What are USDA plant hardiness zones?
Short answer: USDA plant hardiness zones are based on the average annual extreme minimum winter temperature at a location. Each zone spans 10°F, split into 5°F half zones (a and b). Zone 7a, for example, averages a coldest night of 0 to 5°F. The 2023 map uses 1991-2020 data and runs about a quarter zone warmer than the 2012 map.
What does a hardiness zone number mean?
The USDA Plant Hardiness Zone Map is “based on the average annual extreme minimum winter temperature.” Each year has one coldest night. The map averages those yearly lows over 30 years.
Zones are 10°F wide. Each zone splits into two 5°F half zones, labeled “a” (colder) and “b” (warmer). A plant tag that says “hardy to zone 5” generally means it is expected to handle typical zone 5 winter lows.
USDA hardiness zone temperature chart (zones 3-10)
USDA displays the map “as 10-degree F zones and 5-degree F half zones,” and its 2012 release lists zone 12 as 50-60°F and zone 13 as 60-70°F. Stepping down 10°F per zone gives the formula:
Zone n low = (10 × n) − 70 °F
For zone 6: (10 × 6) − 70 = −10°F. Purdue Extension confirms zone 5b as −15 to −10°F, 6a as −10 to −5°F and 7a as 0 to 5°F. Celsius uses °C = (°F − 32) × 5/9.
| Zone | “a” half zone (°F) | “b” half zone (°F) | Full zone (°C, approx.) |
|---|---|---|---|
| 3 | −40 to −35 | −35 to −30 | −40.0 to −34.4 |
| 4 | −30 to −25 | −25 to −20 | −34.4 to −28.9 |
| 5 | −20 to −15 | −15 to −10 | −28.9 to −23.3 |
| 6 | −10 to −5 | −5 to 0 | −23.3 to −17.8 |
| 7 | 0 to 5 | 5 to 10 | −17.8 to −12.2 |
| 8 | 10 to 15 | 15 to 20 | −12.2 to −6.7 |
| 9 | 20 to 25 | 25 to 30 | −6.7 to −1.1 |
| 10 | 30 to 35 | 35 to 40 | −1.1 to 4.4 |
These are averages of each year’s coldest night. They are not the lowest temperature ever recorded there.
What changed in the 2023 hardiness zone map?
| Item | 2012 map | 2023 map |
|---|---|---|
| Averaging period | 1976-2005 | 1991-2020 |
| Weather stations | Fewer | 13,625 |
| Overall shift | Generally one 5°F half zone warmer than the 1990 map | About one quarter zone warmer than 2012 |
USDA says the 2023 map is “generally about one quarter-zone warmer” than 2012 across much of the country. A quarter of a 10°F zone is about 2.5°F on average. Zone boundaries shifted in many areas.
The shifts come from:
- The newer 1991-2020 averaging period (vs. 1976-2005).
- Data from more stations.
- Improved interpolation (mapping) methods. USDA notes these can make some spots cooler, not only warmer.
USDA says the most substantial changes from added data and better methods are in upland areas of Alaska. It also cautions that zone changes are not reliable evidence of global warming, because the map tracks only 30-year averages of one extreme value.
How do I find my hardiness zone by ZIP code?
- Go to planthardiness.ars.usda.gov.
- Type your ZIP code in the “Quick Zip Code Search” box.
- Read your zone and its temperature range. You can also click any spot on the map.
USDA recommends a broadband connection for the interactive map.
What do hardiness zones not tell you?
The map measures winter lows only. USDA lists several things it does not capture:
- Extreme years: Zones use average lows. A rare cold snap can still kill plants that have thrived for years.
- Microclimates: A yard can be warmer or colder than its zone. The map is too coarse to show the smallest ones.
- Odd timing: Early-fall freezes and warm midwinter spells followed by sudden cold can injure plants.
- Summer heat: USDA calls warm-season heat and moisture balance “particularly important” to survival.
- Other conditions: “Wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can greatly affect the survival of plants.”
Zones also do not give planting dates. For spring and fall timing, use your average frost dates. See our last frost date guide.
Quick answers
What is my USDA hardiness zone? Enter your ZIP code in the search box at planthardiness.ars.usda.gov. The result shows your half zone and its temperature range.
What is the difference between zone 7a and 7b? Zone 7a averages a coldest night of 0 to 5°F. Zone 7b averages 5 to 10°F.
Did my zone change in 2023? Possibly. USDA says the 2023 map runs about a quarter zone warmer than 2012 across much of the U.S., and boundaries shifted in many areas.
What years does the 2023 hardiness map use? It averages each year’s extreme minimum temperature from 1991 to 2020.
Do hardiness zones tell you when to plant? No. Zones only describe average winter lows. Use your local frost dates for planting time.