An onion set is a small, immature red onion bulb grown from seed the previous year and kept dormant for replanting. Starting with these miniature bulbs is generally the easiest and fastest way to grow full-sized red onions in a home garden. Red onions are prized for their deep purple-red color and generally milder flavor compared to yellow or white varieties. Utilizing sets minimizes the long growing period required from seed, accelerating the journey to a successful harvest.
Pre-Planting Preparation
Selecting the proper location is the first step toward developing large, healthy bulbs, requiring a spot that receives at least six to eight hours of direct, unfiltered sunlight daily. Onions thrive best in well-drained, loose soil, and heavy clay must be amended significantly with organic materials like compost or aged manure to prevent waterlogging. The soil should maintain a slightly acidic to neutral pH level, ideally between 6.0 and 7.0, as highly acidic conditions can negatively affect bulb formation and nutrient uptake.
A crucial decision involves choosing the correct variety for your latitude, as bulb development in onions is regulated by the hours of daylight. Long-day red onion varieties require 14 to 16 hours of daylight to initiate bulbing and are suited for northern regions. Conversely, short-day varieties need only 10 to 12 hours of light and are best for southern climates. Intermediate-day types are adapted for central latitudes with 12 to 14 hours of daily light. Planting the wrong type will result in the plant growing only green tops without forming a usable bulb.
Step-by-Step Planting Guide
The ideal planting time is early spring, as soon as the soil becomes workable after winter, often weeks before the final expected frost. Planting early allows the plant to develop a large set of green tops, which directly translates to the final size of the onion bulb. The initial growth phase focuses entirely on foliage production before the plant shifts its energy to bulb swelling later in the season.
Proper spacing is necessary to encourage full bulb size, requiring a minimum of four to six inches between each set in the row. If you plan to harvest some immature onions as green scallions, closer spacing can be used, but the remaining bulbs must be thinned to the wider distance for optimal growth. Onion sets should be planted very shallowly, only deep enough so that the top tip of the bulb is just barely covered by a thin layer of soil or even slightly exposed.
Burying the set too deeply inhibits the eventual bulb from expanding properly and can lead to smaller, misshapen onions. After positioning the set root-end down, gently firm the surrounding soil to ensure good contact and eliminate any large air pockets. A thorough, gentle watering immediately after planting will help settle the soil and signal the dormant bulb to begin sprouting new roots and foliage.
Essential Care and Maintenance
Consistent moisture is necessary for developing a large, healthy red onion bulb, as the plants have a shallow root system and are easily stressed by drought. Aim to provide about one inch of water per week, especially during the middle of the growing season when the bulbs are actively swelling. Inconsistent watering can cause the bulbs to split or reduce their final size, requiring regular checks of the soil moisture.
Weed control is important because the shallow roots of the onion cannot compete effectively with aggressive weeds for water and nutrients. Hand-pulling weeds is preferred, as deep cultivation can easily damage the developing bulbs just beneath the soil surface. Applying a light layer of mulch helps suppress weeds and maintain soil moisture, but keep it away from the actual bulb to allow for proper air circulation and expansion.
Onions are considered heavy feeders, requiring a steady supply of nitrogen early in the season to fuel the growth of their foliage. A high-nitrogen fertilizer can be applied every few weeks after the initial planting to encourage maximum top growth. However, fertilization must stop once the bulbs begin to swell and push the soil away, as continued nitrogen application at this stage can delay bulb maturity and lead to softer, poorly storing onions.
Harvesting and Curing
Harvest time is indicated by a natural physiological change in the plant tops, typically occurring in mid to late summer. The main sign is when the foliage begins to yellow and naturally fall over, signaling that the plant has finished transferring energy into the bulb. Harvesting should occur when 50 to 75% of the tops have fallen, as waiting longer increases the risk of the bulbs rotting or attempting to regrow.
After gently lifting the bulbs from the soil, the curing process begins for long-term storage. Curing involves laying the onions in a single layer in a warm, dry, and well-ventilated location, such as a covered shed or garage, away from direct sunlight. This process allows the neck of the onion to dry completely and the outer skin to form a protective, papery layer, sealing the bulb.
The curing period usually takes one to three weeks, depending on humidity and temperature, until the neck is completely dry and shrunken. Once fully cured, the dried foliage can be trimmed, leaving at least an inch of the stem attached to the bulb, which helps prevent moisture and microorganisms from entering. The cured red onions can then be stored in mesh bags or braided together in a cool, dry place.