Mercer Island, WA planting zone & frost dates
Mercer Island is in USDA hardiness zone 9a. The last spring frost usually comes around March 13 and the first fall frost around November 17, for a growing season of about 247 days.
Planting calendar for Mercer Island
Dates are counted from the average last spring frost (March 13) using the typical timing recommended by U.S. extension services. Warm-season crops go out only after the danger of frost has passed and the soil has warmed.
| Crop | When |
|---|---|
| Tomatoes | Start seeds indoors: Jan 16 – Jan 30 Transplant outdoors: Mar 20 – Mar 27 |
| Peppers | Start seeds indoors: Jan 2 – Jan 16 Transplant outdoors: Mar 27 – Apr 3 |
| Eggplant | Start seeds indoors: Jan 2 – Jan 16 Transplant outdoors: Mar 27 – Apr 3 |
| Broccoli | Start seeds indoors: Jan 2 – Jan 16 Transplant outdoors: Feb 13 – Feb 27 |
| Cabbage | Start seeds indoors: Jan 2 – Jan 16 Transplant outdoors: Feb 13 – Feb 27 |
| Onions (sets) | Plant outdoors: Feb 13 – Feb 27 |
| Peas | Sow outdoors: Jan 30 – Feb 13 |
| Spinach | Sow outdoors: Jan 30 – Feb 13 |
| Lettuce | Sow outdoors: Feb 13 – Feb 27 |
| Carrots | Sow outdoors: Feb 13 – Feb 27 |
| Potatoes | Plant outdoors: Feb 13 – Feb 27 |
| Beans | Sow outdoors: Mar 20 – Mar 27 |
| Sweet corn | Sow outdoors: Mar 20 – Mar 27 |
| Cucumbers | Sow outdoors: Mar 20 – Mar 27 |
| Squash & zucchini | Sow outdoors: Mar 20 – Mar 27 |
| Basil | Start seeds indoors: Jan 30 – Feb 13 Transplant outdoors: Mar 20 – Mar 27 |
Fall garden: cool-season crops like lettuce, spinach and kale can be sown again around Sep 22 (8 weeks before the average first fall frost on Nov 17).
Useful for zone 9a gardens
- Seed starting kit with dome — to start seeds indoors before the last frost
- LED grow light for seedlings — stops leggy seedlings
- Frost cloth / plant covers — for late spring and early fall frosts
- Soil thermometer — warm-season crops need soil above 60°F
- Shade cloth — for hot summers
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Frost risk in detail
| 90% chance | 50% chance | 10% chance | |
|---|---|---|---|
| Last 32°F frost in spring is after | Feb 20 | Mar 13 | Apr 1 |
| First 32°F frost in fall is before | Dec 6 | Nov 17 | Nov 1 |
How to read it: in spring there is a 10% chance of a frost after April 1, so that is a safe date for tender plants. In fall there is a 10% chance of frost before November 1. A hard freeze (28°F) usually ends around February 18.
Hardiness zone by ZIP code in Mercer Island
- 98040 zone 9a
Nearby cities
- Bellevue zone 9a, last frost Mar 8
- Columbia City zone 9a, last frost Mar 13
- Newcastle zone 8b, last frost Mar 13
- Bryn Mawr-Skyway zone 9a, last frost Mar 13
- Renton zone 9a, last frost Mar 13
- Tukwila zone 9a, last frost Mar 13
- Kirkland zone 8b, last frost Mar 8
- Seattle zone 9a, last frost Mar 13
Questions
What growing zone is Mercer Island, WA?
Mercer Island is in USDA Plant Hardiness Zone 9a (average extreme winter low 20°F to 25°F), according to the USDA 2023 map.
When is the last frost in Mercer Island?
The average last spring frost (32°F) in Mercer Island is March 13. There is still a 10% chance of frost until April 1.
When can I plant tomatoes in Mercer Island?
Transplant tomatoes outdoors 1–2 weeks after the average last frost: around Mar 20 – Mar 27. Start seeds indoors 6–8 weeks before the last frost.
Frost dates: NOAA U.S. Climate Normals 1991–2020, station Seattle Boeing Fld, WA (USW00024234), 7.4 km (4.6 mi) from the city center, elevation 6 m, based on 18 years of records. Hardiness zone: USDA Plant Hardiness Zone Map 2023 for the ZIP code nearest to the city center. Microclimates (valleys, hills, cities, coasts) can be warmer or colder: check the local forecast before planting. How we calculate the data.