Onion Plants Grow But Don't Form Onions

The variety of onion you should choose depends on where you live.


Even though they bring tears to your eyes, it's difficult to imagine not having onions in the kitchen. These cool-season vegetables can be grown reliably in any climate. As they grow, the plants produce a tuft of leaves above ground as a bulb develops underground. A bulbless onion can still be harvested for its greens, but if your goal is a crop of fully developed onions, you'll need to make sure you have the correct variety and proper growing conditions.

Wrong Conditions

To form a bulb, the underground portion of the onion plant needs ample space in loose soil. Planting in heavy clay soil or planting your onions too closely makes it difficult for the bulbs to grow. Before planting, amend your soil to loosen it or plant your onions in raised beds. Onion plants should have at least three to four inches of space between them to avoid crowding. Onions also need full sun. Planted in partial shade or full shade, the plants won't be able to produce the energy they need to divert energy into bulb formation.

Wrong Variety

Onions grown for their bulbs are available in three varieties: short-day, intermediate-day and long-day. To form a bulb, onion plants must receive a certain amount of daylight. Gardeners in the south, where day length doesn't change much throughout the year, should plant short-day varieties. Gardeners in the north, where summer days are long, should plant long-day varieties. Gardeners near the 37th parallel, roughly the horizontal center of the United States from San Francisco to Washington, D.C., will want to plant intermediate-day varieties. These varieties also have different planting times; if planted at the wrong time of year, they won't form bulbs. Short-day varieties are planted in late fall to early winter, while intermediate-day and short-day varieties are planted in early spring.

Poor Nutrition

For an onion plant to be healthy enough to form a bulb, it needs plenty of nutrients in its early growth stages. If there is too much nitrogen, your onion plants will put on lush above-ground growth, but will fail to produce bulbs. Choose a balanced fertilizer and apply it one month after planting and then once more four weeks later. For the plant to access the nutrients in the fertilizer, the pH of your onion bed's soil needs to be between 6.2 and 6.8. Higher or lower pH levels make it difficult for the roots to take up the nutrients. Onions also need regular water for healthy growth and bulb formation.

Pests and Diseases

Weeds can rob your onions of the nutrients and water they need. If the competition becomes too heavy, your onions won't form bulbs. When weeding, pull gently to avoid disturbing the shallow roots of the onion plants. Onions aren't affected by many pests or diseases. The two main problems are various forms of rot and root maggots that can ruin the developing bulbs. These problems can be solved by planting onions in loose, well-drained soil in sunny locations and by not planting onions in the same bed two years in a row.

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