Mercer Island, WA 98108 Frost Dates
Frost dates for Mercer Island, WA, ZIP 98108
Around 03/13 is the usual last spring frost, about 197 days away. The first fall frost typically arrives around 11/17, about 247 frost-free days later.
Mercer Island sits in USDA Zone 9a (20 to 25°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 9a around Mercer Island has about 12 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 Mercer Island's last frost
03/13 is the middle of 30 years of NOAA records, not a promise. Plant by 02/20 and you're betting against frost in roughly 9 years out of 10. 04/01 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Mercer Island compares
At roughly 247 frost-free days, Mercer Island places 7,793 of 40,283 ZIP codes in this dataset. Within WA, it ranks 150 of 700.
Learn more
local microclimates explains what one yard in Mercer Island can differ from the 98108 normal, and zone vs. frost date explains why Zone 9a and Mercer Island's frost date answer different questions.