Onion Sowing Guide: When & How to Plant Successfully

The best time to sow onions depends on your climate zone: start seeds indoors 8-10 weeks before last frost, or plant sets directly in garden soil when it reaches 50°F (10°C). Proper spacing (4-6 inches apart), soil preparation (well-draining, pH 6.0-7.0), and consistent moisture are critical for successful onion growth and bulb development.

Master Onion Sowing: Your Complete Planting Roadmap

Whether you're a first-time gardener or refining your technique, understanding onion sowing fundamentals transforms your harvest. Onions require precise timing and care during planting to develop those perfect, layered bulbs. This guide delivers actionable steps backed by agricultural research to ensure your onion crop thrives from seed to harvest.

Planning Your Onion Planting Strategy

Successful onion growing starts with matching varieties to your climate. Day-length requirements determine whether onions form bulbs in your region:

Onion Type	Day Length Requirement	Best Zones	Planting Window
Short-Day (Sweet)	10-12 hours	7-11	Oct-Dec (fall planting)
Intermediate-Day	12-14 hours	5-7	Feb-Mar (early spring)
Long-Day (Storage)	14-16 hours	3-5	Mar-Apr (as soil thaws)

According to the USDA Plant Hardiness Zone Map, northern gardeners (zones 3-5) achieve best results with long-day varieties like 'Yellow Sweet Spanish', while southern growers (zones 7-11) should choose short-day types such as 'Texas Grano'. Intermediate varieties work well in transition zones (5-7) with 'Candy' being a reliable performer.

Soil Preparation Essentials

Onions demand well-prepared soil for optimal root development. Two weeks before planting:

• Test soil pH (ideal range 6.0-7.0) using a reliable kit
• Amend heavy clay soils with 3 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

The University of Minnesota Extension confirms onions perform poorly in compacted or waterlogged soils, which cause rot and stunted growth. Avoid planting where other alliums grew recently to prevent disease buildup.

Step-by-Step Sowing Instructions

Follow these precise steps whether planting seeds or sets:

For Onion Seeds (Indoors)

1. Start seeds 8-10 weeks before last frost date
2. Use seed trays with drainage holes and sterile mix
3. Sow seeds 1/4 inch deep, 1 inch apart
4. Maintain soil temperature at 65-75°F (18-24°C)
5. Provide 14 hours of light daily once sprouted
6. Transplant when seedlings reach pencil thickness

For Onion Sets (Direct Garden Planting)

1. Plant when soil temperature reaches 50°F (10°C)
2. Choose sets under 1 inch in diameter for best results
3. Position with pointed end up, 1-2 inches deep
4. Space 4-6 inches apart in rows 12-18 inches apart
5. Water immediately after planting

Research from Oregon State University Extension shows planting depth directly impacts bulb formation—too shallow causes poor root development, while too deep delays emergence. The optimal 1-2 inch depth balances these factors.

Critical Post-Planting Care

What happens after sowing determines your harvest quality:

Water Management

Maintain consistent moisture (1 inch per week) without saturation. Use drip irrigation or soaker hoses to keep foliage dry and prevent disease. Reduce watering 2-3 weeks before harvest to encourage proper bulb curing.

Thinning Schedule

• When seedlings reach 4 inches: thin to 2 inches apart
• At 6 inches tall: final thinning to 4-6 inches apart
• Use thinnings as green onions in salads

Fertilization Timing

Apply balanced fertilizer (10-10-10) when plants reach 6 inches tall, then every 3-4 weeks until bulbs begin forming. Stop fertilizing 4 weeks before expected harvest to prevent excessive top growth.

Troubleshooting Common Sowing Problems

Address these frequent onion-growing challenges promptly:

Double Bulbing

Cause: Temperature fluctuations during early growth
Solution: Plant sets at proper depth; avoid early planting in cold soil

Poor Bulb Formation

Cause: Incorrect day-length variety for your zone
Solution: Match varieties to your region's daylight patterns

Onion Maggots

Prevention: Rotate crops annually; use floating row covers at planting
Control: Apply beneficial nematodes if infestation occurs

Harvesting and Storage Guidelines

Harvest onions when tops naturally fall over and yellow. Cure bulbs for 2-3 weeks in a warm, dry, well-ventilated area before storage. Store cured onions in mesh bags in a cool (32-40°F), dark location with 65-70% humidity. Properly stored onions maintain quality for 4-6 months.

Advanced Sowing Techniques

For gardeners seeking higher yields:

• Succession planting: Sow every 2-3 weeks for continuous harvest
• Interplanting: Grow with carrots (different root depths)
• Cold frames: Extend season in northern climates

When is the best time to sow onion seeds outdoors?

Sow onion seeds outdoors 4-6 weeks before your last expected frost date when soil temperature reaches at least 50°F (10°C). In warmer climates (zones 7+), fall planting (October-November) works best for short-day varieties.

How deep should I plant onion sets?

Plant onion sets 1-2 inches deep with the pointed end facing up. In heavier soils, plant slightly shallower (1 inch), while in sandy soils, plant slightly deeper (2 inches). Proper depth ensures optimal bulb formation and prevents premature bolting.

Can I grow onions from kitchen scraps?

Yes, you can regrow green onions from root ends, but full bulb onions require proper sowing techniques. Cut 1-2 inches from the root end of store-bought onions, plant in moist soil with roots down, and keep watered. This works best for green onions, not for developing full storage bulbs.

Why are my onion plants flowering prematurely?

Premature flowering (bolting) occurs due to temperature fluctuations, planting oversized sets, or incorrect variety selection. To prevent this, plant appropriate day-length varieties for your region, use sets under 1 inch in diameter, and avoid planting too early when cold snaps are possible.

How far apart should I space onion plants?

Space onion plants 4-6 inches apart in rows 12-18 inches apart. Smaller spacing (4 inches) works for green onions, while wider spacing (6 inches) produces larger storage bulbs. Proper spacing ensures adequate room for bulb expansion and reduces disease risk from overcrowding.