Onions are a fundamental crop valued for their versatility. Determining the best planting time depends heavily on your geographical location and the specific variety chosen. Planting at the wrong time, particularly concerning daylight hours, prevents the plant from forming a large, desirable bulb. Success requires aligning the onion’s biological requirements with the natural progression of the seasons in your area.
Understanding Onion Day Length Requirements
Bulb formation in onions is triggered by photoperiodism, the plant’s response to daylight duration, not temperature. Onion varieties are categorized into three main groups based on the minimum daylight hours needed to initiate bulbing. This biological clock is the most important factor for successful bulb development.
Short-Day varieties require the least sunlight, typically bulbing when daylight reaches 10 to 12 hours. These onions are best suited for the Southern United States, generally below 35 degrees latitude, where day length remains relatively short. Planting a Short-Day onion too far north causes premature bulbing, resulting in small, underdeveloped onions.
Intermediate, or Day-Neutral, onions need slightly longer days, triggered by 12 to 14 hours of sunlight. These types are ideal for mid-latitude regions, serving as a transitional variety between northern and southern growing zones. Long-Day onions require the longest daylight period, needing 14 to 16 hours to begin forming a bulb. These varieties are used in the Northern United States and Canada, allowing maximum time for foliage growth before summer days trigger bulbing.
The amount of vegetative growth, or the number of leaves produced before bulbing starts, directly determines the final bulb size at harvest. Each leaf developed translates into one distinct ring in the mature bulb. Planting the correct day-length variety ensures the plant establishes robust top growth before increasing daylight signals the shift of energy into bulb formation.
Timing for Spring Planting
The primary window for planting onions is early spring, aiming to establish the plant before the summer solstice. Whether using sets (small dormant bulbs) or transplants, the goal is to get them into the ground as soon as the soil is workable. This timing typically falls four to six weeks before your area’s last expected hard frost date.
Onion sets and bare-root transplants are cold-tolerant and can survive light frosts, allowing for an early start. The ground temperature should ideally be at least 50°F for rapid root growth, though planting can occur earlier if the soil is not frozen. Positioning the plants early allows them several weeks to develop a strong root system and substantial leaf growth.
Starting from seed requires shifting the planting schedule indoors for an earlier start. Seeds should be sown indoors approximately eight to ten weeks prior to the planned outdoor transplant date. This early sowing, usually in late winter, allows seedlings enough time to reach a pencil-thick diameter, the ideal size for transplanting.
Transplanting seedlings outside should occur when the soil is not waterlogged and the risk of a severe freeze has passed. This early spring timing is important for Long-Day varieties, which must maximize leaf production before summer days initiate bulbing. Delaying planting causes the bulbing trigger to arrive before the plant accumulates enough leaf layers, resulting in smaller bulbs.
Alternative Planting Schedules
While spring is the main planting season for a summer harvest, alternative schedules exist for specific climates. In regions with mild winters, such as the Southern United States, planting Short-Day varieties in the fall is common. These onions are planted in late fall or early winter, allowing them to establish a root system and small amount of foliage before the coldest part of the year.
The plants overwinter slowly, resuming vigorous growth in early spring, leading to an earlier harvest than spring-planted varieties. Successful overwintering requires planting late enough that the plants only establish roots and avoid growing large tops. Large tops increase the risk of bolting (premature flowering) in the spring. This method yields a crop often ready by late spring or early summer.
A separate planting schedule is used for green onions, also known as scallions, which are harvested for their edible leaves and immature stalks rather than a large bulb. Since they are harvested before bulbing, they are less sensitive to day length. They can be planted from seed or sets intermittently throughout the entire growing season, with successive small plantings every few weeks ensuring a continuous supply.
Optimizing the Planting Environment
Once the ideal planting time is determined, preparing the physical environment is necessary. Onions have a shallow root system and require loose, well-draining soil to expand their bulb freely. A sandy-loam or silt-loam soil rich in organic matter is best. Heavy clay soils are a poor choice unless extensively amended to improve aeration and drainage.
Planting requires a location that receives full, direct sunlight for the majority of the day, as the plant uses maximum light for photosynthesis and leaf growth. Onions thrive when the soil pH is slightly acidic to neutral, typically ranging between 6.0 and 6.8. Raised beds are beneficial because they enhance drainage and allow the soil to warm up earlier in the spring.
When planting sets or transplants, set them shallowly into the soil. The roots should be covered, but the tip of the bulb or the top of the transplant’s root plate must rest near the soil surface. Planting too deeply restricts the bulb’s ability to expand, leading to misshapen or smaller bulbs. For optimal size, individual plants should be spaced three to four inches apart in the row, giving each onion sufficient space and nutrients.