Producing large onion heads, or bulbs, depends on both the variety of onion and proper care. Not all varieties produce bulbs. Green onion varieties, for example, may produce plenty of flavorful foliage for harvest as scallions, but the bulbs are small. Onions produce large heads in response to the hours sunlight they get each day, so a large bulbing variety that produces well in one region may not develop good heads in another. Choose onion varieties bred to produce large heads, such as Spanish onions. Purchasing onion sets from a local source helps ensure the selection of a large variety that grows well in your area.
Loosen the soil with a hoe in a bed that gets full sun and has well-draining soil. Do this in spring after the soil dries and the average temperature is above 50 degrees Fahrenheit. Newly planted onions tolerate light freezes and require cooler temperatures to initiate early growth so they can produce large heads.
Plant onion sets 1 inch deep. Transplant indoor-started seedlings at the same depth they were at previously. Space both sets and seedlings three inches apart in the row so the bulbs have room to fully form.
Water the onions with 1 to 2 inches of water per week to help increase head size. Too much water can cause the plants to rot, while too little stresses the onions and may cause the foliage to die back before full bulb formation.
Apply a nitrogen-rich fertilizer halfway through the growing season to further bulb formation. Apply the fertilizer at the package's recommended rate for root or vegetable plants.
Reduce watering by one-half once the foliage begins to yellow. Harvest the onions only after the yellowing foliage begins to fall over.
Things You Will Need
- Onion sets or seedlings
- Nitrogen-rich fertilizer
- Select an onion variety that produces large heads in your area. Plant long-day onion varieties in areas that receive 12 hours of sunlight in midsummer, or intermediate-day varieties in areas that receive 10 to 12 hours of sun in midsummer.
- A thick layer of mulch applied over the onions protects the bulbs from sunlight and prevents the soil from drying too quickly.
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