Onion starters, either small, dormant bulbs (sets) or young plants (transplants), offer a significant advantage over direct-sown seeds for the home gardener. Using starters bypasses the lengthy and often inconsistent germination period, giving the crop a vital head start. This method allows the onions to establish a strong root system and foliage before the critical bulbing period begins. Planting starters is the most reliable path to a successful and timely harvest of full-sized onions in a single growing season.
Preparing the Planting Site
Onions require specific soil conditions and ample sunlight to develop large, healthy bulbs. The planting location must receive full sun, meaning at least six to eight hours of direct light each day; insufficient sunlight results in small bulbs. Soil drainage is equally important because onions have shallow root systems and will rot quickly if they sit in standing water. Choosing a well-drained site or building a raised bed at least four inches high is highly recommended.
The soil texture should be loose and crumbly, ideally a sandy or silt loam, to allow for unrestricted bulb expansion. Heavy clay soils should be amended extensively to improve aeration and drainage. Before planting, incorporate generous amounts of aged manure or well-rotted compost into the top six to eight inches of soil. This enrichment provides the high level of fertility that onions, which are heavy feeders, need for optimal growth.
Timing the Planting and Selecting Healthy Starters
The timing of planting is a critical factor for successful onion cultivation because bulb formation is triggered by the number of daylight hours. Gardeners must select a variety—short-day, long-day, or intermediate-day—that matches their geographical latitude to ensure proper bulbing. Long-day onions require 14 to 16 hours of daylight and are best suited for northern regions, while short-day varieties need only 10 to 12 hours and thrive in the South.
Starters should be planted in early spring, as soon as the soil is workable, typically two to four weeks before the average last frost date. Planting early allows the plant to maximize leaf growth in the cool spring weather before the summer solstice triggers bulbing. When selecting onion sets, choose firm bulbs about the size of a nickel or smaller, as larger sets are more likely to prematurely “bolt,” or go to seed. Transplants should have a healthy, green appearance and a strong root system without signs of fungus or disease.
The Physical Planting Technique
Planting the starter requires attention to proper depth and spacing to maximize bulb development. Onions must be planted very shallowly, as burying the bulb too deeply will inhibit its ability to swell. If using sets, gently press the small bulb into the prepared soil so that the tip is barely covered, with no more than one to two inches of soil over the top. For transplants, bury the roots and only about one inch of the lower, white portion of the plant.
Spacing is paramount for encouraging large, full-sized onions. Place each starter approximately four to six inches apart within the row to give the developing bulbs sufficient room to expand without competing for resources. If the goal is to harvest some as green onions or scallions early in the season, the spacing can be reduced to two inches, with plans to thin out every other plant as they grow.
Once the starter is placed, gently firm the soil around the base of the plant to eliminate any air pockets. Avoid compacting the soil excessively, as the bulb needs loose earth to push against as it expands. For multiple rows, ensure there is 12 to 18 inches of space between them to allow for cultivation, air circulation, and ease of access.
Immediate Post-Planting Care
The first step after planting is a thorough initial watering, which is essential for settling the soil around the roots and minimizing transplant shock. This deep soaking helps the roots establish contact with the soil particles. Even if the soil feels moist, a deep watering is necessary to ensure the moisture penetrates to the full depth of the shallow root zone.
Following the initial watering, maintaining consistent soil moisture is important because onions have shallow roots and dry out quickly. Applying a light layer of organic mulch, such as straw or shredded leaves, immediately after planting helps retain moisture and suppress early weeds. Apply the mulch only between the plants and not directly over the top of the forming bulbs, which need to emerge above the soil line as they mature.