Planting an onion bulb or a small starter called an “onion set” begins a simple journey toward a new harvest. The planted set is a dormant, immature bulb possessing the genetic material needed to complete its life cycle. Once placed in the soil, the onion transitions from its resting state. It focuses energy on establishing a root system and producing new green foliage, culminating in a larger, mature bulb or, sometimes, the production of seeds.
Preparing and Planting the Onion
For a successful harvest, attention to the onion’s specific environmental needs is required. Onions thrive in full sun, needing at least six to eight hours of direct sunlight daily to energize the bulbing process. The soil must be loose and well-draining, ideally a sandy-loam, to prevent excessive moisture that can lead to rot. Adding compost can improve soil texture.
When planting purchased onion sets, place them root-side down and barely cover them with soil. The pointed tip should remain near or just above the surface, as planting too deeply hinders proper bulb development. Space the sets four to five inches apart to give each future bulb adequate room to expand. Sprouting onions from the kitchen should be planted the same way, ensuring the roots are oriented downward.
Vegetative Growth and Care
Once planted, the onion begins the vegetative phase, focusing on producing green shoots. These leaves are the plant’s energy factories, photosynthesizing sugars transported down to fuel the enlargement of the underground bulb. Each green leaf corresponds to a distinct layer within the developing bulb, directly linking foliage health to the final harvest size. Consistent moisture is required for strong growth, as onions have shallow root systems that dry out quickly.
Provide approximately one inch of water per week, factoring in rainfall, to keep the soil evenly moist. Onions are poor competitors, requiring diligent weeding throughout the growing period, usually done by hand or with shallow cultivation to avoid disturbing the roots. Apply a nitrogen-rich fertilizer early in the season to encourage vigorous leaf growth. Cease fertilization once the bulb begins to visibly swell, as continued nitrogen can delay maturity and result in soft bulbs that store poorly.
Recognizing and Managing Bolting
Bolting is a shift in development where the onion prematurely changes focus from bulb enlargement to seed production. This occurs when the biennial onion is stressed, often by prolonged exposure to cold temperatures below 45°F or significant temperature fluctuations. The plant interprets this stress as a need to reproduce, initiating the formation of a flower stalk. Bolting is recognized by a thick, rigid stem emerging directly from the center of the plant, which eventually forms a cluster of flowers.
Once an onion bolts, energy diverts to the flower stalk, and the bulb will not mature into a quality storage onion. The central stalk creates a woody core that makes curing difficult and provides an entry point for rot. The best course of action is to harvest the bolted onion immediately and use it within a week or two, as it will not keep for long-term storage.
Harvesting the New Bulb
Harvest timing is determined by visual cues from the foliage, signaling that energy transfer to the bulb is complete. The onion tops will naturally begin to yellow, soften, and fall over at the neck. Wait until about two-thirds of the green tops have wilted and fallen completely to the ground. At this point, carefully lift the mature bulbs from the soil, ideally by gently loosening the surrounding earth with a fork to avoid bruising.
After harvest, the bulbs require curing for long-term storage, which involves drying the outer layers and neck. Curing should take place in a warm, dry, and well-ventilated location, out of direct sunlight, with temperatures ideally ranging between 75°F and 90°F. Spread the onions in a single layer for two to four weeks. This allows the neck to shrivel and the outer skin to form a tight, protective paper shell. Once the necks are completely dry, the roots can be trimmed, and the tops cut back to about one inch above the bulb, preparing them for cool, dark storage.