When to Plant Basil in Washington
Published: April 21, 2026
Washington gardeners in Zone 4b have a 21-week frost-free window for basil (May 7 to October 1). Warm-season crops like basil 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.
Basil Planting Dates for Washington
| Start seeds indoors | March 26–April 2 |
| Last frost (average) | May 7 |
| Transplant outdoors | May 21–31 |
| Direct sow outdoors | May 21–31 |
| Expect first harvest | June 20 – July 20 |
| First fall frost (average) | October 1 |
Growing Basil in Washington's Climate
Washington's cold Zone 4b climate puts warm-season basil 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 basil needs for aggressive root growth.
Common basil pests to watch for in Washington include Aphids and Spider Mite. The first line of defense is companion planting: Tomato and Pepper 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.
Basil Garden Calendar for Washington
| Month | Task | Notes |
|---|---|---|
| January | — | Dormant season |
| February | — | Dormant season |
| March | Start seeds indoors | Use grow lights; warm soil to 70–75°F for best germination |
| April | — | Dormant season |
| May ← | Transplant outdoors | Soil temp must be 60°F+; nighttime lows above 50°F |
| 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 basil; cover plants on frost nights |
| November | — | Dormant season |
| December | — | Dormant season |
Basil Growing Tips for Washington Gardeners
- •Never expose basil to temperatures below 50°F — even briefly — it turns black and dies.
- •Pinch flower buds immediately as they appear to keep leaves large and productive.
- •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.
- •Harvest by cutting whole stems from the top, leaving lower sets of leaves to branch out.
Companion Planting for Basil in Washington
In Washington's cold climate, planting basil with Tomato and Pepper helps deter pests and improve soil. Avoid planting near Sage and Fennel, which inhibit basil growth through root chemistry or shared pest pressure. See the full basil companion planting guide for the complete list.
Basil 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 basil in Washington?
In Washington (Zone 4b), start basil seeds indoors around March 26–April 2 and transplant outdoors around May 21–31 — 2 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 basil year-round in Washington?
Not year-round — Washington's cold season stops basil production. The basil season runs May 7 to October 1, roughly 21 weeks.
What basil varieties grow best in Washington?
For Washington's Zone 4b, Genovese and Dolce Fresca are fast-growing and bulk up quickly in short seasons.
When do I start basil seeds indoors in Washington?
Start basil seeds indoors in Washington around March 26–April 2, which is 6 weeks before the state's average last frost of May 7. Use grow lights and keep soil temperatures at 70–75°F for fastest germination.