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Hardiness Zones 101: What All Home Gardeners Need to Know

These geographic zones help gardeners choose which plants are most likely to survive and thrive in their yards.

hardiness zones

Photo: USDA

Sometimes called "growing zones" or "gardening zones," hardiness zones refer to a map developed by the USDA that separates the United States into regions based on the average low winter temperatures.

Some plants will adapt to a variety of conditions, including soil type, watering fluctuations, and humidity differences, but most plants will only tolerate a specific degree of chill in the winter. That's where hardiness zones come in—they list the average lowest temp in every area of the country, so gardeners can select perennial plants that will survive the type of winters common to specific regions.

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What are hardiness zones?

The USDA Plant Hardiness Zone Map consists of 13 individual zones that take in all of the contiguous United States, plus Alaska, Hawaii, and Puerto Rico. The first zone lies at the coldest and northernmost areas of Alaska. In these regions, winter temps can dip as low as -60 degrees Fahrenheit. On the southernmost part of the scale lies Zone 13, where winter lows remain above 65 degrees Fahrenheit.

Zones represent low temperatures in 10-degree increments, and each zone is further broken down into two additional segments, "a" and "b," representing 5-degree increments. Not all hardiness maps, such as the Arbor Day map, list all 13 zones, and they might not feature the "a" and "b" breakdowns, but they're relatively similar in design.

hardiness zones

Photo: istockphoto.com

What hardiness zones won't tell you.

Hardiness zone maps are essential for helping gardeners choose plants, but they're not all-inclusive categories. Some vital factors that require consideration are not covered on a hardiness zone map.

Maximum temperature: The average hottest summer temperature isn't found on a hardiness map, but it might be vital to a plant's survival. Gardeners should research the plant species to determine whether it will survive the heat in the region.
Precipitation: Some plants require a lot of water, while others prefer a desert-type environment. The USDA Hardiness Zone map does not include average precipitation amounts.
Average frost dates: Planting too soon in the spring could result in the need to cover plants at night if temps dip below freezing. Likewise, gardeners might extend the fall growing season by covering plants or bringing them into a greenhouse. Average first and last frost dates can be found on The Old Farmer's Almanac by ZIP code.

hardiness zones

Photo: istockphoto.com

Typical hardiness zones by region:

For a closer look at different regions, the USDA offers an interactive tool that allows users to see a compilation of hardiness zones in one of five general regions of the contiguous states.

North Central: From the northern border of the U.S., ranging as far west as South Dakota, as far east as Illinois, and taking in Kansas and Missouri, the North Central region of the nation encompasses four hardiness zones, from 3a to 7b, with minimum winter temps ranging from -40 to -15 degrees Fahrenheit.
North East: Following the eastern seaboard from Maine to Virginia and extending as far west as Kentucky and Michigan, the North East region includes portions of five zones, ranging from 3a at the northern border to 8a at the southernmost tip of Virginia (-40 to -15 degrees Fahrenheit).
North West: This hardiness zone takes in all of Montana and Wyoming and then extends west to the western seaboard, and features a wide range of hardiness zones, from 3a in parts of Montana to 9a along the coast of Oregon and Washington (-40 to -10 degrees Fahrenheit).
South Central: Featuring just four states, Texas, Louisiana, Arkansas, and Oklahoma, the South Central hardiness region encompasses zones from 6a in the northern areas to 10a at the southern tip of Texas (-10 to 15 degrees Fahrenheit).
South East: Taking in Tennessee and Mississippi, extending from there east to the Atlantic Coast, and encompassing all states to the south, the South East region ranges in hardiness from zone 5b to 11a (-15 to 45 degrees Fahrenheit).
South West: Including Colorado and the states to the west and south, the South West hardiness region features a wide range of zones, from 3b to 11a, or from -35 to 40 degrees Fahrenheit.

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hardiness zones

Caption: istockphoto.com

How to use your hardiness zone.

Use the hardiness map to find the average lowest winter temperature in your community and then choose plants that will survive that temp. To simplify the process, most commercial greenhouses provide detailed plant labels that list suitable hardiness zones, the lowest temperatures the plants can survive, and other vital information, such as whether they will grow better in the sun or shade.

Use the zone map as one factor in choosing a plant. However, don't forget to check off all the boxes on its other needs.

hardiness zones

Photo: istockphoto.com

When in doubt, opt for native plants.

Plants that are indigenous to an area are always good choices. These native plants are well suited to the average minimum winter temps and the typical amount of rain, type of soil, and average summer heat in the region.

In addition to being good options for thriving in the area, native plants also play a vital role in xeriscaping, the practice of growing plants that do not require a lot of extra water, fertilizer, or pesticides, because they're well adapted to the region. In this way, using native plants will help conserve water and cut down on the number of chemicals introduced in the environment.

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