Growing onions from seed offers a vast selection of varieties not typically found in pre-grown sets or transplants, often leading to a superior harvest with better storage potential. This method provides complete control over the plant’s early development, which maximizes bulb size and quality. Starting with seed requires a significant head start indoors, demanding patience and precision to ensure robust seedlings. The process begins long before the last frost, requiring early planning to align the onion’s growth cycle with the local climate.
Selecting Onion Variety and Timing
The size and formation of an onion bulb are determined primarily by photoperiodism, the plant’s response to the duration of daylight. Onion varieties are categorized into three types based on the daylight hours required to trigger bulbing.
Short-day onions, best suited for southern latitudes (typically below 35°), begin to form bulbs when daylight reaches 10 to 12 hours. Intermediate-day, or day-neutral, varieties require 12 to 14 hours of light, making them suitable for central latitudes (between 32° and 42°). Long-day onions, grown in northern regions (above 37°), need 14 to 16 hours of daylight to initiate bulb formation. Selecting the correct type for your latitude is paramount; planting the wrong type results in small bulbs or plants that prematurely bolt.
The ideal time to sow seeds indoors is typically 8 to 10 weeks before the last expected spring frost date. Some gardeners start as early as 12 to 16 weeks to achieve larger transplants. This timing ensures the seedlings have sufficient vegetative growth before lengthening days signal the switch to bulb production. Calculating this date accurately provides a long, cool period for leaf growth, which directly translates to bulb layers and final size.
Starting Onion Seeds Indoors
Starting onion seeds requires careful management of light, moisture, and temperature to produce healthy seedlings. Begin with a sterile, fine-textured seed-starting mix in containers that are at least three to four inches deep, as onions do not tolerate being potted up easily. Sow the small, black seeds densely, covering them lightly with about a quarter-inch of mix.
Initial germination is fastest with a soil temperature between 68 and 77 degrees Fahrenheit, often provided by a heat mat. Once the seeds sprout, typically within a week, remove the heat source. Seedlings must be immediately placed under supplemental lighting for 14 to 16 hours daily to prevent them from becoming weak and “leggy.”
The shoots emerge as small green loops and tend to grow tall quickly and flop over, which can hinder air circulation. To promote robust root development and thicker necks, use sharp scissors to trim the green tops back to about three to four inches tall. Perform this trimming regularly whenever the foliage reaches six to eight inches in height, until about a week before moving the seedlings outdoors.
Transplanting and Field Care
The transition from the protected indoor environment requires a gradual process called “hardening off,” which prepares the tender seedlings for sun, wind, and temperature fluctuations. Over seven to ten days, introduce the onion plants to the outdoors for increasing durations, starting with a couple of hours in a shady, sheltered location. Gradually expose them to direct sunlight and wind, bringing them inside if temperatures are expected to drop below freezing.
Onions thrive in loose, well-draining soil that is rich in organic matter, with a slightly acidic to neutral pH range of 6.0 to 7.0. Before planting, incorporate a balanced fertilizer into the soil, as onions are heavy feeders requiring a constant supply of nutrients to form large bulbs. When transplanting, carefully separate the seedlings, which have delicate root systems, and place them into the prepared garden bed.
Plant the seedlings so the roots are covered, but the base of the stem is very near the soil surface. Planting too deep restricts the bulb’s ability to swell and form properly. Space the individual plants four to six inches apart for maximum bulb size, with rows spaced at least a foot apart. Consistent care involves maintaining even moisture during the bulbing phase and aggressive weed control, since onions have shallow root systems that compete poorly for water and nutrients.
Harvesting and Curing Onions
The signal for harvest occurs when the onion plants naturally begin to mature, typically in mid-to-late summer. Maturity is visually confirmed when the green tops turn yellow and approximately half to three-quarters of the foliage naturally flops over. At this point, the plant has stopped putting energy into leaf growth and is completing bulb formation.
About a week before harvest, stop watering the onions to encourage the outer skins to dry out and the necks to thin. Harvest gently, lifting the bulbs from the soil using a garden fork or trowel to avoid bruising the delicate skin. Bruised onions do not store well and should be set aside for immediate consumption.
Curing is necessary for long-term storage, involving drying the outer skin and neck to seal the bulb. Lay the harvested onions in a single layer in a warm, dry, and well-ventilated location, such as a covered porch or shed. Avoid direct sunlight, which can cause sunscald. The curing process is complete after two to four weeks when the neck is completely dry, the outer skin is papery, and the roots are brittle. Once fully cured, trim the tops and roots, and store the onions in a cool, dark place with good air circulation.