When to Sow Onion Sets: Complete Planting Guide

The optimal time to sow onion sets is 2-4 weeks before your last expected spring frost date, when soil temperatures reach 40°F (4°C). For most temperate regions, this means planting between February and April. In milder climates, fall planting (September-October) for overwintering is also effective. Proper timing ensures onions establish strong roots before bulb formation begins.

Planting onion sets at the right moment makes the difference between thriving bulbs and disappointing harvests. As a gardener, you need precise timing guidance based on your specific climate conditions—not generic advice that ignores regional variations. This guide delivers actionable planting schedules backed by horticultural research, helping you maximize your onion yield regardless of your location.

Understanding Onion Sets and Their Planting Requirements

Onion sets are small, partially grown onion bulbs specifically cultivated for planting rather than consumption. Unlike seeds, which require 100-120 days to mature, sets typically mature in 80-100 days, making them ideal for shorter growing seasons. The Royal Horticultural Society confirms that sets offer higher success rates for beginners due to their resilience against variable spring conditions.

Three critical factors determine your perfect planting window:

• Soil temperature (must be at least 40°F/4°C)
• Last frost date for your specific location
• Onion variety (short-day, intermediate-day, or long-day types)

Regional Planting Timelines for Maximum Yield

Your geographic location dramatically impacts when you should sow onion sets. The USDA Agricultural Research Service provides clear regional guidelines based on decades of cultivation data. Understanding your specific climate zone prevents premature planting that leads to bolting (premature flowering) or delayed planting that reduces bulb size.

Climate Zone	Optimal Planting Window	Soil Temperature Requirement	Special Considerations
Northern US/Canada (Zones 3-5)	March to early April	40-50°F (4-10°C)	Wait until soil is workable; use mulch to regulate temperature
Midwestern US (Zones 5-7)	February to March	45-55°F (7-13°C)	Watch for late frosts; plant sets 1¼" deep
Southern US (Zones 7-10)	October to November (fall)	50-65°F (10-18°C)	Use short-day varieties; avoid spring planting
UK/Europe	February to March	40-50°F (4-10°C)	Protect from excessive rain; ensure good drainage

Step-by-Step Planting Process for Success

Following these steps ensures your onion sets establish properly and develop into full-sized bulbs:

1. Prepare the soil 2 weeks before planting by adding 2-3 inches of compost and working it into the top 6 inches of soil. Onion sets prefer loose, well-draining soil with a pH between 6.0-7.0.
2. Check soil temperature using a soil thermometer at 9 AM for three consecutive days. Plant only when readings consistently hit 40°F (4°C) or higher.
3. Plant sets with the pointed end up, 1-1¼ inches deep and 4-6 inches apart in rows spaced 12-18 inches apart. Deeper planting in sandy soils (up to 2 inches) prevents bulbs from pushing out.
4. Water immediately after planting with 1 inch of water, then maintain consistent moisture (about 1 inch per week) until bulbs begin forming.

Avoiding Common Planting Mistakes

Many gardeners encounter problems due to timing errors. The Cornell University Cooperative Extension identifies these frequent issues:

• Planting too early in cold, wet soil causes sets to rot before sprouting
• Planting too late reduces bulb development time, yielding smaller onions
• Misidentifying day-length requirements for your region (long-day varieties need 14-16 hours of daylight)
• Overcrowding prevents proper bulb expansion

For northern gardeners, remember that onion sets need approximately 4-6 weeks of cool weather (45-75°F) for proper leaf development before the 14+ hour days trigger bulb formation. Southern gardeners should select short-day varieties that form bulbs with 10-12 hours of daylight.

Monitoring Growth and Adjusting Care

After planting, track these growth milestones to ensure your timing was correct:

• Weeks 1-2: Sprouts emerge (if not visible within 14 days, check soil temperature)
• Weeks 3-6: Leaf growth accelerates (critical period for consistent watering)
• Weeks 7-10: Bulb formation begins (reduce watering slightly)
• Weeks 11-14: Bulbs reach maturity (stop watering when tops begin falling over)

If your onion tops fall over prematurely (before bulbs have developed), this indicates stress from either temperature fluctuations or inconsistent moisture. The Michigan State University Extension recommends applying a balanced fertilizer when plants reach 6 inches tall to support healthy development.

Special Considerations for Challenging Climates

Certain conditions require adjusted planting strategies:

• Cold climates: Use black plastic mulch to warm soil 2-3 weeks before planting
• Wet springs: Plant in raised beds for better drainage (onion roots rot in standing water)
• Short seasons: Start sets indoors 2-3 weeks before last frost for a head start
• Warm climates: Plant in fall for winter harvest; choose Egyptian walking onions for continuous production

Remember that onion sets have limited cold tolerance—they can survive light frosts (down to 20°F/-7°C) but prolonged freezing temperatures will damage them. The University of California Agriculture and Natural Resources advises covering newly planted sets with row covers if temperatures drop below 25°F (-4°C).

When to Harvest Your Onion Crop

Proper planting timing directly affects harvest success. Most onion varieties reach maturity 80-100 days after planting. Watch for these harvest indicators:

• Natural yellowing and falling of the top foliage
• Neck of the bulb becoming soft and beginning to collapse
• Outer skin turning dry and papery

Harvest during dry weather, and allow onions to cure in a warm, dry, well-ventilated area for 2-3 weeks before storage. Properly cured onions from correctly timed plantings can store for 6-8 months in cool, dry conditions.

Frequently Asked Questions

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

Yes, in USDA zones 7 and warmer, fall planting (September-October) works well for overwintering. Plant sets 2 inches deep 6-8 weeks before first hard frost. They'll establish roots before winter dormancy and resume growth early spring.

What happens if I plant onion sets too early in spring?

Planting too early in cold, wet soil causes sets to rot before sprouting. If sets do sprout but experience prolonged cold below 45°F (7°C), they may bolt (send up flower stalks), which reduces bulb size and storage life. Wait until soil temperature reaches at least 40°F (4°C).

How deep should I plant onion sets?

Plant onion sets 1 to 1¼ inches deep with the pointed end up. In sandy soils, plant slightly deeper (up to 2 inches) to prevent bulbs from pushing out of the ground. Never cover the neck of the set, as this can cause rot.

Do onion sets need full sun or can they grow in partial shade?

Onion sets require full sun (at least 6-8 hours daily) for optimal bulb development. While they'll grow in partial shade, bulbs will be significantly smaller. Insufficient sunlight causes excessive leaf growth at the expense of bulb formation.

How do I know which onion variety to choose for my region?

Select varieties based on your daylight hours: long-day (14-16 hours) for northern regions, intermediate-day for mid-latitudes, and short-day (10-12 hours) for southern regions. Check seed catalogs for regional recommendations, or consult your local cooperative extension office for proven varieties in your area.