Last spring frost
May 4
Frost Dates for Portsmouth, NH
First fall frost
Oct 8
Growing season
157 days
Hardiness zone
6a
In an average year around Portsmouth, NH, the last spring frost (32°F) is about May 4 and the first fall frost is about Oct 8, giving roughly a 157-day growing season. To be safe most years, wait until Apr 23 before setting out tender plants.
Last spring frost
Dates the last frost of spring typically ends. Plant tender crops after the risk has passed for your comfort level.
| Frost intensity | Early (10%) | Average (50%) | Late (90%) |
|---|---|---|---|
| Light (36°F) | May 29 | May 16 | May 3 |
| Freeze (32°F) | May 18 | May 4 | Apr 23 |
| Hard (28°F) | May 7 | Apr 23 | Apr 7 |
| Severe (24°F) | Apr 23 | Apr 7 | Mar 27 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 29 | May 25 | May 22 | May 19 | May 16 | May 13 | May 10 | May 7 | May 3 |
| 32°F | May 18 | May 14 | May 10 | May 7 | May 4 | May 2 | Apr 29 | Apr 27 | Apr 23 |
| 28°F | May 7 | May 2 | Apr 28 | Apr 25 | Apr 23 | Apr 20 | Apr 17 | Apr 12 | Apr 7 |
| 24°F | Apr 23 | Apr 18 | Apr 14 | Apr 10 | Apr 7 | Apr 5 | Apr 3 | Mar 31 | Mar 27 |
| 20°F | Apr 7 | Apr 3 | Apr 1 | Mar 30 | Mar 28 | Mar 24 | Mar 21 | Mar 17 | Mar 12 |
| 16°F | Mar 31 | Mar 27 | Mar 22 | Mar 19 | Mar 16 | Mar 14 | Mar 11 | Mar 8 | Mar 4 |
First fall frost
Dates the first frost of fall typically arrives. Count back by a crop's days-to-maturity to know your planting cutoff.
| Frost intensity | Early (10%) | Average (50%) | Late (90%) |
|---|---|---|---|
| Light (36°F) | Sep 16 | Sep 30 | Oct 10 |
| Freeze (32°F) | Sep 27 | Oct 8 | Oct 24 |
| Hard (28°F) | Oct 5 | Oct 21 | Nov 5 |
| Severe (24°F) | Oct 18 | Nov 3 | Nov 18 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 16 | Sep 21 | Sep 24 | Sep 27 | Sep 30 | Oct 2 | Oct 4 | Oct 6 | Oct 10 |
| 32°F | Sep 27 | Oct 1 | Oct 4 | Oct 6 | Oct 8 | Oct 11 | Oct 14 | Oct 18 | Oct 24 |
| 28°F | Oct 5 | Oct 10 | Oct 14 | Oct 18 | Oct 21 | Oct 24 | Oct 28 | Nov 1 | Nov 5 |
| 24°F | Oct 18 | Oct 25 | Oct 29 | Nov 1 | Nov 3 | Nov 6 | Nov 10 | Nov 13 | Nov 18 |
| 20°F | Nov 2 | Nov 7 | Nov 11 | Nov 14 | Nov 17 | Nov 20 | Nov 23 | Nov 28 | Dec 2 |
| 16°F | Nov 16 | Nov 21 | Nov 25 | Nov 29 | Dec 1 | Dec 3 | Dec 6 | Dec 10 | Dec 16 |
Growing season is measured at 32°F with about 50% probability. Hardiness zone from the 2023 USDA map; frost normals from NOAA 1991-2020.
Where this data comes from
@endif These dates are computed from the nearest official weather station with long-term records: Greenland, Nh, about 3.3 miles away@if(!empty($station['elevation_ft'])), at 95 ft elevation. The further the station and the greater the elevation difference from your yard, the more your local frost timing can vary.
How we calculate these dates
Frost and freeze dates come from the NOAA 1991–2020 U.S. Climate Normals, the National Oceanic and Atmospheric Administration's 30-year averages for each weather station. A date shown at the 50% level is the median: in half of years the frost falls on or before it. The 10% and 90% columns show the early and late ends of the range, so you can plant on the odds you're comfortable with. These are long-term averages, not a forecast for this year - always check your local outlook before planting.
Source: NOAA National Centers for Environmental Information, 1991–2020 Climate Normals (public domain). Harman is not affiliated with or endorsed by NOAA.
Frequently asked questions
When is the last frost in Portsmouth, NH?
On average, the last spring frost (32°F) around Portsmouth, NH is about May 4. In cooler years it can run later, so for a safer bet many gardeners wait until Apr 23.
When is the first frost in the fall?
The first fall frost around Portsmouth, NH typically arrives about Oct 8. Count back by a crop's days to maturity to find the latest date you can plant it.
When can I plant tomatoes and other tender crops?
Tender crops like tomatoes, peppers and basil are damaged by frost, so wait until the last spring frost has passed - around May 4 on average, or Apr 23 to be safe most years.
How long is the growing season here?
The frost-free growing season (between the last spring and first fall 32°F frost) is about 157 days in a typical year.
Are these dates a forecast for this year?
No. They are 30-year averages from the NOAA 1991–2020 Climate Normals, not a prediction for the current year. Use them for planning and always check your local weather forecast before planting or protecting crops.
Local weather for 03870 Planting calendar → New Hampshire frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026