When to Plant Sweet Corn in Washington
Published: April 24, 2026
Washington gardeners in Zone 4b have a 21-week frost-free window for sweet corn (May 7 to October 1). Warm-season crops like sweet corn must be started indoors early to maximize the short window — Washington's late spring and early fall frost dates leave little room for error. This guide gives exact dates for Washington based on your local frost calendar.
Sweet Corn Planting Dates for Washington
| Start seeds indoors | Year-round |
| Last frost (average) | May 7 |
| Direct sow outdoors | May 14–24 |
| Expect first harvest | July 18 – August 12 |
| First fall frost (average) | October 1 |
⚠ Direct sow corn after last frost when soil reaches 60°F. Plant in blocks of at least 4 rows for pollination. Succession plant every 2 weeks for continuous harvest.
Growing Sweet Corn in Washington's Climate
Washington's cold Zone 4b climate puts warm-season sweet corn at the edge of viability — but the edge is workable with good planning. Starting seeds indoors under lights in late winter is the most important step; skipping this puts the whole harvest at risk from the short outdoor growing window.
Season extenders pay for themselves many times over in Washington. Row covers placed over transplants in early spring add 5°F to 8°F of protection and can extend the season two to three weeks on each end. Black plastic mulch warms the soil to the temperatures sweet corn needs for aggressive root growth.
Common sweet corn pests to watch for in Washington include Corn Earworm and Cutworm. The first line of defense is companion planting: Bush Bean and Pumpkin planted nearby discourage several of these pests by confusing host-plant identification or repelling adults before they can lay eggs. Weekly garden walks in the early morning catch problems when they're still manageable.
Sweet Corn Garden Calendar for Washington
| Month | Task | Notes |
|---|---|---|
| January | — | Dormant season |
| February | — | Dormant season |
| March | — | Dormant season |
| April | — | Dormant season |
| May ← | Direct sow seeds | Sow seeds 1/4–1/2 inch deep, 12 inches apart |
| June | Peak growth; water 1 inch/week | Monitor for pests daily; stake or support as needed |
| July | Continue care | Consistent watering; remove yellow leaves |
| August | — | Dormant season |
| September | — | Dormant season |
| October | First frost approaches | Harvest remaining sweet corn; cover plants on frost nights |
| November | — | Dormant season |
| December | — | Dormant season |
Sweet Corn Growing Tips for Washington Gardeners
- •Plant in blocks of at least 4 rows × 4 plants, not single rows — corn is wind-pollinated and single rows produce deformed ears.
- •Sow successively every 2 weeks until 90 days before first fall frost for continuous harvest through summer.
- •Washington's short growing season means start seeds indoors under grow lights — don't rely on windowsill light which produces leggy, weak seedlings that struggle outdoors.
- •Cover young transplants with floating row covers for the first 2–3 weeks outside. The 3–5°F of protection reduces frost damage risk and accelerates early growth.
- •Side-dress with nitrogen when plants are knee-high and again at tasseling — corn is an extremely heavy feeder.
Companion Planting for Sweet Corn in Washington
In Washington's cold climate, planting sweet corn with Bush Bean and Pumpkin helps deter pests and improve soil. Avoid planting near Tomato and Celery, which inhibit sweet corn growth through root chemistry or shared pest pressure. See the full sweet corn companion planting guide for the complete list.
Sweet Corn Planting Dates by City in Washington
Top cities in Washington — select for city-level frost dates and planting calendars.
Frequently Asked Questions
When should I plant sweet corn in Washington?
In Washington (Zone 4b), start sweet corn seeds indoors around Year-round and transplant outdoors around null — 0 weeks after the state's average last frost of May 7.
What is the last frost date in Washington?
Washington's average last frost date is May 7 in the state's most common Zone 4b. Frost dates vary by city — higher elevations and northern counties run 1 to 3 weeks later than the state average. See the city list below for city-level frost dates.
Can I grow sweet corn year-round in Washington?
Not year-round — Washington's cold season stops sweet corn production. The sweet corn season runs May 7 to October 1, roughly 21 weeks.
What sweet corn varieties grow best in Washington?
For Washington's Zone 4b, consult a local nursery or extension office for corn variety recommendations.
When do I start sweet corn seeds indoors in Washington?
Sweet corn does not need indoor starting in Washington — direct sow around May 14–24 once soil reaches 60°F. Indoor starting is not recommended for this crop because sweet corn transplants poorly.