Mercer Island, WA 98118 Frost Dates
98118, near Mercer Island, WA: the frost window
The last spring frost here typically falls around 03/13, about 197 days from today. Fall's first freeze usually shows up around 11/17, roughly 247 days after the last spring frost.
Zone 9a (20 to 25°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 Mercer Island, not a forecast for this year.
Stretching the season past the first frost
Mercer Island's growing season runs out in roughly 12 weeks — a cold frame or greenhouse kit is the usual way to keep cool-weather crops going past the first frost.
The actual range around Mercer Island's last frost
03/13 is the middle of 30 years of NOAA records, not a promise. Getting tender plants in by 02/20 carries real risk, frost has hit that late in about 9 of 10 years here. Wait until 04/01 and only about 1 year in 10 sees frost after that.
Mercer Island against the rest of the dataset
Mercer Island's growing season runs about 247 days, ranking 7,794 of 40,283 US ZIP codes measured here, longest to shortest. Within WA, it ranks 151 of 700.
Learn more
Two guides worth reading on 98118: zone vs. frost date, on why Zone 9a and Mercer Island's frost date answer different questions, and the frost range, on how to read the range around Mercer Island's dates instead of trusting one.