Onion Bulbs: Complete Planting & Growing Guide

Discover exactly when to plant onion bulbs for your climate zone, how to space them properly, and the common mistakes that cause small bulbs or bolting. This guide reveals professional growing techniques that produce larger, sweeter onions with better storage potential - whether you're planting in containers, raised beds, or traditional garden rows.

Onion bulbs are underground storage organs that contain all the nutrients a young onion plant needs to grow. Plant them in early spring or fall 1-2 inches deep and 4-6 inches apart in well-draining soil with full sun exposure. Most varieties mature in 90-120 days and should be harvested when the tops fall over naturally.

Understanding Onion Bulbs: More Than Just Seeds

Unlike many vegetables that grow from seeds, onions primarily develop from bulbs, sets, or transplants. Onion bulbs represent the most reliable planting method for home gardeners seeking consistent results. These compact storage organs contain pre-formed leaf bases and a developing root system, giving your crop a significant head start compared to seed-grown onions.

Professional growers and experienced gardeners prefer onion bulbs because they're less prone to bolting (premature flowering) and produce more uniform crops. The bulb's outer dry skin protects the inner layers from moisture loss and disease during storage before planting.

When to Plant Onion Bulbs: Timing Is Everything

Getting the planting schedule right makes the difference between small, underdeveloped onions and robust, flavorful bulbs. The optimal planting window depends on your climate zone and the specific onion variety you've selected.

Cold climate gardeners (zones 3-5) should plant onion bulbs as soon as the soil can be worked in early spring, typically late March to mid-April. These regions benefit from long-day onion varieties that require 14-16 hours of daylight to form bulbs properly.

Moderate climate gardeners (zones 6-7) have two planting opportunities: early spring (March-April) for summer harvest, or late summer (August) for overwintering and early summer harvest. Choose intermediate-day varieties that form bulbs with 12-14 hours of daylight.

Warm climate gardeners (zones 8-10) achieve best results planting in fall (October-November) for spring harvest. Select short-day varieties that bulb with just 10-12 hours of daylight.

Onion Type	Day Length Requirement	Best Climate Zones	Planting Time	Harvest Time
Long-Day	14-16 hours	3-5	Early spring	Late summer
Intermediate-Day	12-14 hours	6-7	Spring or late summer	Mid to late summer
Short-Day	10-12 hours	8-10	Fall	Spring

Step-by-Step Planting Guide for Maximum Yield

Follow these research-backed steps to ensure your onion bulbs develop into substantial, flavorful crops:

Soil Preparation (2-3 Weeks Before Planting)

Onions thrive in loose, well-draining soil with a pH between 6.0-6.8. Amend heavy clay soils with 3-4 inches of compost and coarse sand to improve drainage. For sandy soils, incorporate 2-3 inches of compost to increase moisture retention. Avoid fresh manure which can cause excessive foliage growth at the expense of bulb development.

Planting Your Onion Bulbs

1. Choose firm, dry bulbs without signs of mold or sprouting
2. Plant with the pointed end up, 1-2 inches deep depending on bulb size
3. Space bulbs 4-6 inches apart in rows 12-18 inches apart
4. Cover with soil and water gently but thoroughly

According to the USDA Agricultural Research Service, proper spacing directly impacts bulb size - overcrowded onions compete for nutrients and produce smaller bulbs. The 4-6 inch spacing allows each plant sufficient room to develop fully while creating enough density to suppress weeds.

Avoiding Common Onion Growing Mistakes

Even experienced gardeners make these preventable errors that compromise onion bulb development:

Watering Inconsistencies

Onions require consistent moisture, especially during bulb formation (when tops reach 6-8 inches tall). Irregular watering causes splitting or double-center bulbs. Aim for 1 inch of water per week, increasing to 2 inches during bulb formation. Mulching with straw helps maintain consistent soil moisture.

Over-Fertilizing with Nitrogen

Excessive nitrogen promotes lush green tops at the expense of bulb development. Apply balanced fertilizer (10-10-10) at planting time, then switch to low-nitrogen fertilizer (5-10-10) once bulbs begin forming. The University of California Cooperative Extension recommends stopping nitrogen applications 4-6 weeks before harvest to improve storage quality.

Ignoring Day Length Requirements

Planting the wrong day-length variety for your region causes poor bulb formation. Long-day varieties planted in southern regions often bolt before forming substantial bulbs, while short-day varieties in northern regions produce only small bulbs. Match your variety to your latitude for optimal results.

Harvesting and Curing for Long-Term Storage

Knowing when and how to harvest determines your onions' storage potential. Most varieties signal harvest readiness when approximately 50% of the tops have fallen over naturally. Never pull onions prematurely based solely on calendar dates.

Follow this professional curing process:

1. Lift bulbs carefully with a garden fork when tops fall
2. Cure in a warm, dry, well-ventilated area for 2-3 weeks
3. Trim tops to 1 inch and roots to ¼ inch after curing
4. Store in mesh bags in a cool (32-40°F), dry (65-70% humidity) location

Properly cured onions can store for 6-12 months depending on variety. Sweet onions like Walla Walla have higher water content and shorter storage life (2-3 months), while storage varieties like Yellow Sweet Spanish can last 8-12 months under ideal conditions.

Troubleshooting Common Onion Problems

Address these frequent issues before they ruin your crop:

Onion Maggots

These small white larvae tunnel through bulbs, causing rot. Prevent infestations by rotating crops annually and using floating row covers at planting. The Cornell University Department of Entomology recommends applying beneficial nematodes (Steinernema feltiae) to the soil at planting time for organic control.

Bolting (Flowering)

When onions send up flower stalks prematurely, bulb development stops. This typically occurs due to temperature fluctuations or planting bulbs that experienced cold storage conditions. Remove flower stalks immediately and use those onions first, as they won't store well.

Small Bulb Size

Several factors cause undersized bulbs: overcrowding, poor soil nutrition, inconsistent watering, or planting the wrong day-length variety. Address these issues early in the growing season for better results next year.

Onion Bulb Growing Timeline: From Planting to Harvest

Understanding the growth stages helps you provide appropriate care at each critical phase:

• Weeks 1-2: Root development and initial top growth
• Weeks 3-6: Leaf production phase (critical for eventual bulb size)
• Weeks 7-10: Bulb initiation and early expansion
• Weeks 11-14: Rapid bulb enlargement
• Weeks 15-18: Maturation and top fall-over (harvest time)

This timeline varies based on variety and growing conditions. Short-day varieties typically mature faster (90-100 days) while long-day storage varieties take longer (110-120 days).

Special Considerations for Container Growing

You can successfully grow onions in containers with these adjustments:

• Use pots at least 8 inches deep with drainage holes
• Choose compact varieties like Patio or Red Baron
• Mix potting soil with 20% perlite for better drainage
• Water more frequently as containers dry out faster
• Fertilize every 3-4 weeks with balanced liquid fertilizer

Container-grown onions require more attentive watering but offer excellent control over soil conditions. They're ideal for gardeners with limited space or poor native soil.

When is the best time to plant onion bulbs in my region?

Plant onion bulbs in early spring as soon as soil can be worked in cold climates (zones 3-5), spring or late summer in moderate climates (zones 6-7), and fall in warm climates (zones 8-10). Match your planting time to your specific climate zone and the day-length requirements of your onion variety for optimal results.

How deep should I plant onion bulbs for best growth?

Plant onion bulbs 1-2 inches deep with the pointed end facing up. Smaller bulbs (less than 1 inch in diameter) should go 1 inch deep, while larger bulbs need 1.5-2 inches of soil coverage. Proper planting depth ensures optimal root development and prevents bulbs from pushing out of the soil as they grow.

Why are my onion bulbs not growing large?

Small onion bulbs typically result from overcrowding, inconsistent watering during bulb formation, excessive nitrogen fertilizer, or planting the wrong day-length variety for your region. Ensure proper spacing (4-6 inches apart), consistent moisture during bulb development, and select varieties appropriate for your latitude's day length requirements.

Can I grow onions from store-bought bulbs?

While possible, store-bought onions often make poor planting stock as they may have been treated with sprout inhibitors or selected for different growing conditions. Garden center onion bulbs are specifically grown and stored for planting success. If using store-bought, choose organic onions without sprout inhibitors and plant immediately after purchase.

How do I know when to harvest onion bulbs?

Harvest onion bulbs when approximately 50% of the tops have fallen over naturally. This typically occurs 90-120 days after planting, depending on variety. Never harvest based solely on calendar dates. After tops fall, wait 7-10 days before harvesting to allow bulbs to fully mature and develop proper storage skins.