When to Plant Onion in Madison County, ID
May in the garden — Madison County, Idaho
May rewards gardeners who work with the weather, not against it. Here's how to stay in step this month in Madison County, Idaho.
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Set out onion seedlings
Water the tray well an hour before you transplant. Roots slide out cleanly and settle in faster.
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Seed onion outdoors
Thin ruthlessly once seedlings are up. Crowded roots mean smaller crops from every plant.
Onions are a fundamental kitchen staple available in yellow, white, and red varieties. Choose long-day, short-day, or intermediate types based on your latitude.
Madison County, Idaho is in USDA Zone 5a. The average last spring frost is May 31 and the first fall frost is September 11, giving you a growing season of approximately 103 days.
At an elevation of 6,385 feet, Madison County receives approximately 19 inches of rainfall annually with predominantly loam soil. Summer highs average 85°F, providing good warmth for Onion during the growing season. With low rainfall, drip irrigation is essential for growing Onion successfully. Mulch heavily to conserve soil moisture.
Madison County Soil Profile
Soil Type
Loam
Soil pH
6.2-7.8
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 Madison County
How your county's soil matches Onion's growing requirements.
Soil pH
Your soil pH (6.2–7.8) overlaps with Onion's range (6.0–7.0), though not a perfect match.
Soil Texture
The loam soil in Madison County is excellent for Onion — good drainage, moisture retention, and nutrient holding capacity.
Organic Matter
Organic matter is moderate (2.3%). Annual compost additions will help Onion.
How to Plant Onion
Fall planting: Sow 10 weeks before your first frost date for a fall harvest.
Plant Water Budget
Water stress score is 8/10 — consider drought-tolerant varieties and mulching
Monthly Watering Guide for Onion
Onion needs approximately 1 inches of water per week (4.3" per month). Here's how your county's rainfall compares month by month.
| Month | Onion Needs | Rainfall | You Supplement | Action |
|---|---|---|---|---|
| Jan | — | 1.1" | 0" | ❄️ Dormant |
| Feb | — | 1" | 0" | ❄️ Dormant |
| Mar | — | 1.9" | 0" | ❄️ Dormant |
| Apr | — | 2.1" | 0" | ❄️ Dormant |
| May | 4.3" | 2.5" | 1.8" | 💧 Light watering |
| Jun | 4.3" | 1.2" | 3.1" | 🚿 Regular watering |
| Jul | 4.3" | 1.6" | 2.7" | 🚿 Regular watering |
| Aug | 4.3" | 2.2" | 2.1" | 💧 Light watering |
| Sep | 4.3" | 1.6" | 2.7" | 🚿 Regular watering |
| Oct | — | 1.6" | 0" | ❄️ Dormant |
| Nov | — | 1.3" | 0" | ❄️ Dormant |
| Dec | — | 1" | 0" | ❄️ Dormant |
Water needs are for active growing months only (May–Sep in Madison County). Supplement amounts are based on average rainfall — actual needs vary with temperature, soil, and mulching.
Onion 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.
Onion Planting Timeline — Madison County, ID
Onion Planting Calendar
| Activity | When | Date Range |
|---|---|---|
| Start Indoors | April 26 | Apr 26 – May 10 |
| Transplant Outdoors | May 31 | May 31 – Jun 14 |
| Direct Sow | May 17 | May 17 – Jun 7 |
| Harvest | August 30 | Aug 30 – Oct 18 |
| Fall Sowing | July 3 | Jul 3 – Jul 17 |
Plant 1" deep · 6" apart · Rows 12" apart
Month-by-Month Timeline
| Month | Activities |
|---|---|
| January | — |
| February | — |
| March | — |
| April | Start Indoors |
| May | Start Indoors Transplant Outdoors Direct Sow |
| June | Transplant Outdoors Direct Sow |
| July | Fall Sowing |
| August | Harvest |
| September | Harvest |
| October | Harvest |
| November | — |
| December | — |
Growing Conditions
☀️ Sun
Full Sun (6-8+ hours)
💧 Water
1"/week · Only during dry spells
📅 Days to Maturity
90–120 days
🧪 Soil pH
Needs 6–7 · Your soil: acceptable
🗺️ USDA Zone
Zone 5a
📆 Growing Season
103 days in Madison County
Growing Tips for Onion in Madison County
Direct sow Onion outdoors after May 31 in Madison County when soil has warmed and frost danger has passed.
Your 103.0-day growing season in Madison County is tight for Onion (90.0-120.0 days to maturity). Start indoors and choose early-maturing varieties.
Common pests for Onion in this region include onion maggots and thrips. Use row covers early in the season and inspect plants weekly.
Madison County receives only 19" of rain annually. Onion needs consistent moisture — install drip irrigation or water deeply 2-3 times per week.
General growing tips
Start seeds indoors 8-10 weeks before transplanting. Match day-length type to your latitude. Stop watering when tops begin to fall over and cure bulbs for 2-3 weeks before storage.
Companion Planting
Good Companions
Avoid Planting Near
Check more plant combinations with our Companion Planting Checker →
Onion in Other Locations
Frequently Asked Questions
When should I plant Onion in Madison County, ID?
Madison County is in Zone 5a with an average last frost of May 31. Plan your Onion planting based on this frost date — see the calendar above for exact timing.
What planting zone is Madison County, ID?
Madison County, Idaho is in USDA Hardiness Zone 5a. The average last spring frost is May 31 and first fall frost is September 11.
Your Madison County Garden Planner — Free
A 24-page printable planner built for Madison County (Zone 5a). Planting dates, a month-by-month schedule, harvest log, seed inventory, and succession charts — all dialed in for your exact growing season.