Proper planting depth is crucial for maximizing onion yield. An onion transplant is a small, pre-grown seedling, usually sold in bundles, which is distinct from planting a dormant bulb (a set) or starting from seed. Correct depth directly influences how the plant’s neck and bulb develop in the soil, ensuring the bulb swells horizontally and achieves its full size potential.
Preparing Onion Transplants for Planting
Before planting, preparation helps transplants establish quickly and reduces relocation stress. Onion transplants often have long roots and foliage that cause excessive moisture loss. Trimming the roots to about a half-inch to one inch encourages the rapid development of new, healthy root hairs.
The green tops of the transplants should also be trimmed, reducing their length to about five or six inches. This practice minimizes the surface area for transpiration, which is the evaporation of water from the leaves. Reducing the leaf length allows the plant to focus energy on root establishment rather than supporting tall foliage, thereby reducing transplant shock.
Soil preparation requires creating a loose, well-draining environment, since onions develop poorly in compacted or heavy clay soils. Working in organic matter, such as compost, improves soil structure and aeration. Because onions are shallow-rooted, a soil pH between 6.2 and 6.8 is best for optimal nutrient uptake and growth.
Determining the Correct Planting Depth and Spacing
Onion transplants must be planted shallowly so the developing bulb sits correctly at the soil line. The target depth is one-half to one inch deep, covering only the roots and the base of the plant. This shallow placement is required for the plant to form a large, mature bulb.
Burying the transplant too deeply restricts the neck of the bulb, preventing outward expansion. This restriction results in smaller, misshapen bulbs or thick-necked onions that do not store well. The goal is for the top of the bulb to push out of the soil as it swells.
Spacing directly affects the final bulb size. For large, full-sized onions, space transplants four to six inches apart. If plants are intended for early harvest as green onions, spacing can be reduced to one to two inches. Sufficient space also promotes air circulation, which reduces the risk of fungal diseases.
Initial Watering and Post-Planting Care
Immediately after planting, a thorough watering is necessary to settle the soil around the trimmed roots and eliminate air pockets. Because onions have a shallow root system, they require consistent moisture throughout their growth cycle. Providing about one inch of water per week, via rainfall or irrigation, is usually sufficient, though sandy soils may require more frequent watering.
During establishment, plants benefit from a balanced or low-nitrogen fertilizer application every few weeks. Onions are heavy feeders, and early nutrients support the foliage growth that powers bulb formation. Mulching with straw or grass clippings helps maintain consistent soil moisture and suppresses weeds. Do not pile mulch directly against the plant stems, as this can trap moisture and encourage rot.