Producing large onions in the home garden

Introduction:

Onion bulb formation begins when a certain day length is reached.  Short-day onion varieties begin to form bulbs when they receive 10 to 12 hours of daylight, intermediate-day onions need 12 to 14 hours of daylight, and long-day varieties require 14 or more hours of daylight.

Bulb size is largely determined by the number and size of the leaves at bulb initiation.  The larger the tops (foliage area) at bulb initiation, the larger the bulbs will be. For larger onion bulbs,plant a long-day variety as early as possible, then water and fertilize regularly throughout the summer.

Selecting the Right Variety:

Long-day onion varieties are the best choice for gardeners in Idaho.  Short-day varieties generally produce small bulbs when grown in northern areas.  Long-day varieties, however, are able to produce large tops before bulb initiation occurs.  As a result, long-day onion varieties typically produce the biggest bulbs.

Some popular varieties include Big Daddy, Walla Walla, White and Yellow Sweet Spanish, and Red Mars. You should use onion sets or transplants for the best results in your garden. You may grow your own (too late to start this year) or purchase them from local garden centers.

Preparing the Soil:

Onions thrive in well-drained, fertile soil. Begin by preparing the planting area by loosening the soil to a depth of at least 6 inches. Incorporate organic matter such as compost or aged manure to improve soil structure and provide essential nutrients for growth. Poorly drained soil will lead to rot and disease.

Planting and Care:

Long-day onions are typically planted in early spring as soon as the soil can be worked. Plant onion sets or transplants approximately 1 inch deep and 4-6 inches apart in rows spaced 12-18 inches apart. Ensure adequate spacing to allow for proper bulb development. Maintain consistently moist soil throughout the growing season. Weed control is essential as onions are not competitive growers. Side-dress onions with ½ pound of nitrogen fertilizer per 100 square feet in mid-May and late June.

Harvesting:

Onions are typically ready for harvest when the tops begin to yellow and fall over. Use a garden fork or trowel to gently lift the bulbs from the soil, taking care not to damage them. Allow the onions to cure in a warm, dry place with good air circulation for 2-3 weeks before storing them in a cool, dry location.