Planting onion starts, also known as transplants, is necessary for a successful harvest of large, well-formed bulbs. These small, pencil-thin green plants are popular because they bypass the lengthy germination period required for seeds. Proper planting depth and spacing are the primary factors that determine whether your efforts yield small scallions or substantial storage onions. Following specific instructions ensures the young plants establish quickly and dedicate their energy toward developing a healthy, mature bulb.
Identifying and Preparing Onion Transplants
An onion start is a young, bare-root seedling, typically sold in bunches, that is distinct from an onion set, which is a small, immature bulb. Upon receiving your starts, remove them from their packaging immediately to prevent rot and keep them in a cool, dry, well-ventilated area until planting time.
Preparing the start involves trimming both the roots and the tops to minimize transplant shock and encourage new growth. Trim the roots down to about half an inch to stimulate fresh root hair generation once placed in the soil. The green tops can be clipped back by about one-third, often to a length of five to six inches, especially if they are long and floppy. This trimming reduces the surface area for moisture loss, helping the plant focus its energy on root development.
Ideal Soil Conditions and Site Selection
Onions perform best in a location that receives full sun, meaning at least six to eight hours of direct sunlight daily for robust bulb formation. The soil structure is equally important for allowing the bulb to expand without restriction. Onions thrive in loose, well-drained, and crumbly soil, ideally a sandy-loam or silt-loam texture. Heavy clay soils can compact easily, which inhibits the uniform swelling of the bulb, so incorporate generous amounts of aged compost or well-rotted manure to improve aeration.
The soil’s chemical composition affects nutrient uptake and plant health. Onions prefer a slightly acidic to neutral pH range, with the ideal being between 6.0 and 6.8. Testing your soil allows you to adjust the pH with amendments like ground limestone to raise the level or peat moss to lower it. Furthermore, because onions have a shallow and inefficient root system, the soil must be rich in organic matter to hold consistent moisture and fertility near the surface.
Optimal Planting Depth and Spacing
They should be planted very shallowly, only deep enough to cover the white root base and the fine roots, which is typically about a half-inch to one inch of soil. Planting too deeply is the most frequent mistake, as the thick layer of soil above the bulb’s neck can physically restrict the developing bulb, preventing it from expanding laterally into a large, round shape. The onion bulb naturally needs to push itself up and out of the soil as it matures, and excessive depth hinders this process.
To plant, use a finger or a small dibbler to create a shallow hole just large enough to accommodate the trimmed roots. Gently set the start into the soil, ensuring the roots point straight down, and then lightly firm the soil around the base of the plant. The neck of the plant, where the green leaves emerge from the white base, should be at or very near the soil surface. For spacing, if you intend to harvest large, mature bulbs, place the transplants approximately four to six inches apart in the row. Closer spacing of two to three inches can be used if your goal is to harvest earlier as small green onions or scallions, with the understanding that the remaining plants will be thinned out to allow the others to reach full size.
Initial Watering and Mulching for Establishment
After placing the onion starts in the ground, an immediate and thorough watering is necessary to settle the soil around the roots. Since the plants have a shallow root system, consistent moisture is important for the first few weeks as they establish themselves. The soil should be kept evenly moist but never waterlogged, which can lead to rot and fungal issues.
Applying a light layer of organic mulch, such as straw or weed-free grass clippings, immediately after planting aids in early establishment. This layer regulates soil temperature, reduces water evaporation, and maintains consistent moisture. Mulch also suppresses weeds, which are a major competitor for the shallow-rooted onion plants. As the bulbs begin to swell and push out of the soil, the mulch should be pulled back slightly to allow the tops of the bulbs to dry in the sun, which promotes curing and helps prevent neck rot.