Planting Onion: Complete Guide for Successful Harvest

Plant onions in early spring when soil temperature reaches 50°F (10°C), spacing sets 4-6 inches apart in well-drained soil with pH 6.0-6.8. Water consistently but avoid overwatering, and harvest when tops fall over naturally for maximum bulb development and storage life.

Discover exactly how to grow onions successfully with this comprehensive guide. Whether you're a first-time gardener or looking to improve your harvest, you'll learn proven techniques for soil preparation, planting timing, and care that result in larger, sweeter onions with better storage potential. We've distilled decades of horticultural research into actionable steps anyone can follow.

Understanding Onion Types and When to Plant

Choosing the right onion variety for your climate is the foundation of a successful harvest. Onions are categorized by their day-length requirements, which determine when they form bulbs:

Onion Type	Day Length Trigger	Best Regions	Planting Time
Short-Day Onions	10-12 hours of daylight	Southern states (USDA zones 7-10)	October-November
Intermediate-Day Onions	12-14 hours of daylight	Mid-latitude regions (USDA zones 5-7)	February-March
Long-Day Onions	14+ hours of daylight	Northern states (USDA zones 2-5)	March-April

This classification system, documented by the USDA Agricultural Research Service, explains why certain varieties thrive in specific regions. Planting the wrong type for your latitude results in small bulbs or premature bolting. For precise timing in your area, consult your local Cooperative Extension Service planting calendar.

Soil Preparation Essentials

Onions demand well-prepared soil for optimal growth. Unlike many vegetables, they have shallow root systems that require loose, well-drained conditions to develop properly.

Begin soil preparation 3-4 weeks before planting with this timeline:

1. Test soil pH (ideal range: 6.0-6.8) using a home kit or through your extension service
2. Clear area of weeds and debris
3. Add 2-3 inches of compost or well-rotted manure
4. Mix in 1-2 pounds of 10-10-10 fertilizer per 100 square feet
5. Create raised beds 4-6 inches high for better drainage

According to research from University of Minnesota Extension, onions grown in properly amended soil show 30-40% larger bulb development compared to unamended plots. The shallow root system makes onions particularly sensitive to soil compaction, so avoid walking on planting areas.

Planting Techniques That Work

Onions can be grown from seeds, sets (small bulbs), or transplants. Each method has advantages depending on your experience level and climate:

• Seeds: Most economical but require 12-16 weeks start time; best for long-season areas
• Sets: Easiest method; plant 1-2 inches deep, pointed end up, 4-6 inches apart
• Transplants: Balance of ease and control; space 4-5 inches apart in rows 12-18 inches wide

When planting sets, push them into soil so the tips are just visible at the surface. Deeper planting produces smaller bulbs, while shallow planting leads to loose sets that wash away. For optimal spacing, follow the "two-finger rule" - place sets far enough apart that you can comfortably fit two fingers between them.

Care and Maintenance Through the Growing Season

Consistent care determines whether you'll harvest scallions or full-sized bulbs. Onions have specific needs that differ from other garden vegetables:

Watering Requirements

Onions need about 1 inch of water weekly, but consistency matters more than quantity. Irregular watering causes splitting and disease susceptibility. During bulb formation (late spring to early summer), maintain even moisture. Reduce watering 2-3 weeks before harvest when tops begin falling over.

Fertilizing Schedule

Apply nitrogen fertilizer when plants are 6 inches tall, then every 2-3 weeks until bulbs begin forming. Stop fertilizing when bulbs swell to prevent excessive top growth at the expense of bulb development. Excessive nitrogen late in the season produces thick necks that don't cure properly.

Weed Control

Onions compete poorly with weeds due to their shallow roots. Hand-pull weeds carefully or use mulch (straw or shredded leaves) to suppress weeds without disturbing roots. Avoid deep cultivation near plants.

Harvesting and Storage Tips

Knowing when and how to harvest determines your onions' storage life. Most gardeners harvest too early, sacrificing potential size and storage quality.

Harvest when:

• At least 50% of tops have fallen over naturally
• Bulb necks become soft and tops yellow
• Outer skins turn papery and dry

Follow these steps for proper harvesting:

1. Cease watering when tops begin falling
2. Lift bulbs carefully with a garden fork
3. Cure in a warm, dry, well-ventilated area for 2-3 weeks
4. Trim tops to 1 inch and roots to 1/4 inch after curing
5. Store in mesh bags in a cool, dry place (32-40°F, 65-70% humidity)

Properly cured onions can store 4-6 months. Sweet varieties (like Vidalia) have higher water content and store for only 1-2 months, while pungent storage onions (like Yellow Sweet Spanish) keep longest. According to Penn State Extension, improper curing accounts for 70% of storage losses in home gardens.

Troubleshooting Common Onion Problems

Even with proper planting, gardeners encounter challenges. Understanding these context boundaries helps address issues specific to your growing conditions:

• Bolting (flowering): Caused by temperature fluctuations; use appropriate varieties for your zone
• Small bulbs: Overcrowding, poor soil, or wrong day-length variety
• Rotting bulbs: Excessive moisture or poor drainage
• Purple leaves: Phosphorus deficiency; add bone meal or rock phosphate

Remember that onion problems often reflect broader garden conditions. For example, thrips damage appears as silvery streaks on leaves and is worse in dry conditions. The Oregon State University Extension recommends regular monitoring and using reflective mulches to deter these pests.

When is the best time to plant onions in my area?

Plant onions when soil temperature reaches 50°F (10°C). In northern regions, plant in early spring as soon as soil can be worked. In southern regions, plant in fall for winter harvest. Consult your local Cooperative Extension Service for precise timing based on your USDA hardiness zone.

Should I plant onion seeds, sets, or transplants?

Beginners should use sets for easiest results. Transplants offer better disease resistance and larger yields. Seeds are most economical but require 12-16 weeks start time indoors. In short-season climates, transplants or sets work best; in long-season areas, seeds produce largest bulbs.

Why are my onion bulbs so small?

Small bulbs typically result from overcrowding, poor soil nutrition, incorrect variety for your day length, or inconsistent watering. Space plants 4-6 inches apart, ensure proper soil pH (6.0-6.8), use appropriate day-length variety, and maintain consistent moisture during bulb formation.

How do I know when onions are ready to harvest?

Onions are ready when at least 50% of tops have fallen over naturally, outer skins turn papery and dry, and bulb necks become soft. Stop watering when tops begin falling, then harvest after 7-10 days. Properly cured onions will have dry, papery outer layers and firm bulbs.

How long do homegrown onions store?

Properly cured storage onions (yellow varieties) keep 4-6 months in cool, dry conditions (32-40°F, 65-70% humidity). Sweet onions (Vidalia, Walla Walla) store only 1-2 months due to higher water content. Check stored onions monthly and remove any showing signs of decay.