When to plant in Mercer Island, WA
USDA Zone 9aEverything below — frost dates, hardiness zone, and what to plant when in Mercer Island, Washington — is derived from the closest NOAA station with complete climate normals.
Mercer Island enjoys a long ~249-day frost-free season — you can succession-sow, fit in a second crop, and grow long-season heat-lovers with room to spare. In zone 9a, frost is a minor factor for Mercer Island — most perennials thrive, and annual vegetables can go out early and stay late.
Frost probability
SEATTLE BOEING FLD · 1991–2020The date the last spring and first fall frost occur, by threshold and probability. A 90% date is later in spring — and earlier in fall — than a 10% date; the 50% · 32°F row is what most gardeners plan around. These are Mercer Island’s own odds, recorded at SEATTLE BOEING FLD.
| Threshold | SPRING 10% | SPRING 50% | SPRING 90% | FALL 10% | FALL 50% | FALL 90% |
|---|---|---|---|---|---|---|
| 36°F | Apr 28 | Apr 3 | Mar 17 | Oct 17 | Oct 31 | Nov 17 |
| 32°F | Apr 1 | Mar 13 | Feb 20 | Nov 1 | Nov 17 | Dec 6 |
| 28°F | Mar 10 | Feb 18 | Jan 7 | Nov 12 | Dec 2 | Dec 30 |
Download this table as CSV ↓ — every threshold and probability, plus this city’s planting-window dates.
What to plant now
TODAY · JULY 19Full-year planting calendar
Each bar is the exact window to take a planting action in Mercer Island, drawn to the day from the local frost dates. The dashed line is today.
Nearby weather stations
3 within 14 km · complete 32°F normalsWhen stations disagree by more than a few days, that spread is real microclimate variation — elevation, water, urban heat. Judge which station best matches your own yard.
Mercer Island planting FAQ
When is the last spring frost in Mercer Island, WA?
On average, the last spring frost in Mercer Island is around March 13 (50% probability at 32°F, from 1991–2020 NOAA normals). Wait until after this date to set out tender plants like tomatoes and peppers.
When is the first fall frost in Mercer Island, WA?
Expect Mercer Island's first fall frost near November 17 — a 50% chance of 32°F by that date. Bring in or cover tender crops ahead of it.
What hardiness zone is Mercer Island in?
Mercer Island is in USDA hardiness zone 9a. In zone 9a, winters are mild — many tender perennials overwinter here.
How long is the growing season in Mercer Island?
Mercer Island has about 249 frost-free days — a long growing season — between the average last spring frost (March 13) and first fall frost (November 17).
When should I plant tomatoes in Mercer Island?
For Mercer Island, sow tomatoes indoors about January 16–January 30 and move the seedlings out around March 20, after the last spring frost.
Never miss a window in Mercer Island
An email when it’s time to start seeds, transplant, and sow — timed to Mercer Island’s frost dates. Double opt-in, one-click unsubscribe, no spam.
Nearby cities
8 within reach- Newcastle · 6 km
- Bellevue · 7 km
- Bryn Mawr-Skyway · 8 km
- Renton · 10 km
- Tukwila · 10 km
- White Center · 11 km
- Seattle · 11 km
- Burien · 13 km
Frost dates recorded at SEATTLE BOEING FLD, 6 km from the city center · 1991–2020 NOAA climate normals · zone from the USDA/PRISM 2023 map. How we compute this.
BlissGarden. "When to Plant in Mercer Island, WA — Frost Dates & Zone 9a." Frost normals: NOAA NCEI U.S. Climate Normals 1991–2020, station USW00024234. Retrieved from https://blissgarden.com/washington/mercer-island.