Knowing the precise moment to harvest onions determines their final quality and storage life. Pulling the bulbs too early means sacrificing size and protective outer layers, while waiting too long can expose them to disease and compromise their dormancy. The transition to bulb maturation is signaled by distinct changes in the foliage, which are identical whether the onion was grown from sets or transplants. Recognizing these physical cues is paramount for achieving a successful, long-keeping harvest.
Visual Cues Indicating Maturity
The most definitive signal that the onion has completed its growth cycle is the natural collapse of the green tops, a process called lodging. This occurs because the neck, the area where the leaves meet the bulb, softens and weakens. The softening indicates that the plant has stopped transferring carbohydrates and nutrients from the leaves to the bulb rings for expansion.
The leaves also begin turning yellow and eventually brown, starting with the older, outer leaves. This shows that the plant is redirecting all remaining energy into forming the protective outer skin layers of the bulb. Wait until 50% to 90% of the onion tops in the patch have fallen over naturally before beginning the harvest. Manually bending the tops over is not advised because it can sever the nutrient transfer too early, potentially resulting in a thick, wet neck that is prone to rot in storage.
Preparing the Onions for Harvest
Once the tops have begun to lodge and yellow, cease all irrigation to the onion patch. Stopping the water supply approximately one to two weeks before the planned harvest date forces the onion to begin the drying process while still in the ground. This deliberate dehydration allows the outer skin layers to thin and dry, which is necessary for long-term storage potential.
Continued watering at this stage can encourage the bulb to take up excess moisture, making the outer skin susceptible to cracking and increasing the risk of mold or bacterial soft rot. If the soil is particularly heavy clay or has experienced recent rainfall, gently loosening the soil around the bulbs can assist in accelerating the drying of the outer layers and simplify the eventual lifting process.
Curing and Storage Preparation
After lifting the bulbs from the ground, the onions require a period of curing. Curing involves drying the neck tissue and the outer papery layers to create a sealed, protective barrier against moisture loss and fungal pathogens like neck rot. The ideal curing environment is warm, dry, and has excellent air circulation, often with temperatures ranging between 75°F and 90°F.
The bulbs should be spread out in a single layer, away from direct sunlight, in a shed or covered porch for two to four weeks. The curing process is complete when the neck is completely dry and tight, and the outer skins are thin, papery, and rustle when handled. Once fully cured, the tops and roots can be trimmed, leaving about an inch of the neck attached. The trimmed, cured onions should then be moved to a final storage location that is cool, dry, dark, and well-ventilated, ideally maintaining temperatures near 32°F to 40°F.