Onion cultivation requires a long growing season for the plant to properly develop a large, mature bulb. Starting onion seeds indoors is a necessary step for gardeners in most climates, as it provides a multi-month head start that the seeds cannot achieve when sown directly into cold spring soil. The single most important factor determining the success of the harvest is the precise timing of this indoor start, ensuring the seedlings reach a specific size just as outdoor conditions become favorable for growth.
The Critical Timing Window for Indoor Starting
The standard recommendation is to sow onion seeds indoors approximately 8 to 12 weeks before the last expected spring frost date. This timing allows seedlings to reach a pencil-thick stage, which is the ideal size for transplanting and maximizing final bulb size. Gardeners must determine their local last frost prediction and count backward two to three months to find their optimal start date.
Onion seeds germinate best when the soil is warm, ideally between 75°F and 85°F, often requiring a heat mat for consistent warmth. Once sprouted, the heat mat is no longer required, as seedlings prefer cooler temperatures but demand very high light. They need 12 to 14 hours of bright light daily. Supplemental grow lights must be positioned a few inches above the foliage to prevent weak, “leggy” growth.
Understanding Onion Types and Day Length Sensitivity
The choice of onion variety is inextricably linked to successful timing because of a biological mechanism called photoperiodism. This is the plant’s response to the duration of daylight hours, which acts as the trigger for the plant to switch from producing leaves to forming a bulb. If the wrong type of onion is planted in a given region, it will either bulb prematurely, resulting in a small harvest, or fail to bulb at all.
Onions are categorized into three main types based on the amount of daylight needed to initiate bulbing.
Short-Day Onions
Short-day onions require 10 to 12 hours of daylight. They are suited for the southern United States, typically below the 35th parallel.
Long-Day Onions
Long-day varieties need 14 or more hours of daylight. They are the correct choice for northern regions, generally above the 42nd parallel.
Intermediate-Day Onions
Intermediate-day onions require 12 to 14 hours of light. They perform best in the mid-latitudes, acting as a transitional type between the two extremes.
A long-day onion planted in a short-day region will not receive enough daylight to trigger bulbing before the summer heat arrives. Conversely, a short-day onion planted in a northern region will start bulbing too early, limiting the final bulb size.
Preparing Seedlings for the Outdoors
A conditioning process must occur before seedlings are permanently moved outside. This transition is known as “hardening off,” which gradually acclimatizes the tender plants to harsher outdoor elements like direct sun, wind, and cool temperatures. Skipping this step can shock the seedlings, severely stunting their growth or even leading to death upon transplanting.
The hardening-off process should take about 7 to 10 days. Begin by placing the seedlings outdoors in a protected, shady location for just one to two hours. The duration of outdoor exposure is incrementally increased each day, with a gradual introduction to direct sunlight and wind.
Before the hardening-off period begins, trim the top green foliage of the seedlings with scissors to a height of about three to five inches. This trimming encourages the plant to thicken its neck and root system, resulting in a sturdier seedling prepared for the main garden bed.
Direct Sowing vs. Transplanting
While starting seeds indoors for later transplanting is the method favored for producing large, uniform bulbs, a gardener can also choose to sow onion seeds directly into the garden soil. Direct sowing generally takes place very early in spring, as soon as the soil is workable and the threat of severe hard frost has passed. Onion seeds require the soil temperature to be at least 50°F for reliable germination, though they can tolerate slightly cooler conditions.
The young, grass-like onion seedlings face intense competition from weeds in the garden bed. Their slow start often leads to a smaller final bulb size compared to transplants. Direct-sown onions also take an extra month or two to reach maturity, meaning the harvest will be significantly later.