks at a time. All of these factors will play a role in how well a plant to thrive.
Moisture has just as much of an impact on plants as temperatures do. The USDA plant hardiness zone maps won’t tell you how often it rains, which season is wettest or driest, or what record rainfall has been recorded. The wetness or dryness of an area, particularly during the colder months, can have a dramatic impact on how plants thrive when they’re stressed by colder than normal temperatures. This is evident in western areas of the U.S., which are much more prone to microclimate conditions that eastern sections of the country.
A Word of Ending
If you’re a novice gardener, you can definitely benefit from the information provided by the USDA zones map. When you’re ready to make plans for the plants you’d like to put in your garden but you’re not sure of your area’s climate, the plant hardiness zone maps will give you answers you need. Likewise, if you’ve moved to a new area, using the USDA maps will help you determine which of the plants you love can be included in your garden. Many gardening experts recommend researching plants you think you’d like to try to grow and then consulting the USDA zones map. You’ll have a much better sense of the plants that can thrive in your garden.