Planting Onion Sets: Complete Step-by-Step Guide

Plant onion sets by placing them 1-2 inches deep with pointed ends up, spacing 4-6 inches apart in early spring as soon as soil can be worked. Choose firm, disease-free bulbs 1/2-1 inch in diameter, plant in well-draining soil with pH 6.0-6.8, and water consistently for best results.

Onion sets offer home gardeners a reliable, beginner-friendly method for growing onions with minimal effort compared to seeds. These small pre-grown bulbs provide a head start on the growing season and deliver consistent results when planted correctly. Whether you're cultivating sweet Vidalias for summer salads or pungent storage onions for winter cooking, understanding the proper planting technique ensures a bountiful harvest.

Planning Your Onion Set Planting

Successful onion growing begins with proper timing and preparation. Onion sets should be planted early in the season when soil temperatures reach 50°F (10°C), typically 4-6 weeks before your last expected frost date. This timing allows bulbs to establish roots before summer heat triggers bulb formation.

Before planting, prepare your garden bed by:

• Testing soil pH (ideal range: 6.0-6.8)
• Amending with 2-3 inches of compost
• Ensuring proper drainage (onions dislike soggy conditions)
• Removing rocks and large debris

Onion Variety	Best Planting Zone	Days to Maturity	Storage Potential
Yellow Storage	3-9	100-120	6-8 months
Red Creole	5-10	90-110	4-6 months
Vidalia	7-9	95-105	2-3 months

According to the University of Minnesota Extension, selecting the right variety for your climate zone significantly impacts success rates. Short-day varieties (like Vidalias) require 10-12 hours of daylight to form bulbs, while long-day types (most storage onions) need 14-16 hours.

Step-by-Step Planting Process

Follow these steps for optimal onion set planting:

1. Prepare sets: Inspect bulbs and discard any soft, moldy, or sprouted specimens. Healthy sets should feel firm with dry outer skins.
2. Determine spacing: Arrange sets 4-6 inches apart in rows 12-18 inches apart, depending on variety size.
3. Planting depth: Place sets with pointed ends up, burying them 1-2 inches deep (deeper in sandy soils, shallower in clay).
4. Cover and firm: Gently cover with soil and press down to ensure good soil contact.
5. Water thoroughly: Provide 1 inch of water immediately after planting to settle the soil.

Unlike seeds, onion sets shouldn't be soaked before planting. The Oregon State University Extension confirms that pre-soaking can increase rot risk without improving germination rates.

Post-Planting Care Requirements

Proper care after planting ensures healthy bulb development:

• Watering: Maintain consistent moisture (1 inch per week), increasing slightly during bulb formation (June-July)
• Weeding: Hand-pull weeds carefully to avoid damaging shallow roots
• Fertilizing: Apply balanced fertilizer when tops reach 6 inches tall
• Pest monitoring: Watch for thrips and onion maggots, especially in warm weather

As your onions grow, you'll notice distinct development stages:

Weeks After Planting	Growth Stage	Care Focus
1-3	Root establishment	Consistent moisture
4-6	Top growth	Weed control, light feeding
7-10	Bulb formation	Increase watering, monitor pests
11-16	Bulb maturation	Reduce watering, prepare for harvest

This growth timeline from the USDA Agricultural Research Service demonstrates why consistent care throughout the season matters—interrupted growth can lead to double bulbs or premature bolting.

Harvesting and Storage Techniques

Harvest onions when tops naturally fall over and yellow (about 90-120 days after planting). Follow these steps:

1. Carefully lift bulbs using a garden fork
2. Cure in a warm, dry, well-ventilated area for 2-3 weeks
3. Trim roots and tops to 1 inch after curing
4. Store in mesh bags in a cool, dry place (32-40°F with 65-70% humidity)

Properly cured storage onions can last 6-8 months. Sweet varieties like Vidalias have thinner skins and shorter storage life (2-3 months). The Royal Horticultural Society recommends checking stored onions monthly and removing any showing signs of decay to prevent spoilage from spreading.

Troubleshooting Common Issues

Even with proper planting, gardeners sometimes encounter these problems:

• Premature bolting: Caused by temperature fluctuations; remove flower stalks immediately
• Small bulbs: Usually from overcrowding or nutrient deficiency
• Rotting sets: Indicates poor drainage or overwatering
• Thick necks: Results from excess nitrogen or late-season watering

Remember that onion sets naturally produce smaller bulbs than seed-grown plants, typically reaching 1.5-2.5 inches in diameter. This makes them ideal for green onions or small cooking onions rather than large storage varieties.

Can you plant onion sets in containers?

Yes, onion sets grow well in containers at least 8 inches deep with drainage holes. Use quality potting mix, plant sets 4 inches apart, and maintain consistent moisture. Container-grown onions may need more frequent watering than garden beds.

How deep should onion sets be planted?

Plant onion sets 1-2 inches deep with the pointed end facing up. In heavy clay soils, plant slightly shallower (1 inch); in sandy soils, plant slightly deeper (2 inches). The tip of the set should be just below the soil surface.

Why are my onion sets not growing?

Common reasons include planting too deep, cold soil temperatures below 40°F, poor drainage causing rot, or using dried-out sets. Ensure sets are firm before planting and maintain consistent moisture without waterlogging the soil.

When should I stop watering onions before harvest?

Reduce watering 2-3 weeks before harvest when tops begin to yellow and fall over. Completely stop watering when half the tops have naturally fallen to encourage proper curing and prevent rot during storage.