Onions require a very long period of growth to develop into large, mature bulbs, making the timing of planting extremely important. Starting onion seeds indoors is a common practice used by gardeners to maximize the growing season, especially in regions with shorter summers. This method allows the plants to reach a substantial size before transplanting outside, which is necessary for achieving a successful harvest of full-sized onions. The precise timing is determined by the plant’s response to light and local climate conditions.
Why Onions Need an Indoor Head Start
Onion bulb formation is highly dependent on the plant’s age and the amount of daylight it receives, a phenomenon known as photoperiodism. The plant first focuses on a vegetative phase, growing green tops and roots, before shifting energy to bulbing. For each leaf an onion grows, one layer is added to the final bulb, meaning more top growth equals a larger onion.
Bulb development takes four to six months from seed to harvest. Since the vegetative phase must occur before bulbing begins, the plant needs a significant head start indoors to develop enough leaves. If the plant lacks a large top before the bulbing trigger is met, the resulting onion will be small.
Determining the Right Onion Type for Your Region
The switch from leaf growth to bulb formation is triggered when the day length reaches a specific number of hours, which varies by onion type. This sensitivity to light makes it necessary to select a variety adapted to your geographical latitude. Onions are categorized into three main types: short-day, intermediate-day, and long-day.
Short-day onions require only 10 to 12 hours of daylight to start bulbing and are best suited for Southern regions, typically USDA Zone 7 and warmer. Long-day onions need 14 to 16 hours of daylight to initiate bulbing, making them the correct choice for Northern latitudes, generally Zone 6 and colder. Intermediate-day onions, or day-neutral varieties, are a transitional type, bulbing with 12 to 14 hours of daylight and performing well in the middle tier of the country, around zones 5 and 6. Planting the wrong type will result in failure; for example, a short-day onion grown in the North will bulb too early, producing tiny bulbs.
Calculating the Optimal Indoor Start Date
The most reliable method for determining the indoor planting date is to count backward from the average last expected frost date for your location. This date marks the point when seedlings can safely be transplanted outdoors. For long-day and many intermediate-day varieties, the ideal window for sowing seeds indoors is 10 to 12 weeks before this final frost date.
For example, if your average last frost is May 1st, start your seeds indoors between February 7th and February 21st. The goal is to produce seedlings about the thickness of a pencil and six inches tall by the time they are transplanted. Starting seeds too early can be detrimental, as prolonged exposure to cold temperatures (below 45–50°F) after emergence can cause them to bolt, or prematurely send up a flower stalk, which ruins the final bulb. Conversely, starting too late means the plants will not have developed enough leaves before summer day length triggers bulbing, resulting in a small harvest.
Necessary Conditions for Healthy Seedling Growth
Once the timing is calculated, providing the correct environment is necessary for developing robust transplants. Seeds should be sown about 1/4 inch deep in shallow containers or trays filled with a sterile seed-starting mix. The medium must be well-draining, though seedlings do not require extensive root space at this stage.
After germination, which occurs best with some warmth, the seedlings require cooler temperatures, ideally 60–65°F, to promote strong growth and prevent bolting. Natural light from a window is almost never sufficient, especially during winter. Supplemental lighting from grow lights is necessary to prevent the seedlings from stretching and becoming leggy. The lights should be positioned just a few inches above the tops of the seedlings and kept on for 14 to 16 hours per day to mimic the long days necessary for healthy vegetative growth.