How Many Onion Sets to a Plant?

Onion Set Information

• Onions are hardy in U.S. Department of Agriculture plant hardiness zones 3 to 9, and are a cold-season crop. Commercial growers and many home gardeners start onion plants from seeds sown inside early in the spring. Other backyard gardeners take the quicker route of planting onion sets. These small "baby" onions are planted before the last spring frost and picked in the early stages as spring, green, or bunching onions, or allowed to reach maturity as full-sized storage onions.

Growth and Development

• A single onion seed produces one plant in the form of a small white bulblet from which emerges a thin green top. Initially, there is little bulb growth while the green top processes and stores food that is eventually redirected to the bulb. Early top development is rapid, and within just a few weeks, the young onions, also called scallions, can be harvested, and both parts from the root to the tip of the green top are edible. Allowed to develop, the sets enlarge beneath the ground until late summer or early fall when they are harvested and processed for long-term storage. Onions that grow to seed should be harvested and used right away.

Culture

• Onions grown from seed should be started indoors and moved to the garden after about 6 weeks, or 4 weeks before the last spring frost as long as the temperature doesn't fall below 20 degrees F. Onion sets appear in nurseries and garden centers in late winter and early spring and are sold in bulk or by the pound in mesh bags. Like young seedlings, sets can be planted outside before the last frost as soon as the ground can be worked. You can also grow your own sets by planting onion seed only 1/4 inch apart and harvesting the small onions when the tops die back. Dried and stored in mesh bags, they can be used to grow green or full-sized onions the following spring.

Multiplier Onions

• Another type of onion called the multiplier (Allium cepa var. aggregatum) produces clusters of small onions from a single bulb. Several small shoots develop from the main bulb and each grows into a medium-sized bulb. Multiplier onions can be harvested and used fresh, stored or saved for future plantings. Also called potato onions, multipliers are planted in early spring for a summer harvest. They can also be left in the ground over the winter if covered with a thick layer of mulch. A single large multiplier bulb can produce up to 10 or 12 bulbs, and bulbs planted in the spring can be harvested like other onions as scallions.