Portsmouth, NH 03840 Frost Dates
Frost dates for Portsmouth, NH, ZIP 03840
Around 05/04 is the usual last spring frost, about 249 days away. 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
About 157 days separate Portsmouth's last and first frost, putting it 33,231 of 40,283 ZIP codes nationally. Within NH, it ranks 87 of 260.
Learn more
zone vs. frost date explains why Zone 6a and Portsmouth's frost date answer different questions, and the frost range explains how to read the range around Portsmouth's dates instead of trusting one.