Editor's Note: This story originally appeared on LawnStarter.
The USDA Plant Hardiness Zone Map is the go-to resource for many gardeners — and it shows it really is getting warmer across the U.S.
How much warmer? The recently updated map shows, on average, temperatures 2.5 degrees warmer than the 2012 map, Chris Daly, director of the PRISM Climate Group at Oregon State University, told NPR.
The map, showing which plants are most likely to survive the coldest winter temperatures at a certain location, is the result of research by USDA, PRISM, meteorologists, horticulturists and other scientists.
Daly says about half the country has shifted into a new zone and half hasn’t. “In some locations, people may find they can grow new types of flowers, fruits, vegetables and plants,” he says.
Read on to learn how to use the map when selecting plants, trees, and even grass for your yard. Note that this map is just the starting point when figuring out what greenery will best thrive in your yard.
What Is the USDA Plant Hardiness Zone Map?
The map reflects weather data collected from 1990 to 2020 throughout the country, including the territory of Puerto Rico. Of particular interest is the average minimum temperature for a region — some flowers, shrubs and trees (like some people) just can’t take the cold.
The American beautyberry, for example, won’t be so beautiful if grown in Minneapolis, Detroit or New York City. It’s best in the South and as far west and north as St. Louis.
The Hardiness Zone Map divides the country into 13 zones: 1a to 13b (which range from minus 60 degrees to 70 degrees Fahrenheit). From that info, the scientists developed zones with a 5-degree range for the lowest temperature.
Average Minimum Winter Temperatures for Each Zone
Here are the average annual minimum winter temperatures for each zone:
- Zone 1a: (-60 to -55 F/ -51.1 to -48.3 C)
- Zone 1b: (-55 to -50 F/ -48.3 to -45.6 C)
- Zone 2a: (-50 to -45 F/ -45.6 to -42.8 C)
- Zone 2b: (-45 to -40 F/ -42.8 to -40 C)
- Zone 3a: (-40 to -35 F/ -40 to -37.2 C)
- Zone 3b: (-35 to -30 F/ -37.2 to -34.4 C)
- Zone 4a: (-30 to -25 F/ -34.4 to -31.7 C)
- Zone 4b: (-25 to -20 F/ -31.7 to -28.9 C)
- Zone 5a: (-20 to -15 F/ -28.9 to -26.1 C)
- Zone 5b: (-15 to -10 F/ -26.1 to -23.3 C)
- Zone 6a: (-10 to -5 F/ -23.3 to -20.6 C)
- Zone 6b: (-5 to 0 F/ -20.6 to -17.8 C)
- Zone 7a: (0 to 5 F/ -17.8 to -15 C)
- Zone 7b: (5 to 10 F/ -15 to -12.2 C)
- Zone 8a: (10 to 15 F/ -12.2 to -9.4 C)
- Zone 8b: (15 to 20 F/ -9.4 to -6.7 C)
- Zone 9a: (20 to 25 F/ -6.7 to -3.9 C)
- Zone 9b: (25 to 30 F/ -3.9 to -1.1 C)
- Zone 10a: (30 to 35 F/ -1.1 to 1.7 C)
- Zone 10b: (35 to 40 F/ 1.7 to 4.4 C)
- Zone 11a: (40 to 45 F/ 4.4 to 7.2 C)
- Zone 11b: (45 to 50 F/ 7.2 to 10 C)
- Zone 12a: (50 to 55 F/ 10 to 12.8 C)
- Zone 12b: (55 to 60 F/ 12.8 to 15.6 C)
- Zone 13a: (60 to 65 F/ 15.6 to 18.3 C)
- Zone 13b: (65 to 70 F/ 18.3 to 21.1 C)
Take USDA Zone 6a and 6b, for instance. USDA Zone 6b’s minimum average temperature is -5 to 0 degrees Fahrenheit. For Zone 6a, it is -10 to -5 degrees Fahrenheit.
How to Use the Hardiness Zone Map
The Plant Hardiness Zone Map shows the average lowest temperature at which plants can survive in your region. So, how can you use this information? Click on the USDA Plant Hardiness Zone Map online and key in your ZIP code to find your plant hardiness zone.
Even better, the online map is ready to accept your GIS (Geographic Information System) for an even more precise reading of your hardiness zone.
The map is an excellent guide for selecting plants that are winter hardy in your area. If you know your average minimum temperature is minus 10 degrees, you want to make sure plants will be hardy to that temperature.
Plant tags in stores, catalogs and online resources almost always give the plant’s hardiness zone. Just what is plant hardiness?
Plant hardiness refers to the roots of a plant and its top growth. Many plants are root hardy, but their tops are killed by cold temperatures or they drop their leaves.
Deciduous plants, such as maple trees, drop their leaves but their roots survive underground. With herbaceous perennials, such as coneflowers, the cold kills plants back to the ground, but the roots survive.
Limitations of the Hardiness Zone Map
The hardiness zone map details just that … winter hardiness. The map doesn’t tell gardeners how much heat a plant can take before it keels over.
Unfortunately, there isn’t an up-to-date plant heat zone map. For the West and Southwest U.S., many gardeners rely on the Sunset Climate Zones, which better breaks down which plants can tolerate the sun and dry climate of these regions.
Other essential planting factors the USDA Plant Hardiness Zone Map does not cover include:
- Humidity
- Elevation
- Nearby bodies of water
- Available sunlight
- Wind
- Precipitation
- Frost dates
- Soil type
- Urban heat (urban areas are significantly warmer than surrounding rural areas).
Why does your specific geographic location, such as if you live near a lake, mountain or valley, matter when selecting plants for your yard? Those topographic formations influence temperatures.
A local resource for recommendations of plants for your regions is your cooperative extension office, or check with someone knowledgeable at your neighborhood garden center.
Does the USDA Plant Hardiness Zone Map Apply to Turfgrass?
Although turfgrass is a plant, you don’t want to depend on the USDA Plant Hardiness Zone Map for choosing the best grass type for your yard. The map you want to use instead is the Transition Zone Map.
If you live up North, the best grass type for your yard will be cool-season grass. Cool-season grasses grow best in areas with low temperatures throughout most of the year.
These grasses green up in early spring, go dormant when temperatures rise in summer, thrive in autumn, and enter winter dormancy when soil temperatures fall below 45 degrees Fahrenheit.
If you live down South, where temperatures are warm most of the year, you’ll want to plant warm-season grass. Warm-season grasses green up in spring, thrive in summer, and enter winter dormancy when soil temperatures drop below 55 degrees Fahrenheit.
The area sandwiched between the Northern and Southern states is the transition zone. You can grow either warm-season or cool-season grass if you live in the transition zone.
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