Portsmouth, NH 03909 Frost Dates
Frost dates for Portsmouth, NH, ZIP 03909
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.
Zone 6a (-10 to -5°F average annual minimum) comes from the USDA Plant Hardiness Zone Map; the frost dates above are the 1991-2020 normal at the nearest reporting NOAA station for Portsmouth, not a forecast for this year.
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.
The actual range around Portsmouth's last frost
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 33,239 of 40,283 ZIP codes nationally. Within NH, it ranks 95 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 03909 normal.