A personalized planting guide from real 30-year climate data and crop agronomy
Your garden in Seattle, Washington (USDA Zone 8b) has a mild maritime climate with cool, wet winters and warm, dry summers. The frost-free window runs from an average last frost of March 20 to an average first fall frost of November 26 — about 251 days total. This number is generous, but not all of it is equally useful for every crop.
Below is the raw climate data (from Open-Meteo ERA5 30-year historical climate + USDA Plant Hardiness Zone Map). Your personalized report also applies the decision engine that turns these numbers into actual planting guidance — because a frost date is a safety boundary, not the start of the planting season for warm-season crops.
The last frost date tells us when hard freezes stop being the main threat — the earliest point you'd even consider putting warm-season plants out. But it does not tell us when to actually plant. Warm-season crops like tomato and basil need the soil to warm up and stay warm after frost risk clears. Cool-season crops like lettuce are the opposite — they grow best in the shoulder seasons, before summer heat arrives, and frost tolerance is not the gate.
Each crop below includes both the agronomy facts (spacing, sun, etc.) and the decision-engine result — the planting window, strategy, and limiting factor. This is what your personalized report does automatically for every crop you select.
Frost tolerance: Tender · Days to maturity: ~75 · Optimal range: 64°F–86°F
Suitability for Seattle Zone 8b: ★★★★☆ — Good. The 251-day growing season supports this crop, but the 1,128 GDD budget is moderate — so early-maturing varieties help.
Sunlight: Full sun (8+ hours) · Spacing: 4 sq ft per plant · Soil pH target: ~6.6
Decision-engine result:
Decision trace: Frost Risk Evaluation (source: Open-Meteo ERA5) → GDD 1,128 supports maturity (source: Open-Meteo ERA5) → Growing season 251 days supports this crop (source: Open-Meteo ERA5). Confidence: High.
Action guidance: Start seedlings indoors 6–8 weeks before the window opens. Transplant only once the soil is consistently warm. Use black plastic mulch to capture heat. Watch for late blight during wet late-summer spells.
Frost tolerance: Very Tender · Days to maturity: ~50 · Optimal range: 64°F–86°F
Suitability for Seattle Zone 8b: ★★★★☆ — Good. Basil is a very-tender warm-season crop and needs more warmth than tomato.
Sunlight: Full sun (6+ hours) · Spacing: 6–12 inches
Decision-engine result:
Action guidance: Pinch off flower buds to encourage bushier growth. Harvest from the top to keep the plant producing. Don't rush transplanting — basil sulks in cool soil and will catch up faster if you wait.
Frost tolerance: Semi-hardy · Days to maturity: ~45 · Optimal range: 54°F–72°F
Suitability for Seattle Zone 8b: ★★★★★ — Excellent. Lettuce is a cool-season crop that grows well in spring and fall.
Sunlight: Full sun to partial shade (4+ hours) · Spacing: 6–10 inches
Decision-engine result:
Action guidance: Direct-sow early spring and again in fall for two harvests. Provide afternoon shade during warm spells to prevent bolting. Succession-sow every 2–3 weeks through early May for a continuous spring harvest.
This calendar translates the decision-engine windows above into month-by-month actions for Seattle Zone 8b. Remember: it is a guide, not a hard rule — your actual soil warm-up, microclimate, and garden conditions determine when each step really happens.
| Month | Tomato (warm-season, tender) | Basil (warm-season, very tender) | Lettuce (cool-season, heat-avoid) |
|---|---|---|---|
| February | Start indoors (6–8 weeks pre-window) | — | — |
| March | Indoor care, hardening off | Start indoors (late March) | Direct sow early — before heat sets in |
| April | Monitor soil temp; prepare beds; black plastic optional | Indoor care | Harvest begins; succession sow through early May |
| May | Transplant — soil warmed, nights mild | Transplant — nights stay above 55°F (13°C) | Full harvest; watch for summer heat |
| June | Grow — full sun, consistent watering | Harvest begins; pinch to extend season | — heat too warm, hold until fall |
| July–August | Harvest — consistent soil moisture | Harvest | — |
| September | Harvest — green fruit ripen indoors as nights cool | Final harvest; cover on cold nights | Direct sow — fall window opens as heat cools |
Seattle-area soils are often loamy to clay-loam with slightly acidic pH. Most vegetable crops tolerate 5.5–7.5, but testing is worth it — the formal report includes soil guidelines calibrated to your specific conditions. A balanced organic fertilizer applied at planting and side-dressed mid-season is a safe general approach.
Seattle's climate means you'll water infrequently but deeply in summer (1–2 inches per week July–September) and rarely in winter. Drip irrigation or soaker hoses are ideal — they deliver water directly to the root zone and reduce fungal disease risk on wet foliage. Consistent watering matters more than quantity: erratic moisture causes blossom-end rot in tomatoes and splits in fruit.
Black plastic mulch, raised beds, and south-facing walls all help warm soil faster in spring. For tomato, basil, and other warm-season crops, testing soil warmth with your hand is more reliable than checking the calendar. If the soil still feels cold at 6 inches deep, wait another week — it's a more trustworthy signal than any frost date.
Every section above — planting windows, suitability, action guidance — comes from TheYardWise Agronomy Rules Engine. The engine combines real 30-year climate data (Open-Meteo ERA5), USDA hardiness zones, and crop agronomy (frost tolerance, GDD needs, optimal temperature ranges). You can inspect the trace below for this sample — your personalized report will include the full trace for every crop you select.
Confidence: High — decision derived from real city climate data and crop physiology.
Data Sources: Open-Meteo ERA5 30-year historical climate · USDA Plant Hardiness Zone Map · TheYardWise Plant Database · Crop Agronomy Database. See theyardwise.com/data-sources for the full methodology.
This is a sample — your plan will be tailored to your exact city, climate, soil, and crop choices. It usually arrives within a few minutes.
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