Portsmouth, NH 03871 Frost Dates
Frost dates for Portsmouth, NH, ZIP 03871
The last spring frost here typically falls around 05/04, about 249 days from today. The first fall frost typically arrives around 10/08, about 157 frost-free days later.
The USDA Plant Hardiness Zone Map puts Portsmouth in Zone 6a (-10 to -5°F average annual minimum); frost dates are NOAA's 1991-2020 station normal, not this year's forecast.
What to do before the first frost
Zone 6a around Portsmouth has about 6 weeks left before the first fall frost — that's the usual window to get a cold frame up if you want to keep harvesting past it.
Why 05/04 isn't a guarantee
05/04 is the middle of 30 years of NOAA records, not a promise. Plant by 04/23 and you're betting against frost in roughly 9 years out of 10. 05/18 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Portsmouth compares
Portsmouth's growing season runs about 157 days, ranking 33,235 of 40,283 US ZIP codes measured here, longest to shortest. Within NH, it ranks 91 of 260.
Learn more
local microclimates explains what one yard in Portsmouth can differ from the 03871 normal, and zone vs. frost date explains why Zone 6a and Portsmouth's frost date answer different questions.