Last frost date in Mercer Island, WA
The average last frost in Mercer Island is March 13. By April 1 the chance of another 32°F night drops to 1 in 10. The first fall frost usually arrives about November 17, which leaves 247 frost-free days. An early freeze before November 1 happens one year in ten.
- Safe after (10% risk)
- Apr 1
- Average first frost
- Nov 17
- Frost-free season
- 247 days
- Hard freeze (28°F) ends
- Feb 18
No frost in the next 6 nights
National Weather Service forecast, updated Oct 22, 12:57 UTC
- ThuOct 2248°
- FriOct 2344°
- SatOct 2446°
- SunOct 2544°
- MonOct 2644°
- TueOct 2743°
Planting calendar for Mercer Island
Windows count forward or back from the average frost dates above. Click a crop for dates across Washington.
TomatoesMar 20–Apr 3
PumpkinsMar 27–Apr 10
SunflowersMar 13–Apr 3
CucumbersMar 27–Apr 3
CornMar 20–Apr 3
Sweet potatoesApr 3–Apr 17
WatermelonMar 27–Apr 10
Green beansMar 20–Apr 10
ZucchiniMar 20–Apr 3
OkraApr 3–Apr 17
PeppersMar 27–Apr 10
BasilMar 20–Apr 3
Butternut squashMar 20–Apr 3
CantaloupeMar 27–Apr 10
EggplantMar 27–Apr 10
PotatoesFeb 13–Feb 27
OnionsJan 30–Feb 20
CarrotsFeb 20–Mar 27
LettuceFeb 27–Mar 20
BroccoliFeb 13–Feb 27
BeetsFeb 13–Mar 27
PeasJan 30–Feb 13
CabbageFeb 13–Feb 27
RadishesFeb 6–Mar 20
Brussels sproutsFeb 27–Mar 13
SpinachJan 30–Feb 20
CilantroFeb 27–Mar 20
KaleFeb 13–Feb 27
Collard greensFeb 13–Feb 27
TurnipsFeb 13–Feb 27
DillMar 13–Mar 27
CeleryMar 20–Mar 27
CauliflowerFeb 27–Mar 13
LeeksFeb 27–Mar 13
Swiss chardFeb 27–Mar 27
ParsleyFeb 27–Mar 20
Tulip bulbsNov 3–Dec 8
GarlicOct 15–Nov 30
Daffodil bulbsOct 15–Nov 30
StrawberriesFeb 13–Feb 27
AsparagusFeb 13–Feb 27Frost probabilities
| Threshold | Spring, 90% chance later | Spring, 50% | Spring, 10% | Fall, 10% chance earlier | Fall, 50% | Fall, 90% |
|---|---|---|---|---|---|---|
| Light frost, 32°F | Feb 20 | Mar 13 | Apr 1 | Nov 1 | Nov 17 | Dec 6 |
| Hard freeze, 28°F | Jan 7 | Feb 18 | Mar 10 | Nov 12 | Dec 2 | Dec 30 |
Read the spring 10% column as the date after which only one year in ten still gets frost. That is the safer date for tomatoes, peppers and basil.
Where these numbers come from
NOAA station Seattle Boeing Field (USW00024234) sits 4.0 miles from the center of Mercer Island at 20 feet. Its averages cover 1991 to 2020. Low spots, valleys and open fields can keep getting frost a week or two after the station stops; city centers and lakeshores usually get less of it.
Questions
When is the last frost in Mercer Island, WA?
The average last frost date in Mercer Island is March 13, based on NOAA 1991–2020 climate normals. There is a 10% chance of frost after April 1.
When is the first frost in Mercer Island?
The first frost in fall usually comes around November 17. One year in ten it arrives before November 1.
How long is the growing season in Mercer Island?
About 247 days between the average last and first frost.
When should I plant tomatoes in Mercer Island?
Plant tomato seedlings outside from March 20 to April 3. Start seeds indoors six to eight weeks earlier.