Onion sets are small, immature bulbs grown from seed in one season to be planted the next. These miniature bulbs bypass the fragile germination stage and mature faster than direct-sown seeds, leading to an earlier harvest. This method is favored for its reliability and ease of handling. The entire process of making sets is essentially a controlled stunting of the plant’s growth cycle.
Initial Planting and Management
Creating onion sets requires planting seeds in a manner that forces them to bulb prematurely, preventing them from reaching full size. This process begins in early spring, as soon as the soil can be worked, in a location that receives full sun. Selecting a short-day or intermediate-day variety, depending on your region’s daylight hours, ensures the plants receive the proper light cues to initiate bulbing.
The most important step is high-density planting to intentionally stress the young plants and limit their resources. Sow the onion seeds thickly, aiming for a density where the plants are only about half an inch apart in all directions. You must not thin the seedlings; this overcrowding is what stunts their growth and results in small, marble-sized bulbs.
To further discourage large bulb development, use a lean soil that is low in nitrogen, which is the nutrient that promotes leafy green growth. As the season progresses into mid-summer, limit watering to encourage the plants to enter dormancy naturally. This restriction of water and nutrients ensures the bulbs remain small, ideally between one-half and three-quarters of an inch in diameter.
Harvesting and Curing the Sets
The time to harvest the immature bulbs is typically in mid-to-late summer, when the onion tops naturally begin to yellow and fall over. This collapse of the foliage signals that the plant is directing its remaining energy into the bulb and preparing for dormancy. Harvest the sets by gently lifting them from the soil, taking care to avoid bruising the small bulbs.
Curing is a necessary step that dries the outer layers and seals the neck of the onion, preventing disease entry during storage. Lay the newly harvested sets out in a single layer in a warm, dry, and well-ventilated location. A temperature range between 68 and 80°F with good air circulation is effective for this drying period.
Allow the sets to cure for one to two weeks, or until the necks are completely dry and the outer skin is papery and tight. Once cured, trim the dried roots close to the bulb and cut the withered tops, leaving a short neck of about a half-inch of dried stem attached. Any sets larger than one inch in diameter should be set aside for immediate consumption, as larger bulbs are prone to bolting when planted the following spring.
Proper Storage Techniques
Successful long-term storage is essential to keep the onion sets dormant and healthy until the next planting season. The primary goal of storage is to prevent the bulb from breaking dormancy and sending up a flower stalk, a process called bolting, which is often triggered by temperature fluctuations. Storing the sets at an intermediate temperature between 45 and 65°F will almost certainly cause them to break dormancy and bolt when planted.
To maintain dormancy, the sets must be kept in a cool, dry environment with temperatures consistently near freezing, ideally ranging from 32 to 36°F. This cold temperature range inhibits the metabolic processes that lead to sprouting.
Place the prepared, cured sets in mesh bags, old stockings, or shallow, slatted boxes to ensure continuous air circulation and prevent moisture buildup. The storage area must be dark and dry, with humidity kept low to prevent mold or fungal growth. Inspect the stored sets periodically throughout the winter and remove any that show signs of sprouting or rot to protect the rest of the batch.