Onions are a rewarding crop, but successful growth depends on precise timing, especially when starting from seed. Growing from seed requires a long, uninterrupted season to ensure large, mature bulbs, unlike planting small, immature “sets.” Starting with seeds offers a wider choice of varieties and often results in a higher-quality, better-storing crop. Seeds require an early indoor start to develop sufficient foliage before outdoor conditions trigger the bulbing process.
Matching Onion Variety to Your Climate
The most fundamental decision for successful onion cultivation is selecting a variety that matches your geographical location. Onion bulb formation is a photoperiodic response, meaning it is triggered by the number of daylight hours, not just the temperature. Planting the wrong type will prevent the bulb from forming correctly.
There are three main categories of onions based on their light requirements. Short-Day onions are best suited for Southern regions, typically below the 35th parallel, and begin bulbing when they receive 10 to 12 hours of light per day. These are often planted in the fall and harvested in the spring in warmer climates.
Long-Day onions require 14 to 16 hours of daylight to trigger bulbing and are the correct choice for Northern regions, generally above the 42nd parallel. These varieties are planted early in the spring, allowing the plants to develop extensive foliage during the long days of summer. The size of the mature bulb is directly related to the amount of foliage developed before bulbing begins.
Intermediate-Day or Day-Neutral onions require 12 to 14 hours of daylight to bulb and are ideal for middle latitudes, often between the 35th and 42nd parallels. A short-day onion planted in the North will start bulbing too early with too little foliage, resulting in a small, disappointing harvest.
The Indoor Start: Calculating Planting Time
The core of successful onion growing from seed lies in calculating the optimal time for the indoor start. The general rule is to sow seeds indoors 8 to 10 weeks before the average last expected frost date in your specific growing region. This timeline provides the seedlings with enough time to reach the ideal size—about the diameter of a pencil and 5 to 6 inches tall—before being transplanted outside.
To start the seeds, use shallow trays or flats filled with a sterile seed-starting mix, planting the tiny seeds about a quarter-inch deep. Consistent moisture is necessary for germination, and a temperature around 65 to 75°F works well until the seedlings emerge. After germination, the seedlings require high light to prevent them from becoming weak and “leggy,” so a strong grow light source placed just a few inches above the plants is often needed.
Trimming Seedlings
A technique often employed is the frequent trimming of the onion seedlings’ tops. When the thin, grass-like foliage reaches about 4 to 6 inches in height, use clean scissors to trim the tops back to about 3 inches. This practice encourages the plant to direct its energy into developing a stronger root system and thicker neck. Trimming also prevents the seedlings from flopping over and tangling, and this process can be repeated every few weeks until the seedlings are ready for the garden.
Hardening Off and Transplanting Outdoors
The second critical timing decision is the transition of the established seedlings from the controlled indoor environment to the unpredictable conditions outdoors. This process, known as “hardening off,” is a gradual acclimation that prepares the delicate plants for direct sunlight, wind, and cooler temperatures. Seedlings that are not properly hardened off can suffer severe transplant shock.
The hardening process should take approximately 7 to 14 days, starting about two weeks before the planned transplant date. Begin by placing the seedlings outdoors in a sheltered, shady location for just a few hours a day, gradually increasing the duration and exposure to direct sunlight over the course of the week. Monitor the plants closely, avoiding exposure to strong winds or sudden, hard temperature drops.
Onions are a cool-weather crop and are quite tolerant of cold, making them one of the first vegetables to be transplanted in the spring. The ideal time to move the hardened-off seedlings to the garden is typically two to four weeks before your last expected spring frost date, as soon as the soil is workable. Onions thrive in cool temperatures, ideally around 50°F, and can withstand light frosts down to about 28°F. Starting them indoors and transplanting is the more reliable method to ensure a mature harvest in regions with shorter growing seasons.