In zone 6a, the zone with the most weather stations in this dataset, the median last spring frost is April 30 and the median first fall frost is October 14, but stations in that one zone typically range from April 23 to May 8 in spring and from October 6 to October 20 in fall.1 The table below gives the same figures for every zone in the lower 48 with enough stations to support a median.
A hardiness zone is not a frost date. The USDA Plant Hardiness Zone Map sorts the country by average annual extreme minimum winter temperature, which is the cold a perennial has to survive rather than the morning a tomato seedling gets caught, so two gardens in the same zone can sit weeks apart on either end of the frost-free season.2 The range columns are the honest part of this table. For the number that applies to your own beds, use the per-ZIP frost lookup or the state-by-state table.
| Zone | Median last spring frost | Typical range, spring | Median first fall frost | Typical range, fall | Frost-free season (days) | Stations | Example city |
|---|---|---|---|---|---|---|---|
| 3b | May 17 | May 14 to May 23 | September 23 | September 19 to September 27 | 129 | 89 | |
| 4a | May 14 | May 10 to May 20 | September 27 | September 23 to September 30 | 136 | 301 | Fargo, North Dakota |
| 4b | May 11 | May 5 to May 21 | September 30 | September 24 to October 4 | 142 | 487 | Rochester, Minnesota |
| 5a | May 9 | May 1 to May 19 | October 2 | September 27 to October 7 | 147 | 721 | Minneapolis, Minnesota |
| 5b | May 4 | April 27 to May 14 | October 9 | October 2 to October 14 | 158 | 791 | Lincoln, Nebraska |
| 6a | April 30 | April 23 to May 8 | October 14 | October 6 to October 20 | 167 | 1044 | Chicago, Illinois |
| 6b | April 24 | April 17 to May 2 | October 20 | October 12 to October 24 | 179 | 1033 | Columbus, Ohio |
| 7a | April 18 | April 10 to April 26 | October 24 | October 17 to October 29 | 189 | 954 | Boston, Massachusetts |
| 7b | April 7 | April 2 to April 15 | October 31 | October 25 to November 4 | 206 | 695 | New York, New York |
| 8a | March 30 | March 24 to April 4 | November 6 | November 1 to November 10 | 221 | 707 | Charlotte, North Carolina |
| 8b | March 19 | March 12 to March 28 | November 13 | November 7 to November 18 | 238 | 713 | Dallas, Texas |
| 9a | March 4 | February 22 to March 14 | November 25 | November 18 to December 3 | 266 | 482 | San Antonio, Texas |
| 9b | February 5 | January 26 to February 23 | December 12 | December 2 to December 25 | 311 | 384 | Houston, Texas |
| 10a | January 17 | January 8 to January 28 | January 1 | December 23 to January 9 | 353 | 178 | Phoenix, Arizona |
| 10b | January 15 | January 5 to January 21 | January 5 | December 28 to January 15 | 360 | 45 | Los Angeles, California |
How these numbers were built
Each weather station carries a 50th percentile last spring frost and first fall frost from the 1991 to 2020 normals, the window NOAA rebuilds once a decade.1 Every ZIP code in the lower 48 is assigned to its nearest station and to its USDA zone, so a station counts toward each zone its ZIPs fall in. A station that never reaches a 50 percent frost date in the normals carries no date at all and is left out of its zone, which is why the warmest rows rest on the fewest stations. A zone row is the median of its stations, taken one column at a time, and the typical range is the middle half of those stations, from the 25th to the 75th percentile. The season column is the median of the per-station season lengths, so it will not always equal the gap between the two date columns to the day.
In the warmest zones frost, when it comes at all, arrives around the turn of the year, so a fall date can land in January and sit only days ahead of the spring one. Zones with fewer than 20 stations are left out rather than shown on a thin sample: 3a (5), 11a (10), 11b (2).
What a zone median hides
The range column is the reason this table exists. A single zone runs from one coast to the other and from valley floor to foothill, and its stations disagree by a week or more about when the last frost falls. The median is where the middle of that spread sits, and even at one station it reads as odds rather than a promise: in half of years frost falls earlier, in half it falls later. The example city in each row links to a page with that city's own station figures, which is the number a garden can actually use.