Plant Hardiness Zone Table (USDA Zones 1-13)
| Zone | From (°F) | To (°F) | From (°C) | To (°C) |
|---|---|---|---|---|
| 1a | -60 | -55 | -51.1 | -48.3 |
| 1b | -55 | -50 | -48.3 | -45.6 |
| 2a | -50 | -45 | -45.6 | -42.8 |
| 2b | -45 | -40 | -42.8 | -40.0 |
| 3a | -40 | -35 | -40.0 | -37.2 |
| 3b | -35 | -30 | -37.2 | -34.4 |
| 4a | -30 | -25 | -34.4 | -31.7 |
| 4b | -25 | -20 | -31.7 | -28.9 |
| 5a | -20 | -15 | -28.9 | -26.1 |
| 5b | -15 | -10 | -26.1 | -23.3 |
| 6a | -10 | -5 | -23.3 | -20.6 |
| 6b | -5 | 0 | -20.6 | -17.8 |
| 7a | 0 | 5 | -17.8 | -15.0 |
| 7b | 5 | 10 | -15.0 | -12.2 |
| 8a | 10 | 15 | -12.2 | -9.4 |
| 8b | 15 | 20 | -9.4 | -6.7 |
| 9a | 20 | 25 | -6.7 | -3.9 |
| 9b | 25 | 30 | -3.9 | -1.1 |
| 10a | 30 | 35 | -1.1 | 1.7 |
| 10b | 35 | 40 | 1.7 | 4.4 |
| 11a | 40 | 45 | 4.4 | 7.2 |
| 11b | 45 | 50 | 7.2 | 10.0 |
| 12a | 50 | 55 | 10.0 | 12.8 |
| 12b | 55 | 60 | 12.8 | 15.6 |
| 13a | 60 | 65 | 15.6 | 18.3 |
| 13b | <65 | 70 | 18.3 | 21.1+ |
| City | USDA zone (2023 map era) |
|---|---|
| Anchorage, Alaska | 5a/5b |
| Minneapolis, Minnesota | 5a |
| Chicago, Illinois | 6a/6b |
| New York, New York | 7b |
| St. Louis, Missouri | 7a |
| Atlanta, Georgia | 8a/8b |
| Portland, Oregon | 8b/9a |
| Seattle, Washington | 9a |
| Houston, Texas | 9b |
| Phoenix, Arizona | 9b/10a |
| Tampa, Florida | 10a |
| Miami, Florida | 11a |
A plant tag says hardy to zone 6. What that number actually encodes is one statistic: the average annual extreme minimum temperature - how cold the single coldest night of winter typically gets, averaged over thirty years. Zone 6 means that floor lands between -10 and 0 degrees Fahrenheit. Nothing about summer heat, rainfall, first frost, or last frost lives in the number; it is a winter-survival rating, and the whole USDA map exists to answer one question - will this perennial's roots endure the coldest night your address usually serves.
Bottom line: the table runs zone 1 (-60 to -50°F on the coldest nights) through zone 13 (65°F and warmer) in ten-degree bands, and the arithmetic is worth memorizing: a zone's lower bound in Fahrenheit is simply ten times the zone number minus seventy - zone 9 starts at 20°F, zone 5 at -20°F. Each band then splits into a/b half-zones five degrees apart, which is why Minneapolis sits at 5a while its leafier, lake-moderated neighborhoods read 5b - and why that letter can decide a magnolia's fate.
Two honest caveats before you plan around a zone. First, the 2023 map update shifted many addresses half a zone warmer than the 2012 edition, because the thirty-year baseline rolled forward - your zone is a moving statistic, not a property line. Second, a zone map cannot see your yard: a south-facing brick wall, a low frost pocket, or a sheltered courtyard routinely swings five degrees or more from the ZIP-code value, and in half-zone gardening, five degrees is the whole game.
How to use
- Look up your ZIP on the official USDA map (linked below), then find your zone row here to see the exact temperature range the letter encodes.
- Check the a/b split for your area - the half-zone is five degrees, and plant tags that say hardy to 6b mean the cold end of zone 6, not the warm end.
- Before planting a marginal perennial, compare the plant's hardiness floor with your zone's minimum, then lean warm: a sheltered spot buys back a half zone, an open windswept lot can cost one.
Frequently asked questions
What is the difference between zone 6a and 6b?
Five Fahrenheit degrees. Zone 6a's extreme minimum averages -10 to -5°F, zone 6b's -5 to 0°F. The letter matters because plant hardiness is a threshold game: a plant rated hardy to -5°F survives the average 6b winter but is expected to die back or die outright in a typical 6a one. Microclimates within a single yard frequently span that same five-degree gap, which is why two neighbors can grow the same borderline plant with opposite results.
Why did my hardiness zone change with the 2023 map?
The 2023 USDA update rolled its thirty-year extreme-minimum baseline forward (1991-2020), and a warmer recent record nudged many locations half a zone warmer than the 2012 map showed. Your climate did not jump overnight - the statistical window did. Practical consequence: plants rated for your old zone are safer than the new number implies, and the trend line matters more than any single update.
Does the zone tell me when to plant?
No - and confusing the two is the classic beginner error. Hardiness zone answers whether a plant survives winter; the planting calendar answers when soil and air warm enough to start. Tomatoes die at the first hint of frost regardless of zone, and zone 9 gardeners still wait out their own last-frost date. Use the zone to pick which perennials come back, and frost-date and seed-starting tools to schedule when anything goes in the ground.
Is there a quick way to convert a zone to a temperature?
Yes - one line of arithmetic covers the whole table: multiply the zone number by ten and subtract seventy to get the band's lower bound in Fahrenheit. Zone 3: 30 - 70 = -40°F. Zone 8: 80 - 70 = 10°F. Zone 13: 60°F. The band runs ten degrees up from there, and the a/b split divides it at the five-degree mark. Celsius has no such neat rule, so the table carries converted values per row.
Does the zone rating cover summer heat?
Not at all - and for heat-limited plants that is the more important half. A zone 6 Vermont summer and a zone 6 (high-elevation equivalent) summer can share a winter floor while differing wildly in July peaks and humidity. The American Horticultural Society's heat-zone system rates the other direction by days over 86°F. Peonies, for instance, survive the winter across a wide cold range but sulk where winters never chill or summers never cool - the USDA zone alone cannot see that.