The onion is one of the world’s most popular garden crops, but its successful growth depends entirely on precise timing. Unlike many other vegetables, the onion plant’s transition from leafy green growth to a mature, harvestable bulb is strictly controlled by the length of the day. Planting time is the most important factor for achieving a large, well-formed bulb, and the correct window depends heavily on the gardener’s geographic location and the specific variety chosen.
Understanding Onion Day-Length Needs
The fundamental science governing onion growth is called photoperiodism, which is a plant’s response to the duration of light exposure. An onion only begins to form a bulb when daylight hours reach a specific threshold. This process stops the production of new leaves and diverts energy downward. Gardeners must select a variety that matches the peak day length of their region to ensure the plant grows large green tops before bulbing is triggered.
Onion varieties are classified into three groups based on the minimum light requirement needed to initiate bulbing. Short-day onions require 10 to 12 hours of daylight to begin forming a bulb. These varieties thrive best in the Southern United States, generally between 25° and 35° north latitude.
Moving northward, intermediate-day or day-neutral onions are suited for transition zones, needing 12 to 14 hours of light to start bulbing. This group performs well in the middle latitudes of the US, approximately between 32° and 42°. Finally, long-day onions require 14 to 16 hours of daylight and are the choice for the Northern US, typically above 37° north latitude.
If a gardener plants a short-day onion in a long-day region, the plant will start bulbing too early, resulting in small, disappointing bulbs. The size of the final bulb is directly related to the number of leaves the plant develops before the bulbing signal is received. Choosing the correct day-length type for your specific latitude determines the ultimate success of the crop.
Spring Planting Windows by Onion Type
The traditional time for planting onions to achieve a full-sized bulb is in the spring, allowing the plant to maximize leafy growth before the summer solstice triggers bulbing. The exact timing depends on whether the gardener is planting from seeds, small bulbs called sets, or young transplants. For all methods, the goal is to plant as early as possible once the ground is workable.
Planting onion seeds requires the earliest start. Seeds must be sown indoors approximately 8 to 10 weeks before the last expected frost date, usually between late December and February. Starting seeds this early ensures the seedlings are robust enough to be transplanted outside as soon as the weather allows.
Once the threat of a hard freeze has passed, seedlings are transplanted, or small onion sets are planted directly into the garden. Onions are cold-tolerant and should be planted four to six weeks before the last expected frost. They are one of the first crops to go into the ground in early spring. A rule of thumb is to plant once the soil temperature is consistently above 50°F.
Transplants and sets are placed in the soil with the pointed tip just below the surface, allowing the roots to anchor quickly. This early planting ensures the plants can focus on developing a large quantity of foliage during the cool, moist spring period. The more leaves the plant develops, the more layers the bulb will have. The main spring planting leads to a harvest window that typically begins in mid-summer, when the tops naturally begin to fall over.
Overwintering and Fall Planting
An alternative strategy for growing onions, particularly in milder climates, involves planting them in the fall for an earlier spring or summer harvest. This method is primarily used in regions with short-day conditions, such as the Southern US, or in zones where the ground does not freeze solid for long periods.
The key to successful overwintering is precise timing in late summer or early fall, typically late August through September. Small sets or transplants must be planted late enough to establish a robust root system and some green growth before cold weather slows development. The plant must not begin the bulbing process before winter dormancy sets in.
This pause in growth allows the plant to resume development very early the following spring, giving it a head start over spring-planted varieties. Specific “overwintering” varieties are used because they are more cold-hardy and less prone to bolting (sending up a flower stalk) after cold exposure. Gardeners in colder regions often use heavy mulch, such as straw, to protect the shallowly planted bulbs from heaving out of the ground during freeze-thaw cycles.