When to Plant Sweet Corn in Napa County, CA
May in the garden — Napa County, California
Here's what deserves your attention in Napa County, California this month. Everything below is tailored to Zone 9b and timed around your local frost dates.
-
Start harvesting sweet corn
Check every 1–2 days. Many of these get tough or go to seed if you wait too long.
Looking ahead to June
- First harvests: sweet corn
Sweet corn is bred for high sugar content in its kernels, which convert to starch rapidly after harvest. Modern supersweet varieties hold their sweetness longer.
Napa County, California is in USDA Zone 9b. The average last spring frost is February 26 and the first fall frost is November 26, giving you a growing season of approximately 273 days.
At an elevation of 449 feet, Napa County receives approximately 35 inches of rainfall annually with predominantly loam soil. Summer highs average 88°F, providing good warmth for Sweet Corn during the growing season.
Napa County Soil Profile
Soil Type
Loam
Soil pH
6.7-7.5
Drainage
Well Drained
Plant Planting Risk Windows
Percentages indicate frost risk at transplant. The 70% safe window means there is a 30% chance of frost after transplant — suitable for cold-hardy crops or gardeners with frost protection. The 90% safe window is best for tender plants.
Soil Compatibility in Napa County
How your county's soil matches Sweet Corn's growing requirements.
Soil pH
Your soil pH (6.7–7.5) is more alkaline than Sweet Corn prefers (6.0–7.0). Add sulfur or peat moss to lower pH.
Soil Texture
The loam soil in Napa County is excellent for Sweet Corn — good drainage, moisture retention, and nutrient holding capacity.
Drainage
Drainage is adequate for Sweet Corn.
Organic Matter
Organic matter is moderate (3.0%). Annual compost additions will help Sweet Corn.
How to Plant Sweet Corn
Succession Planting Sweet Corn
Sow every 6.9 weeks. Last sowing by Aug 28 to harvest before frost.
Plant Water Budget
Water stress score is 6/10 — consider drought-tolerant varieties and mulching
Monthly Watering Guide for Sweet Corn
Sweet Corn needs approximately 1.5 inches of water per week (6.5" per month). Here's how your county's rainfall compares month by month.
| Month | Sweet Corn Needs | Rainfall | You Supplement | Action |
|---|---|---|---|---|
| Jan | — | 7.5" | 0" | ❄️ Dormant |
| Feb | 6.5" | 7.8" | 0" | ✅ Rainfall sufficient |
| Mar | 6.5" | 5.8" | 0.7" | 💧 Light watering |
| Apr | 6.5" | 2.3" | 4.2" | 🚿 Regular watering |
| May | 6.5" | 0.8" | 5.7" | 🚿 Regular watering |
| Jun | 6.5" | 0.2" | 6.3" | 🚿 Regular watering |
| Jul | 6.5" | 0" | 6.5" | 🚿 Regular watering |
| Aug | 6.5" | 0" | 6.5" | 🚿 Regular watering |
| Sep | 6.5" | 0.3" | 6.2" | 🚿 Regular watering |
| Oct | 6.5" | 1.3" | 5.2" | 🚿 Regular watering |
| Nov | 6.5" | 3.3" | 3.2" | 💧 Light watering |
| Dec | — | 5.6" | 0" | ❄️ Dormant |
Water needs are for active growing months only (Feb–Nov in Napa County). Supplement amounts are based on average rainfall — actual needs vary with temperature, soil, and mulching.
Sweet Corn Heat Requirements (GDD)
What are Growing Degree Days (GDD)?
Growing Degree Days measure the total warmth your plants receive during the growing season. Think of it as a "heat bank" — every day above 50°F deposits warmth that helps your plants grow.
Each plant needs a certain amount of accumulated heat to mature. If your county provides more GDD than the plant needs, it's a great fit. If it's close, you may want to choose faster-maturing varieties or start seeds indoors to get a head start.
Sweet Corn Planting Timeline — Napa County, CA
Sweet Corn Planting Calendar
| Activity | When | Date Range |
|---|---|---|
| Direct Sow | March 5 | Mar 5 – Mar 26 |
| Harvest | May 7 | May 7 – Jun 18 |
Plant 1" deep · 12" apart · Rows 36" apart
Month-by-Month Timeline
| Month | Activities |
|---|---|
| January | — |
| February | — |
| March | Direct Sow |
| April | — |
| May | Harvest |
| June | Harvest |
| July | — |
| August | — |
| September | — |
| October | — |
| November | — |
| December | — |
Growing Conditions
☀️ Sun
Full Sun (6-8+ hours)
💧 Water
1.5"/week · 2-3 times/week
📅 Days to Maturity
60–90 days
🧪 Soil pH
Needs 6–7 · Your soil: too_alkaline
🗺️ USDA Zone
Zone 9b
📆 Growing Season
273 days in Napa County
Growing Tips for Sweet Corn in Napa County
Direct sow Sweet Corn outdoors after February 26 in Napa County when soil has warmed and frost danger has passed.
Common pests for Sweet Corn in this region include corn earworm and corn borers. Use row covers early in the season and inspect plants weekly.
General growing tips
Plant in blocks of at least 4 rows for wind pollination. Direct sow after soil is warm. Isolate supersweet varieties from other corn types to prevent cross-pollination.
Companion Planting
Good Companions
Avoid Planting Near
Check more plant combinations with our Companion Planting Checker →
Sweet Corn in Other Locations
Frequently Asked Questions
When should I plant Sweet Corn in Napa County, CA?
Napa County is in Zone 9b with an average last frost of February 26. Plan your Sweet Corn planting based on this frost date — see the calendar above for exact timing.
What planting zone is Napa County, CA?
Napa County, California is in USDA Hardiness Zone 9b. The average last spring frost is February 26 and first fall frost is November 26.
Your Napa County Garden Planner — Free
A 24-page printable planner built for Napa County (Zone 9b). Planting dates, a month-by-month schedule, harvest log, seed inventory, and succession charts — all dialed in for your exact growing season.