When a home gardener talks about growing onions from bulbs, they are typically referring to planting onion sets. These are small, immature bulbs grown from seed the previous year and harvested while still dormant. Utilizing onion sets bypasses the lengthy germination and early growth period required for seeds, offering a significant head start. Planting these dormant bulbs in the spring allows them to quickly resume growth and focus energy on forming a full-sized bulb within a single season.
Choosing and Preparing Onion Sets
Selecting the correct variety determines the success of the onion crop. Onion growth is highly sensitive to photoperiodism, meaning plants begin to form a bulb only after the day length reaches a specific number of hours. Gardeners must select between short-day, intermediate-day, or long-day varieties based on their geographical latitude. Short-day onions initiate bulbing with 10 to 12 hours of daylight and suit southern climates, while long-day varieties require 14 to 16 hours of light and thrive in northern regions. Intermediate-day types suit central latitudes, requiring 12 to 14 hours of daily light to begin bulbing.
The quality of the initial set impacts the final harvest, so look for firm bulbs showing no signs of mold or softening. Sets over three-quarters of an inch in diameter are more likely to “bolt,” or send up a flower stalk, which halts bulb development. Before planting, gently trim away any excessively long, dried roots or loose, papery outer layers. This preparation ensures the bulb establishes itself quickly once placed in the soil.
Planting Technique and Timing
Onions are a cool-weather crop that should be planted in early spring as soon as the ground is workable. This timing typically falls four to six weeks before the last expected hard frost date. They require full sun and soil that is loose, well-draining, and rich in organic matter, ideally with a pH between 6.0 and 6.8. Heavy clay soils must be amended with compost to improve drainage, as onions will not tolerate sitting in wet conditions.
When planting, place the sets shallowly into the prepared soil, with the pointed end facing up. Cover the bulb with only one to two inches of soil, leaving the tip of the set visible above the surface. Planting too deep restricts the bulb’s ability to swell and mature properly. Space each set four to six inches apart to give the bulbs ample room to expand.
Essential Growing Maintenance
Onions possess a shallow root system, making them highly dependent on consistent moisture throughout the growing season. They require about one inch of water per week, especially once the bulbs begin to enlarge. Allowing the soil to dry out and crack negatively affects the final size and quality of the harvest. Avoid overwatering, as this encourages rot in the developing bulbs.
Since the roots are shallow, onions struggle when competing with weeds. Frequent, shallow weeding is necessary to prevent competition for water and nutrients. This weeding should be done carefully by hand to avoid disturbing the delicate root systems. Onions are heavy feeders and require a good supply of nitrogen early on to promote vigorous leaf growth.
The size of the mature bulb is directly related to the amount of top growth established before bulbing begins. Applying a nitrogen-rich fertilizer every few weeks supports this initial growth phase. Once the base of the plant starts to visibly swell and the bulb begins to push the soil away, stop all nitrogen applications. Continuing to fertilize with nitrogen at this stage results in thick, fleshy necks that do not dry down properly for storage.
Harvesting and Curing for Storage
Onions are ready for harvest when the tops begin to naturally yellow, brown, and fall over. This collapse signals that the plant has finished its main growth cycle and is directing remaining energy into the bulb. Waiting until roughly two-thirds of the tops have collapsed yields the best flavor and size. Gardeners should resist the urge to bend the tops over manually, as this stunts bulb development.
About two weeks before the expected harvest date, stop watering the plants entirely. This withholding of moisture allows the outer skins to begin drying and thickening, which is important for storage. To lift the bulbs, gently loosen the surrounding soil with a garden fork to prevent bruising before carefully pulling the onions by their tops. Bruising the onion makes it susceptible to rot and significantly reduces its storage life.
After lifting, the bulbs must be cured for long-term keeping. Curing involves spreading the onions in a single layer in a warm, dry, and well-ventilated area, such as a covered porch or shed. The ideal curing temperature is between 75 and 80 degrees Fahrenheit. The process takes two to three weeks until the necks are completely dry and papery. Once fully cured, the brittle roots can be trimmed off, and the dried tops should be cut back to one inch above the bulb before moving the onions to a cool, dark storage location with good air circulation.