Replanting onions using small seedlings or partially grown sets is an effective way to produce large, mature bulbs for harvest. Transplanting bypasses the lengthy and often inconsistent germination period required when starting from seed outdoors. This approach focuses the plant’s energy on bulb development right from the start, significantly increasing the likelihood of a successful yield. A systematic method ensures the young plants establish quickly and thrive in their new environment.
Preparing the Onion Material and Soil
Preparation begins with selecting a site that receives full sun exposure throughout the day. Onions are heavy feeders and require at least six to eight hours of direct sunlight daily for optimal bulb formation. The ground should be workable, and planting should occur four to six weeks before the last expected spring frost date in your region, after the danger of a hard freeze has passed.
The soil must be loose, crumbly, and well-draining, as heavy clay or compacted soil will inhibit the development of large, round bulbs, often resulting in misshapen or smaller yields. Working in aged compost or well-rotted manure improves aeration and drainage while also enriching the nutrient content. Onions prefer a slightly acidic to neutral soil pH, ideally ranging between 6.0 and 6.8.
Before planting, prepare the onion starts (small, grass-like seedlings) to reduce transplant shock and encourage root growth. Trim the tops of the seedlings down to about four to six inches to prevent them from becoming top-heavy and minimize water loss. A minor trim of the roots to about half an inch stimulates the rapid growth of new feeder roots once planted.
The Replanting Process
The replanting technique requires precision in both depth and spacing to allow for proper bulb expansion. A shallow planting depth is necessary for both onion sets and seedlings, as burying the neck too deeply impedes the bulbing process. Onion sets should be placed with the pointed tip facing upward, covered with about one inch of soil, or just below the surface so the tip is barely visible.
Seedlings should be set at a uniform depth, ensuring the root mass is covered but the point where the leaves emerge from the stem is near the soil line. The most common recommendation for producing large, mature bulbs is to space each plant four to six inches apart within the row. This spacing provides adequate room for the bulb to swell without crowding its neighbors.
Rows should be spaced approximately 12 to 18 inches apart to allow for proper airflow, maintenance, and easy access for weeding. After placing the onion material into the prepared holes, gently firm the soil around the base to eliminate any air pockets that could dry out the delicate roots. Avoid excessive compaction of the surrounding soil, which would restrict the eventual expansion of the bulb.
Post-Transplant Care
Immediate and consistent moisture is required after transplanting to help the plants settle and initiate new root growth. Water the area thoroughly right away to ensure the soil makes good contact with the trimmed roots or the base of the sets. The soil should remain evenly moist but never waterlogged, as excessive water can lead to rot. Onions require about one inch of water per week, including rainfall, throughout their growth cycle.
Diligent weed control is necessary since onions cannot compete well with other plants, but their shallow roots are easily damaged by deep cultivation. Hand weeding or using a very shallow hoe is the safest method. Onions are heavy feeders, requiring high-nitrogen fertilizer to promote vigorous leaf growth, as each leaf layer contributes to the final bulb size. Apply a balanced fertilizer, such as 10-20-10, at planting, followed by a high-nitrogen side-dressing every two to three weeks until the bulbing process begins.