7 Secrets To Planting Onions That Grow Massive Bulbs (The 2024 Expert Guide)

Growing your own onions, whether from small bulbs called "sets," tiny transplants, or starting from seed, is one of the most rewarding garden projects. However, many gardeners struggle to get those softball-sized bulbs they dream of because they miss one crucial, updated detail: choosing the right variety for their location. This 2024 expert guide cuts through the confusion, providing a comprehensive, step-by-step breakdown of how to plant onions successfully, ensuring a bountiful harvest of sweet, pungent bulbs.

The key to success lies in understanding the onion’s life cycle, particularly its sensitivity to day length, a concept known as photoperiodism. By following a few simple rules on timing, soil preparation, and feeding, you can avoid the most common mistakes and enjoy a year's supply of homegrown onions.

The Critical First Step: Understanding Long-Day vs. Short-Day Onions

The single biggest mistake gardeners make is planting the wrong type of onion for their latitude. Onions are triggered to stop growing leaves and start forming a bulb based on the number of daylight hours they receive—a process called bulbing. Choosing the correct type is non-negotiable for a successful harvest.

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• Long-Day Onions: These varieties require 14 to 16 hours of daylight to trigger bulbing. They are best suited for northern latitudes (typically above 40° N, roughly USDA Zones 6 and colder). If planted in the South, they will only produce small scallions.
• Short-Day Onions: These varieties only require 10 to 12 hours of daylight to bulb. They thrive in southern latitudes (typically below 35° N, roughly USDA Zones 7 and warmer). If planted in the North, they will bulb too early and remain small.
• Intermediate (Day-Neutral) Onions: A flexible middle ground, these require 12 to 14 hours of daylight and perform well in the mid-latitudes (between 35° N and 40° N).

Popular Onion Varieties by Type (Entities)

• Long-Day Examples: ‘Walla Walla Sweet’, ‘Yellow Spanish’, ‘Red Wethersfield’, ‘Copra’ (a great storage onion).
• Short-Day Examples: ‘Granex’ (Vidalia type), ‘Texas Grano’, ‘Red Creole’.
• Intermediate Examples: ‘Red Candy Apple’, ‘Super Star’, ‘Candy’ (a popular hybrid).

Choosing Your Planting Method: Sets, Transplants, or Seeds

You have three main options for starting your onion crop, each with its own pros and cons. The most common and easiest methods for beginners are using sets or transplants.

1. Onion Sets (Small Bulbs)

Onion sets are small, immature bulbs that were grown the previous year. They are the easiest to plant and offer the quickest path to a harvest. However, they are prone to bolting (producing a flower stalk) and tend to be pungent, which is great for cooking but not ideal for sweet eating. They are typically planted in early spring.

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2. Onion Transplants (Slips)

These are small, pencil-thin seedlings, often sold in bundles. They are the most reliable method for producing large, full-sized bulbs and are the preferred choice for growing sweet onions. Transplants are planted in the early spring, as soon as the soil can be worked.

3. Onion Seeds

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Starting from onion seeds gives you the widest variety choice and is the most cost-effective, but it requires the most effort. Seeds must be started indoors 8 to 10 weeks before the last expected frost, usually in late winter (January or February), and then transplanted outdoors in the spring.

The Step-by-Step Planting Process

Successful planting hinges on proper timing, soil preparation, and spacing. Onions are a cool-weather crop and should be planted early in the spring, as soon as the soil temperature reaches 50°F.

1. Soil Preparation and Location

Onions are heavy feeders and require loose, rich soil. They will not thrive in compacted, heavy clay soils.

• Sunlight: Choose a location with full sun (at least 6-8 hours of direct light per day).
• Soil Type: The ideal medium is a loose, well-draining sandy loam soil that is rich in organic matter, such as well-rotted compost.
• pH Level: Onions prefer a slightly acidic to neutral soil pH, ideally between 6.0 and 7.0.
• Raised Beds: Planting in raised beds is highly recommended as it ensures excellent drainage and prevents soil compaction.

2. Planting Depth and Spacing

Correct spacing is crucial for bulb size; overcrowding will result in small, underdeveloped onions. When planting transplants or sets:

• Depth: Plant them shallowly, so the top of the bulb or the root line of the transplant is barely covered by the soil, or just above the soil level. Planting too deep is a common mistake that restricts bulb formation.
• Spacing: Space each onion 3 to 4 inches apart within the row.
• Rows: If planting in rows, space the rows 12 to 18 inches apart.

Fertilization and Watering: The Secret to Bulb Size

Onions are heavy feeders throughout their vegetative (leaf-growing) phase. Consistent watering and a strategic fertilization schedule are essential for producing large bulbs.

Fertilization Schedule:

1. Pre-Planting: Work in a balanced fertilizer, such as a 10-10-10 or 10-20-10, or organic amendments like bone meal and compost, 2–3 weeks before planting. This supports strong root development.
2. During Growth (High Nitrogen): Once the green tops are actively growing (about 2-3 weeks after transplanting), switch to a high-nitrogen fertilizer, such as blood meal or a chemical fertilizer like 21-0-0 (ammonium sulfate).
3. Frequency: Side-dress the onions with the high-nitrogen fertilizer every 2 to 3 weeks until the bulbs begin to swell (usually around mid-June).
4. Stop Fertilizing: Stop all nitrogen fertilization once the bulbs start to push the soil away and swell. Too much nitrogen late in the season can delay maturity and lead to poor storage quality.

Watering:

Onions have shallow root systems and require consistent moisture, especially during the bulbing phase. Aim for about one inch of water per week, either from rain or irrigation. Consistent moisture prevents the soil from drying out and cracking, which can stunt growth. Using a layer of organic mulch can help retain soil moisture and suppress weeds.

Pest Control, Harvesting, and Curing for Long-Term Storage

A successful garden includes knowing how to handle common threats and how to finish the job for long-term storage.

Common Pests and Diseases

While relatively pest-resistant, onions can be affected by a few key issues:

• Onion Thrips: Tiny, slender insects that suck the sap from the leaves, causing silvering or streaking. Control them with strong sprays of water (overhead irrigation) or insecticidal soap.
• Neck Rot: A fungal disease that causes a soft decay at the neck of the bulb, often appearing after harvest. Prevention is key: ensure good air circulation, use crop rotation, and properly cure the bulbs.
• Disease Prevention: Select disease-resistant varieties like 'Red Baron' or 'White Sweet Spanish' and ensure proper spacing to allow for airflow.

Harvesting and Curing

Onions are ready to harvest when the green tops begin to yellow and fall over naturally. This is the signal that the bulb has finished growing.

1. Harvest: Gently loosen the soil around the bulbs with a fork and pull them out of the ground.
2. Curing: This is the most crucial step for long-term storage. Spread the onions in a single layer on a clean, dry surface, ideally in a cool, well-ventilated, and shaded location (like a garage or covered porch).
3. Duration: Allow the onions to cure for two to four weeks until the outer skins are papery and the neck is completely dry.
4. Storage: Once cured, trim the tops and roots. Store them in mesh bags or braid them and hang them in a cool (32-36°F), dark, and dry area. Do not store onions with potatoes, as both will spoil faster.