Portsmouth, NH 03905 Frost Dates
Frost dates for Portsmouth, NH, ZIP 03905
Typically, the last spring freeze lands around 05/04, roughly 249 days out. The first fall frost typically arrives around 10/08, about 157 frost-free days later.
Portsmouth sits in USDA Zone 6b (-5 to 0°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.
What to do before the first frost
Zone 6b 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.
How much risk comes with planting early here
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
At roughly 157 frost-free days, Portsmouth places 33,238 of 40,283 ZIP codes in this dataset. Within NH, it ranks 94 of 260.
Learn more
the frost range explains how to read the range around Portsmouth's dates instead of trusting one, and local microclimates explains what one yard in Portsmouth can differ from the 03905 normal.