How to Plant Onion Bulbs: Complete Step-by-Step Guide

Plant onion bulbs 1-2 inches deep and 4-6 inches apart in well-draining soil with pH 6.0-7.5, ideally 4-6 weeks before your last spring frost date. Choose firm, disease-free bulbs without sprouts, plant with pointed end up, and water thoroughly after planting for best results.

Ready to grow your own flavorful onions? Whether you're a beginner gardener or looking to perfect your technique, this guide delivers exactly what you need to know about planting onion bulbs successfully. Forget confusing advice—here's a clear, step-by-step approach that works for home gardeners everywhere.

Why Onion Bulbs Outperform Seeds for Home Gardeners

While you can grow onions from seeds, planting onion bulbs (also called sets) offers significant advantages for most home gardeners. Bulbs establish faster, mature earlier, and generally produce larger onions than seed-grown plants. According to the University of Minnesota Extension, onion bulbs typically reach harvest 50-70 days after planting, compared to 100-120 days for seed-started plants.

Planting Method	Time to Harvest	Success Rate	Best For
Onion Bulbs (Sets)	50-70 days	85-90%	Beginners, short-season climates
Transplants	75-95 days	75-80%	Experienced gardeners
Seeds	100-120 days	60-70%	Cooler climates, specialty varieties

Perfect Timing: When to Plant Onion Bulbs in Your Region

The optimal planting window depends on your climate zone. In most regions, plant onion bulbs as soon as the soil can be worked in spring—typically when soil temperatures reach 50°F (10°C). The USDA Plant Hardiness Zone Map provides critical guidance:

• Zones 3-5: Plant in late March to early April (4-6 weeks before last frost)
• Zones 6-7: Plant in mid-March
• Zones 8-10: Plant in late fall or winter for spring harvest

Avoid planting too early when soil is still waterlogged, as this causes bulbs to rot. Wait until the ground is workable but still cool—onion bulbs establish best in cool conditions before summer heat arrives.

Soil Preparation: Creating the Ideal Environment

Onions thrive in well-draining soil rich in organic matter. Follow these preparation steps 2-3 weeks before planting:

1. Test your soil pH (ideal range: 6.0-7.5). Amend acidic soil with garden lime.
2. Remove rocks and debris that could deform bulb growth.
3. Work in 2-3 inches of compost or well-rotted manure.
4. Create raised beds 4-6 inches high for better drainage in heavy soils.

The Penn State Extension emphasizes that proper soil preparation prevents common issues like double bulbs and misshapen onions. Avoid fresh manure, which can cause excessive top growth at the expense of bulb development.

Step-by-Step Planting Process

Follow these precise steps for successful onion bulb planting:

1. Select Quality Bulbs

Choose firm bulbs without sprouts, mold, or soft spots. Smaller bulbs (about 1 inch diameter) typically produce larger mature onions than larger sets. Avoid bulbs that have begun to split, as they're more prone to bolting.

2. Proper Planting Depth and Spacing

Plant bulbs 1-2 inches deep with the pointed end up. Space bulbs 4-6 inches apart in rows 12-18 inches apart. Deeper planting (2 inches) works better in sandy soils, while shallower planting (1 inch) suits clay soils.

3. Watering Immediately After Planting

Water thoroughly after planting to settle the soil around bulbs. Provide 1 inch of water weekly during active growth, increasing to 2 inches as bulbs begin to swell.

Growth Timeline: What to Expect After Planting

Understanding the onion growth cycle helps you provide appropriate care:

Weeks After Planting	What's Happening	Care Required
0-2	Root development	Maintain consistent moisture
3-6	Top growth accelerates	Weed carefully, apply nitrogen fertilizer
7-10	Bulb formation begins	Increase watering, stop nitrogen
11-14	Bulb swelling phase	Provide 2 inches water weekly
15-18	Natural top fall-over	Reduce watering, prepare for harvest

Avoiding Common Onion Planting Mistakes

Even experienced gardeners make these preventable errors:

• Planting too deep: Bulbs planted deeper than 2 inches produce smaller onions
• Overcrowding: Spacing bulbs closer than 4 inches reduces bulb size
• Using nitrogen-heavy fertilizer during bulb formation: Promotes leaf growth at expense of bulbs
• Ignoring day-length requirements: Short-day varieties won't bulb properly in northern zones

The Oregon State University Extension notes that day-length sensitivity is critical—short-day onions (for southern states) form bulbs with 10-12 hours of daylight, while long-day varieties (for northern states) require 14-16 hours.

Harvesting and Curing Your Homegrown Onions

Harvest when about half the tops have naturally fallen over and turned brown. Carefully dig bulbs without bruising them. Cure onions in a warm, dry, well-ventilated area for 2-3 weeks until necks are completely dry and papery. Properly cured onions store for 4-6 months in mesh bags at 32-40°F with 65-70% humidity.

For immediate use, you can harvest "green onions" just 3-4 weeks after planting, when tops reach 6-8 inches tall. These immature onions provide delicious mild flavor for salads and garnishes.

Can I plant onion bulbs in the fall for spring harvest?

Yes, in zones 6 and warmer, plant onion bulbs in fall (October-November) for spring harvest. Choose long-day varieties and plant 2 inches deep. Mulch heavily to protect from extreme cold. Fall-planted onions typically mature 2-3 weeks earlier than spring-planted crops.

Why are my onion bulbs splitting or producing flowers?

Splitting or flowering (bolting) usually occurs when bulbs experience temperature fluctuations. Planting too early in cold soil or using bulbs that have begun sprouting can trigger this. Choose varieties suited to your climate zone and plant at the proper time to minimize bolting risk.

How do I prevent onion maggots from destroying my crop?

Rotate planting locations annually and use floating row covers immediately after planting. The Cornell University Cooperative Extension recommends applying beneficial nematodes to soil before planting as an organic control method. Avoid planting near previous allium crops to break the pest cycle.

Should I remove the papery outer layer from onion bulbs before planting?

No, keep the protective papery layer intact. This natural coating prevents moisture loss and protects against disease. Only remove outer layers if they're moldy or damaged. Healthy bulbs with intact coverings establish better and resist rot during early growth stages.