Onion sets are small, immature onion bulbs grown from seed the previous year, offering a simple and fast-track method for producing full-sized onions. Unlike starting from tiny seeds or fragile transplants, sets are dormant bulbs that provide a significant head start in the garden. Achieving a successful harvest depends on proper planting technique, particularly the depth at which the set is placed. Correct planting ensures the onion focuses its energy on developing a large, edible bulb rather than a thick neck or excessive foliage.
Timing and Soil Preparation
The optimal time to plant onion sets is in early spring, as soon as the soil is workable and approximately four to six weeks before the final spring frost. Onions are a cool-weather crop, and planting them early allows for maximum vegetative growth before the summer heat triggers the bulbing process. This extended leaf development is essential because final bulb size is directly proportional to the number and size of the green leaves present.
Onions thrive in loose, well-drained soil that is rich in organic matter, preferring a slightly acidic to neutral pH between 6.0 and 7.0. Heavy clay soils retaining excessive moisture should be amended with well-rotted compost or aged manure to improve aeration and drainage. Before planting, the soil bed should be tilled or loosened to a depth of at least eight inches to accommodate the onion’s shallow, fibrous root system.
Sunlight is a major factor influencing bulb size, as onions require full sun exposure, meaning at least six to eight hours of direct sunlight daily for optimal development. Onions are heavy feeders, so soil preparation must include initial nutrition. Incorporating a balanced fertilizer with a slightly higher middle number, such as 10-20-10, provides the necessary phosphorus and potassium for healthy root establishment.
Planting Depth and Spacing Requirements
Onion sets should be planted shallowly at a depth of about one to one and a half inches. This technique encourages the developing bulb to form on top of the soil line, necessary for proper expansion. The goal is to bury the set just enough so that its basal plate, where the roots emerge, is secured in the soil, with the pointed tip barely visible at the surface.
Planting the set too deep inhibits the bulb’s natural tendency to push upward and outward, which can lead to the formation of a thick neck. A thick neck is undesirable because it delays the curing process and reduces the storage life of the mature onion. When placing the set, ensure the flatter, root-producing end is facing down and the pointed end is oriented upward.
Spacing is a major factor controlling the final size of the harvestable bulb. For growing large, mature dry onions, sets should be spaced four to six inches apart within the row. Closer spacing, such as two inches apart, results in smaller bulbs or can be used if the intent is to harvest them as green onions early in the season. Rows should be spaced twelve to eighteen inches apart to allow for maintenance, air circulation, and light penetration.
Post-Planting Establishment Care
Once the onion sets are situated, consistent care drives successful bulb enlargement. Onions are shallow-rooted and require constant, even moisture for optimal growth; the soil should never be allowed to dry out completely. A general guideline is to provide about one inch of water per square foot per week, requiring supplemental watering during periods of low rainfall.
Weed control is essential because the onion’s shallow root system makes it a poor competitor against surrounding weeds for water and nutrients. Shallow, gentle hand-weeding is preferred over deep cultivation, which can easily damage the roots. To suppress weeds and maintain soil moisture, apply a light layer of organic mulch, such as straw or shredded leaves, around the plants.
Onions are heavy feeders throughout their vegetative growth phase. After the initial planting fertilizer, a high-nitrogen fertilizer should be side-dressed every two to three weeks to support leaf production. Stop all nitrogen applications once the onion begins to push the soil away and the bulb-swelling process is underway, as continued nitrogen can lead to thick necks and poor storage quality.