The table below gives the median last spring frost, first fall frost, and frost-free season length for each of the 48 contiguous states and the District of Columbia, from 30-year NOAA climate normals.1 It is built from the 313 cities covered by the per-city frost pages on this site, and it reads as odds rather than a promise: in half of years frost falls earlier than the median, in half it falls later.
A hardiness zone is a different measurement and will not give you 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 still sit weeks apart on either end of the frost-free season.2
| State | Median last spring frost | Median first fall frost | Frost-free season (days) | Cities tracked |
|---|---|---|---|---|
| Alabama | March 13 | November 15 | 247 | 4 |
| Arizona | January 27 | December 13 | 321 | 10 |
| Arkansas | March 30 | November 3 | 218 | 4 |
| California | January 9 | December 18 | 354 | 54 |
| Colorado | May 2 | October 8 | 158 | 7 |
| Connecticut | April 17 | October 27 | 193 | 4 |
| Delaware | April 7 | November 3 | 210 | 4 |
| District of Columbia | March 29 | November 8 | 224 | 1 |
| Florida | January 20 | January 13 | 360 | 17 |
| Georgia | March 13 | November 17 | 249 | 6 |
| Idaho | April 27 | October 17 | 174 | 4 |
| Illinois | April 19 | October 23 | 187 | 5 |
| Indiana | April 22 | October 22 | 184 | 4 |
| Iowa | April 26 | October 14 | 171 | 4 |
| Kansas | April 10 | October 29 | 202 | 5 |
| Kentucky | April 4 | November 1 | 211 | 4 |
| Louisiana | February 19 | November 30 | 284 | 4 |
| Maine | April 29 | October 13 | 167 | 4 |
| Maryland | April 1 | November 6 | 219 | 4 |
| Massachusetts | April 23 | October 21 | 181 | 5 |
| Michigan | April 27 | October 25 | 181 | 5 |
| Minnesota | April 28 | October 12 | 165 | 4 |
| Mississippi | March 1 | November 26 | 271 | 4 |
| Missouri | April 4 | October 31 | 209 | 5 |
| Montana | May 10 | October 1 | 144 | 4 |
| Nebraska | April 21 | October 17 | 180 | 4 |
| Nevada | February 1 | December 8 | 310 | 4 |
| New Hampshire | May 2 | October 11 | 162 | 4 |
| New Jersey | April 4 | November 6 | 216 | 4 |
| New Mexico | April 13 | October 28 | 198 | 4 |
| New York | April 24 | October 26 | 185 | 5 |
| North Carolina | March 30 | November 8 | 225 | 7 |
| North Dakota | May 10 | October 1 | 144 | 4 |
| Ohio | April 19 | October 27 | 190 | 6 |
| Oklahoma | April 2 | November 4 | 215 | 4 |
| Oregon | March 25 | November 11 | 231 | 4 |
| Pennsylvania | April 17 | October 28 | 193 | 4 |
| Rhode Island | April 11 | October 30 | 202 | 4 |
| South Carolina | March 10 | November 29 | 264 | 4 |
| South Dakota | May 7 | October 3 | 148 | 4 |
| Tennessee | April 5 | October 31 | 208 | 6 |
| Texas | March 7 | November 20 | 259 | 32 |
| Utah | April 13 | October 27 | 197 | 4 |
| Vermont | May 1 | October 13 | 166 | 4 |
| Virginia | March 24 | November 15 | 236 | 7 |
| Washington | March 17 | November 12 | 242 | 6 |
| West Virginia | April 15 | October 28 | 196 | 4 |
| Wisconsin | April 28 | October 16 | 171 | 4 |
| Wyoming | May 15 | September 30 | 140 | 4 |
How these numbers were built
Each city figure is the 50th percentile date from the 1991 to 2020 normals at the quality-controlled station nearest that city, which is the window NOAA rebuilds once a decade.1 Each state figure is then the median of the city values under it, taken one column at a time. Because the columns are computed separately, the season column is the median of the season lengths of the cities themselves rather than the gap between the two date columns, and the two will not always agree to the day.
In the warmest states frost, when it comes at all, arrives around the turn of the year, so the two dates can sit days apart in January rather than half a year apart. Florida is the clear case: both of its medians land in January, with the fall date about a week ahead of the spring one.
What a state median hides
The number of tracked cities behind a row varies a great deal. California rests on 54, most states on four, and the District of Columbia on one. A single median flattens a state that runs from the coast to the mountains, and in the states with the fewest cities it is a thin average rather than a map.
For your own beds, read the per-city frost pages, which give the 10th, 50th and 90th percentile dates for the station nearest a ZIP. That band is the more useful number, because it says not only when frost typically arrives but how far the date swings from one year to the next.