Growing Onions: Complete Guide from Planting to Harvest

Onions grow best when planted in early spring in well-drained soil with pH 6.0-6.8, spaced 4-6 inches apart, and receiving 1 inch of water weekly. Harvest when tops naturally fall over, typically 100-170 days after planting depending on variety.

Discover exactly how to grow perfect onions in your garden with this comprehensive guide. Whether you're a first-time gardener or looking to improve your harvest, you'll learn proven techniques for planting, caring for, and harvesting onions that yield larger bulbs with better flavor and storage potential.

Planning Your Onion Growing Journey

Successful onion growing starts with proper planning. Onions are cool-season crops that thrive in temperatures between 55°F and 75°F (13°C-24°C). The timing of your planting depends on your USDA hardiness zone:

USDA Hardiness Zone	Best Planting Time	Recommended Onion Type
Zones 3-5	Early spring (as soon as soil can be worked)	Long-day varieties
Zones 6-7	Late winter to early spring	Intermediate-day varieties
Zones 8-10	Fall planting	Short-day varieties

This planting schedule information aligns with recommendations from the USDA Agricultural Research Service, which emphasizes matching onion varieties to your specific daylight conditions for optimal bulb formation.

Choosing Your Onion Varieties

Understanding the three main onion types is crucial for success in your region:

• Short-day onions (10-12 hours of daylight): Best for southern regions (Zones 8-10), form bulbs when days are shorter
• Intermediate-day onions (12-14 hours): Suitable for middle latitudes (Zones 6-7)
• Long-day onions (14+ hours): Ideal for northern regions (Zones 3-5), require longer daylight to form bulbs

Popular varieties include 'Texas Sweet' (short-day), 'Candy' (intermediate-day), and 'Walla Walla' (long-day). For storage onions that last through winter, choose 'Yellow Sweet Spanish' or 'Red Zeppelin'.

Soil Preparation Essentials

Onions need well-drained, loose soil rich in organic matter. Prepare your garden bed 2-3 weeks before planting:

1. Test your soil pH (ideal range is 6.0-6.8)
2. Add 2-4 inches of compost to improve soil structure
3. Mix in balanced organic fertilizer (10-10-10) at 1½ pounds per 100 square feet
4. Create raised beds 4-6 inches high for better drainage

According to research from University of Minnesota Extension, onions grown in properly prepared soil show 30% larger bulb development and significantly reduced disease incidence compared to those in unamended soil.

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 1/4 inch deep in seed trays with sterile potting mix. Keep soil moist and provide 14 hours of light daily. Transplant outdoors when seedlings are pencil-thick.

Using Onion Sets

Sets are small, partially grown bulbs that offer the easiest method for beginners. Plant sets 1-2 inches deep, pointed end up, 4-6 inches apart. This method produces harvestable onions faster but may result in smaller bulbs.

Transplanting Seedlings

For the largest harvest, start seeds indoors then transplant hardened-off seedlings. Space plants 4-6 inches apart in rows 12-18 inches apart. This method requires more effort but yields the biggest, most uniform bulbs.

Watering and Feeding Schedule

Consistent moisture is critical for onion growth. Follow this timeline for optimal results:

Growth Stage	Watering Needs	Fertilizing Recommendations
First 3-4 weeks after planting	1 inch per week	Nitrogen-rich fertilizer every 2 weeks
Bulb formation stage	1½ inches per week	Switch to phosphorus-potassium fertilizer
3 weeks before harvest	Gradually reduce watering	No additional fertilizer

Overwatering during bulb formation causes splitting, while underwatering results in small bulbs. The Oregon State University Extension notes that inconsistent watering is the leading cause of poor onion yields among home gardeners.

Common Problems and Solutions

Be prepared to address these frequent onion growing challenges:

Pests

• Thrips: Tiny insects causing silvery streaks on leaves. Control with insecticidal soap or neem oil.
• Onion maggots: White larvae that eat roots. Prevent with row covers and crop rotation.

Diseases

• Pink root: Purple discoloration of roots. Manage with crop rotation and proper spacing.
• Downy mildew: Fuzzy growth on leaves. Prevent with good air circulation and avoid overhead watering.

Cultural Issues

• Bolting (flowering): Caused by temperature fluctuations. Remove flower stalks immediately.
• Small bulbs: Usually from overcrowding, poor soil, or incorrect variety for your region.

Harvesting and Storage Techniques

Knowing when and how to harvest determines your storage success:

1. Watch for natural top fall-over (about 100-170 days after planting)
2. Cure onions by leaving them in the garden for 7-10 days if weather permits
3. If rain threatens, move to a dry, shaded area with good air circulation
4. Cut tops to 1 inch when necks are completely dry
5. Store in mesh bags in a cool (32°-40°F), dry (65-70% humidity), dark place

Properly stored onions can last 3-5 months. Sweet onions have shorter storage life than pungent varieties. The University of Wisconsin Extension reports that onions stored at optimal conditions maintain 95% of their nutritional value for up to 120 days.

Troubleshooting Your Onion Crop

Address these common issues for better results:

• Problem: Tall, lush green growth but small bulbs
  Solution: You've over-fertilized with nitrogen. Reduce nitrogen and increase potassium.
• Problem: Bulbs forming too early
  Solution: You planted too early in spring. Wait until soil temperature reaches 50°F.
• Problem: Rotting at the base
  Solution: Improve drainage and avoid overhead watering as bulbs mature.

Maximizing Your Onion Harvest

For the best results, follow these professional tips:

• Practice crop rotation - don't plant onions in the same spot more than once every 3 years
• Use mulch to maintain consistent soil moisture and temperature
• Stop watering 3 weeks before harvest to improve storage quality
• Choose varieties specifically bred for your climate zone
• Thin plants early to ensure proper spacing for bulb development

With proper care, a single square foot of garden space can yield 4-6 full-sized onions. Remember that patience pays off - onions develop slowly at first but accelerate bulb formation as days lengthen.

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).

Can I grow onions in containers?

Yes, onions grow well in containers that are at least 8-10 inches deep. Use a quality potting mix, ensure good drainage, and keep soil consistently moist. Container-grown onions need more frequent watering than garden-grown ones.

Why are my onion tops falling over?

Onion tops naturally fall over when bulbs have matured and it's time to harvest. This is a normal part of the growth cycle, not a problem. Wait 7-10 days after tops fall before harvesting for best storage results.

How do I prevent onions from rotting in storage?

Proper curing is essential. Dry onions thoroughly before storage (2-3 weeks in a warm, dry, well-ventilated area). Store in a cool (32°-40°F), dry (65-70% humidity), dark place with good air circulation.

What's the difference between green onions and regular onions?

Green onions (scallions) are harvested young before bulb formation, while regular onions are allowed to mature and form bulbs. Green onions can be any onion variety harvested early, typically 20-30 days after planting.