Onion sets are small, immature onion bulbs grown from seed the previous year and cured for storage. They offer a significant advantage over starting from seed for home gardeners because they are easier to plant, offer a reliable head start on the growing season, and have a higher success rate of developing into full-sized bulbs.
Determining the Ideal Planting Window
Onion sets are cold-tolerant, making them one of the first crops planted in the spring garden. The optimal time to plant is typically two to four weeks before the average last hard frost date in your region. This early planting window allows the onion plants to establish robust, leafy top growth before the summer’s lengthening daylight hours trigger the bulb formation process.
Planting too late can lead to smaller, underdeveloped bulbs because the plants will not have enough time to grow sufficient foliage before bulbing begins. Conversely, planting too early risks exposing the sets to prolonged cold snaps, which can stress the plant and cause it to “bolt,” or prematurely send up a flower stalk.
The ideal soil temperature for planting sets is above freezing, and they can be planted as soon as the soil is workable in early spring. Temperatures should not be expected to dip below 20°F. Exposure to sustained temperatures below 55°F after growth begins can trigger the bolting response, channeling the plant’s energy into seed production instead of bulb development.
Preparing the Site and Sets for Planting
Proper site selection requires full sun, meaning at least six to eight hours of direct sunlight daily, as light exposure is directly related to the size of the final bulb. Onions require loose, fertile, and well-draining soil, ideally a sandy-loam, because heavy clay soils can retain too much water and restrict bulb expansion. The preferred soil pH for optimal onion growth is slightly acidic to neutral, falling between 6.0 and 6.8.
To prepare the soil, loosen it to a depth of eight to ten inches, then incorporate aged compost or manure to improve texture and fertility. Onions are heavy feeders, so a pre-planting application of a complete fertilizer with a higher middle number, such as a 10-20-10 blend, helps encourage strong root establishment. Before planting, inspect the onion sets and discard any that show signs of mold, damage, or are excessively large, as sets greater than three-quarters of an inch in diameter are more likely to bolt.
Planting Techniques for Successful Growth
The physical act of planting onion sets is straightforward, but attention to depth and spacing is necessary for maximizing bulb size. Sets should be planted shallowly, just deep enough so that the pointed tip is barely covered with soil, or even slightly visible above the surface. Burying the sets too deeply will inhibit the natural swelling of the bulb near the soil surface.
For growing full-sized, dry onions, space the sets about two to four inches apart within the row, with rows spaced twelve to eighteen inches apart. If the goal is to harvest immature green onions, a closer spacing of two inches can be used, with every other plant thinned out later to allow the remaining ones to grow to maturity. The root side of the set should be oriented downward into the prepared soil.
Post-Planting Care and Management
Immediately following planting, gently water the area thoroughly to help the soil settle around the sets and stimulate them out of dormancy. Because onions are shallow-rooted, they require consistent moisture, particularly during dry periods, generally needing about one inch of water per square foot per week. Maintaining this consistent soil moisture is important because allowing the soil to dry out and then flooding it can stress the plants and contribute to bolting.
Weed control is paramount because the thin onion leaves do not shade out competing growth, and weeds will quickly steal nutrients and moisture. Weeding should be done by hand or with a shallow cultivator to avoid disturbing the onion’s delicate, surface-level root system.
Onions are heavy feeders throughout their growth cycle, so side-dressing with a nitrogen-rich fertilizer every two to three weeks is recommended for foliage growth. This regular feeding should cease once the bulbs begin to swell and push the soil away, as continued nitrogen application at this stage can encourage more leaves instead of a larger bulb.