Onions are a garden staple, and successful bulb development relies on planting them at the correct depth. Gardeners typically start onions in one of three ways: from small, dormant bulbs called sets, from young transplants grown from seed, or by sowing seeds directly into the soil. The method chosen dictates the precise depth required, as planting too deep or too shallow significantly affects the final harvest.
Why Planting Depth Impacts Onion Growth
The depth at which an onion is placed directly influences two major factors: the plant’s stability and its ability to form a proper bulb. Onions have a shallow, fibrous root system. A slightly deeper placement ensures the root crown is adequately anchored, preventing the young plant from being dislodged by rain or falling over.
The most important effect of depth is on bulb expansion. Onions are designed to form their bulbs near the soil surface, where they can push outward unhindered as they swell. Planting the onion too deep restricts this natural process, forcing the bulb to expand within a confined space. This deep placement often results in smaller bulbs and “neck thickening,” which severely reduces the onion’s ability to cure and store long-term.
Specific Planting Depth for Onion Sets, Transplants, and Seeds
Onion Sets
Onion sets are best planted very shallowly. The small bulb should be covered with only about one-half to one inch of soil, ensuring the pointed tip remains barely covered or even slightly exposed. This shallow depth allows the bulb to begin swelling immediately above the soil line without resistance from dense soil.
Transplants
Young onion transplants should be planted at the same depth they were growing in their trays, or slightly shallower. Cover the roots completely, but the base of the stem, where the bulb will begin to form, should be no more than one inch below the soil surface. Planting transplants deeper than this can hinder bulb formation and encourage excessive leafy growth.
Seeds
For gardeners sowing onion seeds directly, the depth is the shallowest, typically a quarter to a half-inch deep. This minimal coverage is sufficient to ensure good seed-to-soil contact while allowing the tiny seedling to emerge quickly. Seeds are sown thickly and require thinning once the seedlings are a few inches tall, leaving the remaining plants spaced four to six inches apart for proper bulb development.
Immediate Steps After Planting
Once the onions are placed at their correct, shallow depth, initial care is necessary to promote establishment. First, water the area thoroughly to settle the soil around the roots and eliminate air pockets. Lightly firming the soil with your hand around the base of transplants or sets provides additional stability.
A light layer of organic mulch can be beneficial immediately after planting, but it must be applied with caution. The mulch helps to retain consistent soil moisture and regulate temperature. Avoid mounding mulch or soil against the neck of the plant, as this defeats the purpose of shallow planting and can impede proper bulb formation later in the season.