The onion (Allium cepa) is botanically a biennial plant, but it is typically cultivated as an annual crop for its bulb in a single growing season. Achieving large, well-formed bulbs depends on precise planting timing that aligns with specific environmental conditions. Incorrect timing can cause the plant to prematurely flower (bolting) or fail to form a substantial bulb. The goal of proper timing is to encourage maximum leaf growth before the plant shifts its energy into developing the underground bulb.
Understanding Day Length and Onion Varieties
The primary factor determining whether an onion variety will produce a large bulb is the daily duration of light it receives, a biological response known as photoperiodism. Onions spend the initial part of their growth cycle developing foliage, with each leaf representing a layer within the eventual bulb. Once the plant senses a specific number of daylight hours, a hormonal signal is triggered that halts leaf production and begins the process of bulb enlargement. This light requirement separates bulbing onions into three distinct groups based on the latitude where they are grown.
Short-Day Onions
These require the fewest hours of light, typically initiating bulb formation once the daylight reaches 10 to 12 hours. These varieties are best suited for the southern United States and other lower latitudes with milder winters.
Long-Day Onions
These require the most sunlight, typically needing a photoperiod of 14 to 16 hours. This requirement makes them the correct choice for northern latitudes, where summer days are significantly longer. Planting a long-day variety in the South will result in small, undeveloped bulbs.
Intermediate-Day Onions
Also called day-neutral, these varieties require between 12 and 14 hours of daylight to bulb. They are well-adapted for middle latitudes, as they can tolerate a wider range of light conditions. Choosing the correct day-length variety for a specific geographical location is the most important decision for successful onion cultivation.
Optimal Planting Windows by Region
The target planting date is calculated to ensure the onion plant develops a large, robust top before the natural increase in daylight hours triggers bulbing. Maximum leaf growth yields a larger bulb, since the size of the top growth directly correlates to the number of layers in the bulb. The outdoor planting window, therefore, varies significantly across regions.
Southern Regions (Short-Day)
In Southern Regions suitable for short-day onions, the planting window typically opens in late fall or early winter, between October and December. Planting at this time allows the plants to establish a full root system and significant top growth during the mild winter months. The plants then have a large enough structure to support rapid bulbing when the 10-to-12-hour day length threshold is met in early spring.
Northern Regions (Long-Day)
For Northern Regions where long-day varieties thrive, planting occurs in the spring, as soon as the soil is workable. This usually means setting plants out four to six weeks before the last expected frost date. The cold-tolerant plants establish themselves and grow vigorously during the cool spring, maximizing their leaf count. The goal of this early spring planting is to have a sizable plant mass ready to respond when the long summer days (14 to 16 hours) arrive to initiate bulbing. Delaying planting in the North will result in plants that are too small, causing them to bulb prematurely and leading to small, stunted onions.
Intermediate Regions (Intermediate-Day)
In Intermediate Regions, the adaptability of intermediate-day varieties allows for planting in early spring, similar to the North. Gardeners can plant as soon as the soil can be worked, giving the plants a long, cool spring period to develop a substantial top before the 12-to-14-hour day length arrives. This timing balances the need for early root establishment with avoiding prolonged exposure to extreme cold that could induce bolting.
Adjusting the Schedule Based on Planting Method
The selection of planting material—seeds, sets, or transplants—does not change the target outdoor planting date for a given region, but it significantly alters the preparation schedule.
Transplants
The most straightforward method is planting transplants, which are young, grass-like seedlings purchased or grown to be planted directly into the garden bed on the target date. Transplants are often favored by home gardeners because they provide a head start and result in the best yields of large, mature bulbs.
Seeds
Starting onions from seed requires the longest lead time and is the most work, but it offers the widest choice of varieties. For a spring outdoor planting, seeds must be started indoors approximately eight to twelve weeks before the target planting date to produce healthy, pencil-thick seedlings. This indoor period allows the seedlings to reach a transplantable size before the weather permits outdoor planting.
Onion Sets
Onion sets are small, immature bulbs from the previous season that are dried and replanted. They are planted directly into the garden on the target date, similar to transplants. However, because sets have already experienced one growing season, they are biologically primed for their second-year cycle and are at a higher risk of bolting, especially if the sets themselves are too large. For this reason, sets are often considered a better choice for growing green onions rather than the largest possible mature bulbs.