An onion set is a small, dormant bulb grown from seed the previous season. Utilizing these sets eliminates the lengthy germination period required for seeds, making them a popular choice for home gardeners. Successful onion cultivation relies on precise timing to ensure the plant maximizes foliage growth before bulb development begins. Incorrect planting time can lead to bolting or result in small, underdeveloped bulbs.
Timing Planting Based on Climate and Frost
The optimal window for planting onion sets is in the late winter or very early spring, as soon as the soil is workable. Onions are a cool-season crop that thrives in cooler temperatures and can tolerate a mild frost. It is recommended to plant the sets four to six weeks before the date of the last expected hard frost.
Monitoring the ground temperature provides a more reliable metric than relying solely on the calendar. Onion sets establish best when the soil temperature is consistently between 35°F and 55°F. Planting in this cool range encourages the plant to focus energy on developing a strong root system and robust green tops.
Planting sets too early exposes the young plants to prolonged periods of near-freezing temperatures. This extended cold exposure can trick the plant into prematurely triggering a reproductive cycle. Consequently, the onion will bolt, sending up a flower stalk and diverting energy away from bulb enlargement.
Conversely, delaying planting until the soil is too warm shortens the vegetative growth phase. Once warmer weather arrives, the plant quickly shifts focus to bulbing regardless of foliage size. Since the final bulb size is directly related to the number and size of the leaves developed, planting late results in a smaller harvest.
Selecting the Right Onion Type for Your Region
A successful harvest depends on selecting an onion variety suited to the local geography. Onion varieties initiate bulb formation in response to a specific duration of daylight hours, a phenomenon called photoperiodism. Choosing the wrong type for your latitude is a common reason for a lack of bulb development.
The US is divided into three zones corresponding to the type of onion that will form a bulb. Long-day onions require the longest daylight hours, typically needing between 14 and 16 hours of light to start bulbing. These varieties are appropriate for northern latitudes, roughly north of a line running from San Francisco to Washington, D.C.
Short-day onions begin to form bulbs when the daylight reaches only 10 to 12 hours. This makes them ideal for southern states, where the difference between winter and summer daylight length is less pronounced. Short-day varieties are often planted in the fall to mature during the spring in these warmer zones.
Intermediate or day-neutral onions require 12 to 14 hours of light for bulb initiation. These varieties are well-suited for the central band of the country, often corresponding to USDA hardiness zones 5 and 6.
The plant grows foliage throughout the cooler spring months, with each leaf representing a potential layer of the mature bulb. Once the required day length is achieved, the plant stops producing new leaves and the existing leaf bases swell to form the storage tissue.
Step-by-Step Planting of Onion Sets
Once the correct timing and variety have been determined, the planting process is straightforward. Ensure the planting bed consists of loose, well-draining soil, as compacted soil restricts bulb expansion. Incorporating compost or aged manure prior to planting helps improve soil structure and provide necessary nutrients.
The onion sets should be planted very shallowly, barely covering the bulb below the soil surface. A planting depth of about one inch is ideal, as setting them too deep prevents the bulb from properly expanding. The pointed end, the growing tip, must be oriented upward and should be just visible above the soil line.
Proper spacing between each set is necessary to allow for maximum bulb growth without competition. Aim to space the bulbs four to six inches apart within the row.