Onion sets are small, immature red onion bulbs grown from seed the previous year, cured and stored for planting. Using these dormant bulbs offers a significant advantage over planting seeds or transplants, providing a head start on the growing season. Planting these bulbs in early spring, as soon as the soil can be worked, allows for faster establishment and a reliable harvest of fully formed red onions. This method bypasses the slower germination and vulnerable seedling stage, making it an excellent choice for home gardeners.
Selecting the Site and Preparing the Soil
Successful red onion cultivation begins with selecting a site that receives a minimum of six to eight hours of direct sunlight each day. Onions are heavy feeders and require this full-sun exposure to produce the energy needed for large, flavorful bulbs. The soil must also offer exceptional drainage, as waterlogged conditions can lead to bulb rot and plant failure.
A loose, fertile soil structure, ideally a sandy-loam or silt-loam, is necessary to accommodate the onion’s shallow and non-aggressive root system. To achieve this, thoroughly loosen the soil to a depth of at least eight to ten inches before planting. Incorporating well-rotted compost or aged manure significantly improves both the fertility and the texture of the planting medium.
Before planting, mix in a balanced, low-nitrogen fertilizer, such as a 10-20-10 formula, which is higher in phosphorus and potassium. While onions require high fertility, excessive nitrogen encourages leaf growth at the expense of bulb development. Adding a phosphorus source, like bone meal, supports robust root growth, which is a precursor to a large, healthy bulb.
Planting Technique: Depth and Spacing
Planting the red onion set is crucial for ensuring the development of a round, mature bulb. Sets should be planted shallowly, just deep enough so the base of the bulb is covered and the pointed tip is barely visible or slightly protruding above the soil line. Planting the set any deeper than one to two inches can significantly hinder the bulb’s natural expansion process, resulting in smaller, elongated onions.
When placing the sets, orient the flat, root end downward and the pointed tip upward. Gently push the small bulb into the prepared soil without forcing it excessively, which could damage the emerging root plate. Placing the sets too deeply or pushing them hard into compacted soil can create an obstruction that prevents the bulb from properly swelling as it matures.
To harvest large, mature red onions, space the sets approximately four to six inches apart within the row. This spacing provides enough room for the bulb to expand to its full size without competing with its neighbors. If harvesting immature green onions, a closer initial spacing can be used, but thin them later to the wider spacing required for bulb development.
Initial Post-Planting Care
Immediately after planting, provide a thorough, deep watering to help settle the soil around the small bulbs and eliminate any air pockets. This initial moisture is important for activating the dormant roots and encouraging the first signs of growth. Maintaining consistent soil moisture is a requirement during the early establishment phase, as the shallow root system makes the plants sensitive to drying out.
Red onions require approximately one inch of water per week, either from rainfall or supplemental irrigation, to support leaf development. The soil should never become waterlogged, emphasizing the need for excellent drainage. Because the onion’s root system is non-aggressive, it cannot effectively compete with weeds for water and nutrients, making vigilant weeding mandatory.
Immediately pull any weeds that appear, using a shallow cultivation technique to avoid disturbing the onion’s delicate root zone. If multiple shoots emerge from a single set, or if the initial planting was closer than four inches, thinning is necessary to ensure the remaining plants reach their full bulb potential. Thinned plants can be utilized as fresh green onions, providing an early bonus harvest.