Mercer Island, WA frost dates
Last spring frostMar 13median date
First fall frostNov 17median date
Growing season249 daysfrost-free
Hardiness zone9aZone 9a
How Mercer Island compares
- In 8 years out of 10, the last spring frost in Mercer Island falls between February 20 and April 1, a 40-day window. The first fall frost falls between November 1 and December 6.
- The median last frost is 39 days earlier than the Washington median of Apr 21.
- Its 249-day growing season is longer than 89% of the 491 Washington places with a regular frost season (state median 182 days).
- The last frost is 10 days later than the zone 9a median of Mar 3. Hardiness zones measure winter cold, not season length, so use these dates rather than a zone 9a chart.
- The weather station records an average of 30 nights a year at or below 32°F.
- Compared with Bellevue, WA, 7 km away, the last frost here is 5 days later than there and the growing season is 11 days shorter.
Frost probability
| Chance of frost after | Date |
|---|---|
| 90% (early) | February 20 |
| 50% (median) | March 13 |
| 10% (safe for tender plants) | April 1 |
| Chance of frost before | Date |
|---|---|
| 10% (early) | November 1 |
| 50% (median) | November 17 |
| 90% (late) | December 6 |
Frost here means 32°F (0°C). A median date means half of years have their last frost earlier and half later.
Planting calendar for Mercer Island, WA
Mercer Island, WA frost dates — last frost Mar 13, first frost Nov 17, zone 9a. whenisthelastfrost.com
Crop
JFMAMJJASOND
Start indoors: Dec 19 to Jan 2Start indoors: Dec 19 to Jan 2Transplant outside: Feb 13 to Feb 27
Start indoors: Jan 2 to Jan 16Transplant outside: Mar 27 to Apr 10
Start indoors: Jan 2 to Jan 16Transplant outside: Mar 27 to Apr 10
Start indoors: Jan 16 to Jan 30Transplant outside: Mar 20 to Apr 3
Start indoors: Jan 16 to Jan 30Transplant outside: Feb 20 to Mar 6Sow outdoors: Feb 13 to Mar 20Plant for fall: Sep 22 to Oct 20
Start indoors: Jan 16 to Jan 30Transplant outside: Feb 13 to Feb 27Sow outdoors: Feb 13 to Feb 27Plant for fall: Sep 8 to Sep 29
Start indoors: Jan 16 to Jan 30Transplant outside: Feb 27 to Mar 13Plant for fall: Aug 11 to Sep 1
Start indoors: Jan 16 to Jan 30Transplant outside: Feb 20 to Mar 6Plant for fall: Aug 11 to Sep 1
Start indoors: Jan 16 to Jan 30Transplant outside: Feb 27 to Mar 13Plant for fall: Aug 11 to Sep 1
Start indoors: Jan 30 to Feb 13Transplant outside: Apr 3 to Apr 17Sow outdoors: Apr 3 to Apr 17
Start indoors: Jan 30 to Feb 13Transplant outside: Mar 20 to Apr 3
Sow outdoors: Jan 30 to Feb 20Plant for fall: Aug 25 to Sep 15
Sow outdoors: Jan 30 to Feb 20Plant for fall: Sep 22 to Oct 13
Sow outdoors: Feb 6 to Mar 20Plant for fall: Oct 6 to Oct 27
Start indoors: Feb 13 to Feb 20Transplant outside: Mar 27 to Apr 3Sow outdoors: Mar 20 to Apr 3Plant for fall: Sep 1 to Sep 15
Start indoors: Feb 13 to Feb 20Sow outdoors: Mar 20 to Apr 3Plant for fall: Sep 8 to Sep 22
Start indoors: Feb 13 to Feb 20Sow outdoors: Mar 20 to Apr 3
Start indoors: Feb 13 to Feb 20Transplant outside: Mar 27 to Apr 10
Sow outdoors: Feb 20 to Mar 20Plant for fall: Aug 25 to Sep 8
Sow outdoors: Feb 20 to Mar 20Plant for fall: Sep 8 to Sep 22
Sow outdoors: Feb 20 to Mar 13
Sow outdoors: Feb 27 to Mar 27Plant for fall: Sep 8 to Sep 22
Sow outdoors: Feb 27 to Mar 20Plant for fall: Sep 22 to Oct 13
Sow outdoors: Mar 20 to Apr 10Plant for fall: Sep 1 to Sep 15
Sow outdoors: Mar 20 to Apr 3
Transplant outside: Mar 27 to Apr 17
Plant for fall: Nov 3 to Dec 8
Start indoorsTransplant outsideSow outdoorsPlant for fallMedian frost dates
When to plant vegetables in Mercer Island, WA
| Crop | Start indoors | Transplant | Sow outdoors | Fall crop |
|---|---|---|---|---|
| Onions | Dec 19 – Jan 2 | Feb 13 – Feb 27 | — | — |
| Peppers | Jan 2 – Jan 16 | Mar 27 – Apr 10 | — | — |
| Eggplant | Jan 2 – Jan 16 | Mar 27 – Apr 10 | — | — |
| Tomatoes | Jan 16 – Jan 30 | Mar 20 – Apr 3 | — | — |
| Lettuce | Jan 16 – Jan 30 | Feb 20 – Mar 6 | Feb 13 – Mar 20 | Sep 22 – Oct 20 |
| Kale | Jan 16 – Jan 30 | Feb 13 – Feb 27 | Feb 13 – Feb 27 | Sep 8 – Sep 29 |
| Broccoli | Jan 16 – Jan 30 | Feb 27 – Mar 13 | — | Aug 11 – Sep 1 |
| Cabbage | Jan 16 – Jan 30 | Feb 20 – Mar 6 | — | Aug 11 – Sep 1 |
| Cauliflower | Jan 16 – Jan 30 | Feb 27 – Mar 13 | — | Aug 11 – Sep 1 |
| Okra | Jan 30 – Feb 13 | Apr 3 – Apr 17 | Apr 3 – Apr 17 | — |
| Basil | Jan 30 – Feb 13 | Mar 20 – Apr 3 | — | — |
| Peas | — | — | Jan 30 – Feb 20 | Aug 25 – Sep 15 |
| Spinach | — | — | Jan 30 – Feb 20 | Sep 22 – Oct 13 |
| Radishes | — | — | Feb 6 – Mar 20 | Oct 6 – Oct 27 |
| Cucumbers | Feb 13 – Feb 20 | Mar 27 – Apr 3 | Mar 20 – Apr 3 | Sep 1 – Sep 15 |
| Zucchini & Summer Squash | Feb 13 – Feb 20 | — | Mar 20 – Apr 3 | Sep 8 – Sep 22 |
| Winter Squash & Pumpkins | Feb 13 – Feb 20 | — | Mar 20 – Apr 3 | — |
| Melons & Watermelon | Feb 13 – Feb 20 | Mar 27 – Apr 10 | — | — |
| Carrots | — | — | Feb 20 – Mar 20 | Aug 25 – Sep 8 |
| Beets | — | — | Feb 20 – Mar 20 | Sep 8 – Sep 22 |
| Potatoes | — | — | Feb 20 – Mar 13 | — |
| Swiss Chard | — | — | Feb 27 – Mar 27 | Sep 8 – Sep 22 |
| Cilantro | — | — | Feb 27 – Mar 20 | Sep 22 – Oct 13 |
| Green Beans | — | — | Mar 20 – Apr 10 | Sep 1 – Sep 15 |
| Sweet Corn | — | — | Mar 20 – Apr 3 | — |
| Sweet Potatoes | — | Mar 27 – Apr 17 | — | — |
| Garlic | — | — | — | Nov 3 – Dec 8 |
Dates come from the median frost dates and common extension-service timing. Adjust for your soil temperature, microclimate and the varieties you grow.
About these dates
Based on 1991–2020 NOAA climate normals for the Seattle Boeing Fld, WA weather station, 6 km away (elevation 20 ft (6 m)).
Frost dates near Mercer Island
- Bellevue Mar 8 · Nov 23
- Medina Mar 8 · Nov 23
- Seattle Mar 13 · Nov 17
- Renton Mar 13 · Nov 16
- Seahurst Mar 2 · Nov 23
- Kirkland Mar 8 · Nov 23
- Sammamish Mar 30 · Nov 7
- Redmond Mar 8 · Nov 23
- Issaquah Mar 30 · Nov 7
- Kent Mar 20 · Nov 10
Frequently asked questions
When is the last frost in Mercer Island, WA?
The median last spring frost in Mercer Island, WA is March 13. There is still about a 10% chance of frost after April 1, so wait until then to plant frost-tender crops like tomatoes and peppers.
When is the first frost in Mercer Island, WA?
The median first fall frost is November 17, with about a 10% chance of frost before November 1.
How long is the growing season in Mercer Island, WA?
About 249 days between the median last spring frost and first fall frost.
What planting zone is Mercer Island, WA?
Mercer Island, WA is in USDA hardiness zone 9a.