Onions are a popular garden addition, and successfully growing them into large, mature bulbs begins with proper planting. Getting the depth right for your chosen starting material is the first step toward a successful harvest and optimal bulb formation. Planting too deep can hinder the bulb’s ability to swell, while planting too shallow may lead to drying out or instability. Understanding the specific depth requirements for different onion starting materials—sets, transplants, and seeds—ensures the plant can establish a robust root system and maximize the growing season.
Planting Depth for Onion Sets and Bulbs
Onion sets, which are small, immature bulbs harvested and replanted the following season, require a very shallow planting depth. The goal is to barely cover the set with soil, ensuring the bulb’s neck—the pointed tip—is either just covered or slightly exposed. Typically, this means planting them only about one to two inches deep into the soil.
Planting onion sets too deeply can negatively affect the development of the mature bulb. When too much soil covers the emerging bulb, it can encourage neck rot or suppress the natural swelling process. The onion naturally prefers to push upward as it expands, and burying it excessively makes this process more difficult.
The proper technique involves making a small hole or furrow and gently pressing the bulb in, root-side down, so the base is covered but the tip remains near the surface. This shallow placement promotes a healthy neck and prevents excess moisture from accumulating where the leaves emerge.
Depth Requirements for Onion Transplants and Seeds
Onion transplants, which are small, bare-root seedlings often sold in bunches, require a planting depth slightly different from that of sets. These young plants should be set into the ground so that the roots are fully covered and the fragile stem is supported. A depth of about one to two inches is generally recommended for transplants.
Planting transplants at a depth of one inch ensures that the root system is well-established in the soil without burying the neck too deeply. Burying the stem junction too far down can encourage water retention and lead to rot in the sheath where the leaves emerge. You can use a dowel or trowel to create a small hole and gently place the transplant, pressing the soil back around the roots.
Growing onions from seed is another method, which requires the shallowest planting of all. Onion seeds should be sown directly into the garden bed at a depth of only one-quarter to one-half inch. This extremely shallow depth allows the small seeds to germinate and the seedlings to push through the soil easily. Unlike sets or transplants, seeds require a thinning process once they sprout to achieve the desired spacing for bulb growth.
Essential Spacing and Soil Preparation
Proper horizontal spacing is as important as vertical depth for cultivating large, well-formed onion bulbs. To achieve full-sized onions, plants should be spaced approximately four to six inches apart within the row. This distance is necessary because the bulb expands laterally, and crowding restricts its ability to swell to its full genetic potential. Closer spacing of two to three inches can be used if the intent is to harvest immature onions as scallions or green onions.
The soil environment must be carefully prepared to support the shallow planting and subsequent bulb expansion. Onions require loose, well-draining soil, ideally a sandy loam, to prevent compaction around the forming bulb. Heavy clay or compacted soil will physically restrict the bulb’s growth, regardless of the initial planting depth.
Incorporating aged compost or other organic matter into the planting area improves soil texture and drainage while providing necessary nutrients. Since onions are heavy feeders with shallow root systems, loose soil is crucial for both root development and unhindered bulb expansion.