Onion sets are small, immature bulbs from the previous season, offering the most straightforward method for growing a successful crop. Unlike starting from seeds or delicate transplants, sets are easy to handle and establish quickly, providing a significant head start. They are dormant plants ready to resume growth, making them the preferred option for a reliable harvest and bypassing the long indoor starting period required for seeds.
Preparing the Planting Site
Planting onion sets should occur in early spring, as soon as the soil is workable, often a few weeks before the final expected spring frost. Onions require a location that receives full sun, needing at least six to eight hours of direct sunlight each day for optimal bulb development.
Onions have shallow, inefficient root systems, so the soil quality must be loose and crumbly, offering both excellent drainage and high fertility. Heavy clay or compacted soils should be avoided because they retain too much moisture, which can lead to rot. The preferred soil texture is a sandy to silty loam, with a slightly acidic to neutral pH range, ideally between 6.0 and 6.8.
Incorporating several inches of aged compost or well-rotted manure improves the soil structure, boosting aeration and drainage. Onions are heavy feeders, so a balanced fertilizer, such as a 10-20-10 blend, can be mixed into the top few inches of soil at planting time. This provides immediate nourishment for the shallow root systems.
Step-by-Step Planting Guide
The physical act of planting the sets is simple, but correct orientation and depth are necessary for proper bulb formation. Each set has a pointed end, which is the sprout tip, and a flatter end where the roots emerge. The set must be planted with the pointed tip facing upward and the root end facing down into the soil.
Plant the small bulb shallowly, pressing it into the loose soil so it is covered by only about one inch of soil. Planting too deeply will inhibit the onion’s ability to form a large, round bulb.
For growing full-sized bulbs, space the sets approximately four to six inches apart within the row. If you plan to harvest some immature onions as green onions, you can initially space them closer, around two inches apart, and then thin them out later. Rows should be spaced about 12 to 18 inches apart to allow for easy access for cultivation and to ensure adequate airflow. After planting, a thorough watering helps the soil settle around the newly placed sets.
Essential Post-Planting Care
Consistent moisture is important for onions, especially during dry spells, because their root systems are shallow and not efficient at seeking out water. Aim to provide about one inch of water per week, including any rainfall, keeping the soil consistently moist but never waterlogged. Watering is particularly important as the bulbs begin to swell, though it should be reduced significantly closer to harvest time.
Weed control is important because onions cannot compete well with other plants for moisture and nutrients. Hand-weeding is often preferred over hoeing to avoid damaging the shallow root system or the forming bulbs. Applying a light layer of mulch, such as straw or grass clippings, can help suppress weeds, conserve soil moisture, and maintain a cooler soil temperature.
Onions are heavy nitrogen feeders, requiring regular applications to encourage the growth of robust foliage, which directly translates to bulb size. Begin fertilizing with a nitrogen-rich formula about three weeks after planting, and continue applying it every two to three weeks. It is important to stop all nitrogen applications once the bulbs begin to push the soil away and start expanding, as continued feeding at this stage can result in thick necks that store poorly.
Knowing When to Harvest
The time to harvest a mature onion is indicated by specific visual cues from the foliage, typically occurring 90 to 120 days after planting. The first sign of maturity is the softening of the neck, followed by the natural yellowing and collapsing of the leafy tops. This collapse signals the plant is ready for the final stage of bulb development.
Once most of the tops have fallen over, the bulbs should be carefully lifted from the soil, using a garden fork to prevent bruising, which can encourage rot in storage. The lifted onions should then enter the curing process, which is essential for developing the dry, papery outer skin that allows for long-term storage. Curing involves laying the bulbs out with their tops still attached in a warm, dry, and well-ventilated location, such as a garage or shed.
The curing period lasts for two to three weeks, or until the necks are completely dry and the outer scales rustle like paper. After curing is complete, the roots and the dry tops are trimmed away, leaving about an inch of stem above the bulb. Any onions with thick, soft necks or signs of damage should be set aside for immediate use, as they will not store well.