Portsmouth, NH 03870 Frost Dates
03870, near Portsmouth, NH: the frost window
Typically, the last spring freeze lands around 05/04, roughly 249 days out. Fall's first freeze usually shows up around 10/08, roughly 157 days after the last spring frost.
Portsmouth sits in USDA Zone 6a (-10 to -5°F average annual minimum). The dates above are NOAA's 1991-2020 normal at the nearest reporting station, a 30-year average, not a forecast.
Stretching the season past the first frost
Portsmouth's growing season runs out in roughly 6 weeks — a cold frame or greenhouse kit is the usual way to keep cool-weather crops going past the first frost.
Why 05/04 isn't a guarantee
05/04 is the middle of 30 years of NOAA records, not a promise. Getting tender plants in by 04/23 carries real risk, frost has hit that late in about 9 of 10 years here. Wait until 05/18 and only about 1 year in 10 sees frost after that.
Portsmouth against the rest of the dataset
At roughly 157 frost-free days, Portsmouth places 33,234 of 40,283 ZIP codes in this dataset. Within NH, it ranks 90 of 260.
Learn more
Two guides worth reading on 03870: local microclimates, on what one yard in Portsmouth can differ from the 03870 normal, and zone vs. frost date, on why Zone 6a and Portsmouth's frost date answer different questions.