What Promotes Onion Bulb Growth?

Choosing Your Onions

• Onions are divided into short-day types, which begin to produce bulbs when day length reaches 10 to 12 hours, and long-day types, which only produce bulbs when day length reaches 14 to 16 hours. Short-day onions are best suited to states south of the 36th parallel, which runs from the Virginia-North Carolina border on the East Coast across in a straight line to the middle of California on the West Coast. Long-day onions are better in states north of this line. Ask local gardeners or garden centers for advice about which onion varieties grow well in your area.

Planting

• Plant out onion sets or transplants in early spring once temperatures reach 50 degrees Fahrenheit. This gives them as much growing time as possible. Sow two rows in a raised bed 4 inches high and 20 inches wide, leaving at least three inches between plants. Plant sets 2 inches deep with the pointed end upward. Plant seedlings 2 inches deep.

Fertilizing

• Onions benefit from a high-phosphate fertilizer while they are small and producing leaves. Dig in half a cup of super phosphate fertilizer per 10 feet of row prior to planting. Fertilize the plants three weeks after planting with a high-nitrogen fertilizer and subsequently every two or three weeks. Stop fertilizing once the onion neck where the bulb and leaves join becomes soft.

Watering

• Onions have shallow roots and need moist soil at all times. Aim to soak the top inch of soil every time you water. Reduce watering once the onion tops start to topple over.

Care

• Remove any weeds that compete with your onions for light and nutrients. Weed with care as onion plants have very shallow roots that are vulnerable to disturbance and damage.