Growing onions successfully depends heavily on planting them at the correct depth, which varies based on the type of planting material used. Getting the depth wrong can hinder bulb formation. Understanding the differences between the common starting materials—sets, transplants, and seeds—is the first step toward a bountiful harvest. Proper planting depth is important, as is the eventual spacing, which determines the final size of the mature bulbs.
Understanding Onion Planting Material
Home gardeners typically start their onion crop using one of three main forms: sets, transplants, or seeds. Onion sets are small, dormant bulbs, usually less than an inch in diameter, grown from seed the previous year and dried for storage. Planting sets offers the quickest path to a mature onion bulb, making them a popular choice for beginners.
Transplants are small, live plants, often sold in bunches, that look like tiny green onions with roots and a small stem. These young plants, usually six to eight weeks old, provide a head start over seeds and are favored for growing specific varieties not available as sets. Starting from seed is the most economical method, though it requires the longest growing season and is often done indoors several weeks before the last frost.
The Exact Planting Depth and Spacing
The planting depth for an onion set should be shallow, only deep enough to anchor the small bulb and prevent it from being pushed out by frost or early growth. Press the set into the soil so the tip, or neck, is just barely covered or even slightly exposed, aiming for a depth of one to two inches at most.
When planting onion transplants, bury the roots and approximately one inch of the lower stem in the soil to replicate their original growing depth. Dig a small hole or furrow and gently place the transplant so the white base is covered, leaving the green tops exposed to sunlight. Direct-sown seeds require the least depth, typically planted only one-quarter to one-half inch below the soil surface.
Spacing is equally important for maximizing bulb size, with a recommendation of four to six inches between plants for large, mature bulbs. Closer spacing, such as two inches apart, will yield smaller bulbs or can be used if the intent is to harvest them earlier for green onions. Rows should be spaced about 12 to 18 inches apart to allow for cultivation and adequate air circulation.
Essential Soil Preparation and Site Selection
Onions are shallow-rooted and highly sensitive to soil conditions, requiring proper preparation before planting. They thrive best in a location that receives full sunlight, ideally a minimum of six to eight hours of direct light daily, to fuel bulb development. The soil must be loose, crumbly, and well-draining, such as a sandy-loam or silt-loam, to allow the bulb to expand without resistance.
Heavy clay or compacted soil restricts the swelling of the bulb, often leading to misshapen or underdeveloped onions. To improve structure, organic matter like well-rotted manure or compost should be thoroughly incorporated into the planting bed. Onions also prefer a slightly acidic to neutral soil pH, typically ranging between 6.0 and 6.8.
Initial Care After Planting
Immediately following planting, a thorough watering is necessary to settle the soil and eliminate air pockets around the roots. Consistent soil moisture is important because the onion’s shallow root system makes the plant susceptible to drying out quickly. The soil should be kept evenly moist, but never waterlogged, which can lead to root rot.
Applying a light layer of mulch, such as straw or herbicide-free grass clippings, helps conserve moisture and suppresses weeds. Weeds compete directly with the shallow-rooted onions for nutrients and water, so their control is a priority. Careful, shallow cultivation with a hoe can control weeds, but deep digging should be avoided to prevent damage to the developing bulbs.