How to Grow Onions: Complete Step-by-Step Guide

Discover exactly how to grow onions successfully with this comprehensive guide. You'll learn the optimal planting times for your region, proper soil preparation techniques, effective pest management strategies, and harvesting methods that maximize storage life. Whether you're using seeds, sets, or transplants, this guide provides actionable steps for growing flavorful, healthy onions in any garden space.

Onions rank among the most versatile and widely used vegetables in home gardens. With proper technique, you can enjoy fresh, flavorful onions throughout the growing season and beyond. This guide covers everything from selecting the right varieties for your climate to harvesting and storing your bounty.

Understanding Onion Types and Timing

Before planting, it's crucial to understand the three main onion types based on daylight requirements:

• Short-day onions (require 10-12 hours of daylight): Best for southern regions (USDA Zones 7 and south)
• Intermediate-day onions (require 12-14 hours): Suitable for middle regions (USDA Zones 5-7)
• Long-day onions (require 14+ hours): Ideal for northern regions (USDA Zones 3-5)

According to the USDA Plant Hardiness Zone Map, planting times vary significantly by region. Northern gardeners should plant in early spring as soon as soil can be worked, while southern gardeners achieve best results with fall planting for spring harvest.

Onion Type	Best Regions	Planting Time	Harvest Time
Long-day	Northern US & Canada	Early spring	July-August
Intermediate-day	Mid-United States	Late winter/early spring	June-July
Short-day	Southern US	Fall	April-May

Soil Preparation Essentials

Onions thrive in well-drained, loose soil with plenty of organic matter. Prepare your garden bed 2-3 weeks before planting:

• Test soil pH (ideal range: 6.0-6.8)
• Remove all rocks and debris that could impede bulb formation
• Amend with 2-3 inches of compost or well-rotted manure
• Add balanced organic fertilizer according to package instructions

For growing onions in containers, use a high-quality potting mix with excellent drainage. Choose containers at least 8 inches deep with multiple drainage holes.

Planting Methods Compared

You have three options for starting your onion crop, each with advantages:

Planting from Seeds

Start seeds indoors 8-10 weeks before last frost date. Sow ¼ inch deep in seed trays with sterile mix. Keep soil moist and provide 14 hours of light daily. Transplant outdoors when seedlings reach pencil thickness.

Using Onion Sets

For how to grow onions from seed alternatives, sets offer easier establishment. Plant sets 1-2 inches deep, pointed end up, 4-6 inches apart. This method produces earlier harvests but may bolt more readily in hot weather.

Transplanting Seedlings

Hardened seedlings establish quickly. Plant at same depth as in nursery pots, spacing 4-6 inches apart in rows 12-18 inches apart. Water thoroughly after planting.

Ongoing Care for Healthy Plants

Proper care ensures robust growth and prevents common problems:

• Watering: Provide 1 inch of water weekly, increasing during bulb formation. Avoid overhead watering to prevent disease.
• Weeding: Hand-pull weeds carefully to avoid damaging shallow roots. Mulch with straw to suppress weeds.
• Fertilizing: Apply nitrogen-rich fertilizer every 2-3 weeks until bulbing begins, then switch to phosphorus-potassium formula.

According to research from Cornell University Cooperative Extension, consistent moisture is critical during the bulbing stage (when tops reach 6-8 inches tall). Fluctuations in water availability can cause splitting or poor bulb development.

Managing Pests and Diseases

Prevention beats treatment for most onion problems:

• Onion maggots: Use floating row covers at planting. Rotate crops annually.
• Thrips: Spray with insecticidal soap when pests appear. Maintain adequate moisture.
• Pink root: Avoid overwatering. Plant resistant varieties like 'Copra' or 'Stuttgarter'.
• Downy mildew: Water at soil level. Provide good air circulation between plants.

For organic onion growing tips, interplant with carrots or beets, which repel many onion pests. Avoid planting near beans or peas, which can stunt onion growth.

Harvesting and Curing Onions

Timing your harvest correctly ensures maximum storage life:

• Watch for natural top fall-down (when 50-75% of tops have fallen)
• Cure onions in a warm, dry, well-ventilated area for 2-3 weeks
• Trim roots to 1 inch and tops to 1-1½ inches after curing
• Store in mesh bags or braided in a cool, dry, dark place

Research from University of California Agriculture and Natural Resources shows that proper curing significantly extends storage life. Onions cured at 75-80°F with good airflow develop protective outer layers that prevent moisture loss and decay.

Troubleshooting Common Issues

Address these frequent onion growing challenges:

• Onions not bulbing: Check day length requirements. Long-day varieties won't bulb properly in southern regions.
• Premature flowering (bolting): Caused by temperature fluctuations. Remove flower stalks immediately.
• Small bulbs: Overcrowding, poor soil fertility, or insufficient water during bulbing stage.
• Rotting bulbs: Excessive moisture or poor drainage. Improve soil structure before next planting.

Storing Your Homegrown Onions

Proper storage maintains quality for months:

• Store at 32-40°F with 65-70% humidity
• Check monthly and remove any showing signs of decay
• Long-day storage varieties (like 'Copra') keep 6-8 months
• Short-day varieties typically store 2-3 months

For how to store homegrown onions successfully, avoid storing near potatoes, which release moisture and gases that accelerate onion spoilage. Never store onions in plastic bags, which trap moisture.

Frequently Asked Questions

When is the best time to plant onions?

Planting time depends on your region. Northern gardeners should plant in early spring as soon as soil can be worked. Southern gardeners achieve best results planting in fall for spring harvest. Check your USDA hardiness zone for precise timing.

How deep should I plant onion sets?

Plant onion sets 1-2 inches deep with the pointed end facing up. Space them 4-6 inches apart in rows 12-18 inches apart. Planting too deep can delay growth, while planting too shallow may cause sets to dry out.

Why are my onions not forming bulbs?

Onions may not bulb due to incorrect day-length variety for your region, overcrowding, poor soil fertility, or inconsistent watering during bulbing stage. Ensure you've selected the right variety type (short-day, intermediate-day, or long-day) for your location.

How do I know when onions are ready to harvest?

Onions are ready when about 50-75% of the tops have naturally fallen over. The necks should feel soft and beginning to collapse. Stop watering when tops start falling to begin the curing process in the ground.