The common onion, Allium cepa, is a widely grown plant for the home garden. Cultivating this biennial plant for its edible bulb in its first year requires attention to specific environmental factors and planting techniques. Understanding the onion’s growth requirements and following straightforward steps leads to a successful harvest. This guide simplifies the process of growing onions.
Choosing Your Onion Type and Source
Selecting the correct variety based on day-length sensitivity is essential for successful onion growing. Onions are categorized into three groups based on the daylight hours required to trigger bulb formation. Long-day onions need 14 to 16 hours of daylight, suiting them for northern latitudes above 37 degrees north. Short-day onions require 10 to 12 hours of light and are best for southern regions below 35 degrees north latitude. Intermediate, or day-neutral, types are versatile, bulbing with 12 to 14 hours of daylight and performing well in the middle latitudes.
Once the correct day-length type is chosen, a gardener must decide whether to start from seeds, sets, or transplants. Seeds offer the widest selection of varieties but require the longest growing time, often started indoors 10 to 15 weeks before the last frost. Transplants are small seedlings that provide a head start and often result in larger bulbs than sets. Onion sets are small, dormant bulbs that are the easiest and fastest method for beginners. However, sets offer limited variety and are more prone to bolting, which halts bulb growth.
Preparing the Site and Timing the Planting
Onions are shallow-rooted and require a location that receives a minimum of six to eight hours of direct sunlight daily. The soil must be loose, well-draining, and rich in organic matter to allow for unrestricted bulb expansion and moisture retention. Heavy clay soils that retain too much water can lead to bulb rot and should be amended with compost or well-rotted manure to improve aeration. Onions prefer a slightly acidic to neutral soil pH, ideally ranging from 6.0 to 7.0.
The timing of planting is linked to the desired bulb size. For the largest bulbs, onions should be planted in early spring as soon as the soil is workable, typically four to six weeks before the last expected frost. This early start allows the plants to establish a robust root system and maximize leaf growth before the bulbing process begins. In warmer climates, short-day varieties may be planted in the fall for an early spring harvest.
The Step-by-Step Planting Process
The objective of planting is to keep the root system intact and the bulb base near the surface. When planting sets, push the small bulb into the loose soil just deep enough so the base is covered and the pointed tip is visible, typically one to two inches deep. Sets should be spaced four to six inches apart to give each developing bulb sufficient room to swell. Closer spacing of two inches can be used if the goal is to harvest immature green onions.
Transplants should be planted with the roots buried and the lower one inch of the plant covered with soil. Trimming the top growth back to about four inches before planting reduces transplant shock and encourages root development. If starting from seed, sow the seeds shallowly, about a half-inch deep, and then thin the resulting seedlings to four to six inches apart once they reach a few inches in height. Thinning is important because each onion plant only produces a single bulb, and overcrowding severely limits the final size.
Ongoing Care and Curing
Onions are shallow-rooted, requiring consistent moisture and weed control for healthy bulb development. The plants need about an inch of water per square foot per week, especially during bulb enlargement, but overwatering must be avoided as it can promote rot. Since onions compete poorly with other plants for moisture and nutrients, weeding should be done frequently and shallowly to avoid disturbing the near-surface roots.
Onions are heavy feeders and benefit from a nitrogen-rich fertilizer applied every two to three weeks early in the season to support leaf growth. Fertilizing should cease once the bulbing process begins, indicated by the soil cracking as the developing bulb pushes the soil away. Onions are ready for harvest when the green tops turn yellow and begin to fall over, with about half of the foliage lying flat. After harvesting, the bulbs must be cured by spreading them in a single layer in a warm, dry, and well-ventilated location, ideally between 75 and 90 degrees Fahrenheit, for two to four weeks until the necks are dry and the outer skin is papery. Curing seals the onion for long-term storage.