When to Plant Onion Seeds: Complete Zone-by-Zone Guide

Plant onion seeds 4-6 weeks before your last expected spring frost date for outdoor planting. Start seeds indoors 8-10 weeks before transplanting when soil temperatures reach 50°F (10°C), with optimal germination occurring between 60-75°F (15-24°C). Timing varies by USDA hardiness zone and onion type (short-day, intermediate-day, or long-day varieties).

Knowing exactly when to plant onion seeds transforms your gardening success rate. Whether you're a first-time gardener or expanding your vegetable patch, planting at the right moment ensures robust bulbs and maximizes your harvest. This guide delivers precise, zone-specific timing backed by agricultural research so you can plant with confidence—no guesswork required.

Understanding Onion Types and Their Planting Requirements

Not all onions grow the same way. Your planting schedule depends entirely on which variety you're growing, as each responds differently to daylight hours:

Onion Type	Day Length Requirement	Best Growing Regions	Planting Window
Short-Day Onions	10-12 hours of daylight	Southern US (Zones 7-10)	Oct-Dec for spring harvest
Intermediate-Day Onions	12-14 hours of daylight	Mid-US (Zones 5-7)	Feb-Mar for summer harvest
Long-Day Onions	14+ hours of daylight	Northern US (Zones 2-5)	Mar-Apr for late summer harvest

This USDA Plant Hardiness Zone Map reference shows why matching onion type to your region matters. Planting long-day varieties too far south causes premature bulbing and small yields, while short-day types in northern zones won't bulb properly due to insufficient daylight triggers.

Calculating Your Exact Planting Date

Follow this three-step process to determine your perfect onion seed planting schedule:

1. Find your last frost date using the USDA's interactive map or local extension office data
2. Count backward 4-6 weeks for direct outdoor sowing
3. For indoor starts, count back 8-10 weeks before last frost date

Soil temperature matters as much as calendar dates. Onion seeds won't germinate properly in cold soil below 50°F (10°C). Use a soil thermometer to verify conditions—the University of Minnesota Extension confirms optimal germination occurs between 60-75°F (15-24°C).

Zone-Specific Planting Timelines

Your USDA hardiness zone determines whether to start seeds indoors or directly sow outdoors. This planting timeline shows when to plant onion seeds by zone:

• Zones 2-4: Start seeds indoors Feb 15-Mar 1; transplant Apr 1-May 15
• Zones 5-6: Start seeds indoors Feb 1-Mar 15; transplant Mar 15-Apr 30
• Zones 7-8: Direct sow Jan 15-Feb 28; indoor starts Dec 1-Jan 15
• Zones 9-10: Direct sow Oct 15-Dec 1; indoor starts Sep 1-Oct 15

These recommendations align with The Old Farmer's Almanac planting data collected from cooperative extension services nationwide. Gardeners in microclimates should adjust based on local conditions—coastal areas may plant 2-3 weeks later than inland regions at the same latitude.

Preparing Soil for Successful Onion Growth

Onions demand well-prepared soil for optimal bulb development. Follow these preparation steps 2-3 weeks before planting:

• Test soil pH (ideal range: 6.0-7.0) using a home kit or extension service
• Amend heavy clay soils with 3-4 inches of compost and coarse sand
• Work in 2-3 inches of aged manure or balanced organic fertilizer
• Create raised beds 4-6 inches high for improved drainage

Avoid fresh manure which can cause excessive top growth at the expense of bulb development. The Oregon State University Extension notes onions require consistent moisture but will rot in waterlogged conditions—proper soil preparation prevents this common failure point.

Step-by-Step Planting Instructions

Whether starting seeds indoors or direct sowing, follow these precise planting techniques:

Indoor Seed Starting (Recommended for Short Seasons)

1. Fill seed trays with sterile seed-starting mix
2. Sow seeds 1/4 inch deep, spacing 1 inch apart
3. Water gently and maintain 65-75°F (18-24°C) soil temperature
4. Provide 14 hours of light daily once sprouted
5. Harden off seedlings 7-10 days before transplanting

Direct Outdoor Sowing (Best for Mild Climates)

1. Prepare soil as described above
2. Sow seeds 1/2 inch deep in rows 12-18 inches apart
3. Cover lightly with soil and water gently
4. Thin seedlings to 2-4 inches apart when 4 inches tall
5. Mulch with straw once established to control weeds

Proper spacing is critical—onions need room to develop full-sized bulbs. The Penn State Extension research shows overcrowded onions produce 30-50% smaller bulbs due to competition for nutrients.

Troubleshooting Common Planting Mistakes

Avoid these frequent errors that reduce onion yields:

• Planting too early in cold soil causes poor germination—wait until soil consistently reaches 50°F
• Overcrowding seedlings results in small bulbs—thin aggressively to recommended spacing
• Inconsistent watering creates double bulbs—maintain even moisture (1 inch per week)
• Planting wrong variety for your zone—verify day-length requirements before purchasing seeds

If seeds fail to germinate after 10 days, check soil temperature and moisture levels. Onion seeds remain viable for 1-2 years when stored properly, but germination rates decline significantly after the first year according to Association of Official Seed Analysts data.

Post-Planting Care for Maximum Bulb Development

After planting, follow this care schedule to ensure healthy onion growth:

• First 4 weeks: Water 1 inch per week, keep weeds controlled
• Weeks 5-8: Apply balanced fertilizer, increase watering to 1.5 inches weekly
• Weeks 9-12: Reduce watering as bulbs mature, stop fertilizing
• Final 2 weeks: Bend tops over to signal bulbs to stop growing

Watch for these maturity signs indicating harvest time: tops falling over naturally, necks softening, and outer skins becoming papery. Harvest when 50-75% of plants have fallen over for best storage quality.

Can I plant onion seeds in the fall for spring harvest?

Yes, in zones 7 and warmer, plant short-day onion seeds in October or November for spring harvest. In colder zones, fall planting risks winter kill unless you use heavy mulch protection.

How deep should I plant onion seeds?

Plant onion seeds 1/4 to 1/2 inch deep. Planting too deep prevents germination, while too shallow causes drying out. Cover lightly with soil and water gently after sowing.

What temperature kills onion seedlings?

Extended exposure below 28°F (-2°C) will kill onion seedlings. Protect young plants with row covers when temperatures drop below 32°F (0°C), especially during the first 3-4 weeks after transplanting.

How long do onion seeds take to germinate?

Onion seeds typically germinate in 7-14 days at optimal soil temperatures of 60-75°F (15-24°C). Germination slows significantly in cooler soil below 50°F (10°C), taking 2-3 weeks or failing completely.