Successful red onion growth relies heavily on correct planting depth and environment. These flavorful, crimson-skinned vegetables require precise placement in the soil to signal the plant to swell its bulb rather than growing a long neck. The exact depth required depends on the form of the onion you are planting.
Planting Depth Based on Starting Material
The depth for planting red onions changes based on whether you are using onion sets, transplants, or seeds. Onion sets are small, immature bulbs from the previous year, and they require a shallow depth, typically 1 to 2 inches. Place the set so the pointed tip is just barely covered with soil, or even slightly visible above the surface. Planting these too deep will impede the horizontal growth needed for the bulb to expand properly.
If you are beginning with seeds, the planting depth is much shallower, usually between 1/4 and 1/2 inch deep. Seeds need to be near the surface to receive enough warmth and light to germinate successfully. Once the seeds sprout, the initial planting depth is less of a concern than the subsequent thinning process.
Onion transplants, which are small, slender plants often sold in bunches, should be planted about 1 to 2 inches deep. The goal is to cover the root system and the lower portion of the white neck but not much more. Similar to sets, burying the transplants too deeply will encourage the plant to focus its energy on growing a longer stem rather than developing a large, mature bulb.
Essential Soil Preparation and Location
Beyond planting, the condition of the soil is important for red onion development. Onions are shallow-rooted, so they thrive in loose, well-draining soil, with a sandy loam or silt loam texture being ideal. The soil structure must allow for easy expansion of the maturing bulb and prevent water from lingering around the roots.
Poor drainage is detrimental to the crop, as onions are susceptible to rot in soggy conditions. Improve the environment by incorporating generous amounts of aged compost or other organic matter before planting. A soil pH level that is slightly acidic to neutral, ideally between 6.0 and 7.0, supports the best nutrient uptake.
The physical location of the planting site is also a factor in bulb size. Red onions require a location that receives full sun, which means at least six hours of direct sunlight per day. Adequate light exposure triggers the physiological process of bulbing. Preparing the site thoroughly by addressing these soil and light needs creates the proper foundation for healthy growth.
Spacing and Arrangement
The final size of red onions is influenced by the horizontal space provided to each plant. For mature, full-sized bulbs, individual plants should be spaced approximately 4 to 6 inches apart within the row. Planting them too close results in smaller bulbs because the plants compete for limited nutrients and expansion space.
If planting from seeds or sets, you must thin them early in the season if they are placed closer together initially. Thinning involves removing some seedlings to achieve the final desired spacing; the removed plants can be used as green onions. This process ensures that the remaining red onion plants have the necessary room to swell into large bulbs.
The rows themselves should be spaced 12 to 18 inches apart to allow for proper air circulation and ease of cultivation. This spacing prevents overcrowding, which can lead to moisture retention and increased risk of fungal disease. Correct arrangement and spacing translates into a more uniform and substantial onion harvest.