Understanding the USDA Plant Hardiness Zone Map

What is a USDA hardiness zone?

The USDA Plant Hardiness Zone helps gardeners, landscapers, and growers understand what plants will survive through the winter in a specific US location. There are 13 primary zones in the US (including Puerto Rico). Each zone is separated by 10 degrees of temperature difference. The zones are further divided into ‘a’ and ‘b’ segments. The ‘a’ and ‘b’ segments have 5 degrees of difference between them. The higher the zone, the warmer it tends to be in that area. Let’s look at an example using zones 5 and 6.

In the chart below, you can see that there is a 10-degree F range within a zone and a 10-degree difference between the two zones. Also, the ‘a’ and ‘b’ zones are further subdivided in 5-degree increments. I live in zone 6b and the minimum extreme temperature range that I can expect is between -5°F to 0 °. I can also grow plants that are hardy in lower number zones (e.g. 1-6) but not higher zones (e.g. 7-13).

Zone	Minimum Temperature Range	Sub zones	Minimum Temperature Ranges
Zone 5	-20°F to -10°F	Zone 5a	-20°F to -15°F
		Zone 5b	-15°F to -10°F
Zone 6	-10°F to 0	Zone 6a	-10°F to -5°F
		Zone 6b	-5°F to 0

The zones represent the average annual extreme minimum temperatures within a geographic area. To find your zone, click on this link for the USDA zone finder and enter your zip code.

Who developed the US hardiness zone map?

The US National Arboretum in Washington, DC issued the first hardiness map in 1960. In 1990 and 2012, the US Department of Agriculture (USDA) updated the maps. For the 2012 update, the USDA used 30 years of weather data gathered from 1976 through 2005.

How are hardiness zones used?

USDA plant hardiness zone is used to predict a plant’s ability to survive the winter. Plant and seed retailers will list the zone(s) in which a plant can survive to assist gardeners with plant selection. However, the map is meant as a starting point. Other critical factors for plant growth include:

• Sunlight – Plants require either full sun, partial sun or shade
• Daylength – Daylength regulates many plant functions including the start of dormancy in fall
• Soil – Soil provides the oxygen, water, and nutrients that plants need to thrive

Hers what you need to do:

• Look up your zone via the USDA zone finder 
• When purchasing plants or seeds, compare the zone on the plant label with your zone. If the plant zone is equal to or lower than your zone, the plant has a good chance of surviving winters in your area.

It is also interesting to note that some plants are perennials in one zone and annuals in another. A good example is the herb rosemary. Rosemary is an evergreen perennial in zones 9 and higher. However, in colder zones, rosemary is usually grown as an annual or brought indoors for the winter.

Other gardening climate references

Western United States: Uses a 24 zone climate system created by Sunset Magazine.

Australia:  The Australian National Botanic Gardens developed its system for Australian conditions. https://www.anbg.gov.au/gardens/research/hort.research/zones.html