Onions are a popular and widely cultivated vegetable crop. Determining the precise month to plant them depends entirely on the gardener’s geographical location and the specific form of the onion being planted. Starting with seeds, small dormant bulbs called sets, or young plants known as transplants significantly impacts the required timing. Successfully growing a large, mature onion bulb requires precise timing relative to local climate conditions and specific biological requirements.
Understanding Onion Types and Day Length
Onion classification is based on the amount of daylight required to initiate bulb formation, known as bulbing. This photoperiodic response means the plant must accumulate specific daylight hours before it stops producing foliage and shifts energy into swelling the underground bulb. Long-Day onions require 14 to 16 hours of daylight, making them suited for northern regions of the United States and Canada.
Short-Day onions need only 10 to 12 hours of light to begin bulbing, making them the choice for southern climates. Intermediate-Day varieties need around 12 to 14 hours of light and perform well in the middle latitudes. Gardeners must select the appropriate day-length type for their region, as planting an unsuitable variety results in a small, immature bulb.
The starting material also influences the required lead time. Seeds need the longest growth period and are often started indoors several weeks before the final planting date. Sets, which are small, immature bulbs, are the easiest and fastest way to establish a crop. Transplants, which are young plants already several weeks old, offer a good balance of maturity and reliability for bulb production.
Pinpointing Your Ideal Planting Window
The fundamental rule for successful onion growth is ensuring the plant develops substantial foliage and root mass before daylight hours trigger bulbing. In northern or Long-Day regions, the goal is to plant as early in the spring as the soil can be worked, typically from early March through mid-April. Planting sets or transplants during this window allows the plants to develop sufficient green tops before the long summer days initiate bulb swelling. This early establishment is necessary because bulb size is directly proportional to the number of leaves the plant develops before the photoperiodic switch.
Northern growers often start onion seeds indoors during January or February, providing eight to ten weeks of growth before transplanting outside. The young plants should be pencil-thick when moved to the garden to ensure they quickly take advantage of the lengthening spring days. Planting too late in the spring will cause bulbing to occur while the plants are still small, resulting in undersized mature bulbs.
In southern or Short-Day regions, the planting strategy often shifts to a fall or winter schedule to avoid intense summer heat. Gardeners typically plant sets or transplants from September through November, allowing slow growth throughout the cooler months. This fall planting aims for a harvest in the following spring, often in April or May, after the plants have accumulated the necessary light hours.
If fall planting is not feasible in the South, an alternative is planting Short-Day varieties very early in the spring, typically in January or February. This timing ensures establishment before the onset of the hottest weather, which can stress the plants. Intermediate-Day regions generally follow the northern spring planting schedule, aiming for a March planting date.
Gardeners in these transitional zones can choose Intermediate-Day varieties planted in early spring or Short-Day varieties planted in the fall. Onions are cold-tolerant and can be planted approximately four to six weeks before the local average last frost date. This early timing ensures maximum vegetative growth before the photoperiodic switch occurs.
Preparing the Planting Site and Planting Methods
Regardless of the chosen planting month, site preparation is a crucial step for achieving large, healthy bulbs. Begin by selecting a location that receives full, unobstructed sunlight throughout the day. Onions are shallow-rooted and compete poorly with weeds, so the planting bed must be cleared of all competing vegetation before planting. Onions thrive in loose, well-drained loamy soil and struggle in heavy, compacted clay that impedes bulb expansion.
Incorporating organic matter like compost improves soil structure and drainage before planting. Onions are heavy feeders, requiring a consistent supply of nutrients, particularly nitrogen, during their initial vegetative growth phase. A balanced fertilizer high in nitrogen should be worked into the top six inches of soil prior to planting to support strong, leafy tops.
When planting sets, which are small bulbs, position them so the pointed tip is just visible above the soil surface. Planting sets too deeply can restrict the expansion of the new bulb, resulting in misshapen or small harvests. The goal is to anchor the set while allowing the majority of the bulb to rest at the soil line.
Transplants and seedlings started indoors should be trimmed to about five or six inches in length before planting. These young plants should be buried just deep enough to cover the root structure and the white base, ensuring the green stem remains above the soil. Proper spacing is paramount to achieving a desirable bulb size, requiring plants to be spaced four to six inches apart in all directions.
A minimum of 12 inches between rows is recommended to allow for adequate air circulation and access for weeding. If planting seeds directly outdoors, sow them thinly and later thin them to the proper four to six-inch spacing when the seedlings are a few inches tall. Failure to thin seedlings promptly results in a dense cluster of small, undeveloped onions.
Essential Care and Harvesting Cues
Once established, consistent cultural practices ensure the maximum possible yield. Onions require consistent moisture throughout the growing season, especially during the period when the bulb is actively swelling after the day-length trigger has occurred. Irregular watering can lead to splitting or poor bulb development, so the soil should be kept uniformly moist without becoming waterlogged.
Weed control is a high priority, but cultivation must be shallow to avoid damaging the fine root system near the soil surface. Supplemental nitrogen fertilizer should only be applied during the initial foliage growth phase and must be stopped entirely once bulbing begins. Applying nitrogen too late encourages the plant to produce green tops instead of focusing energy on bulb enlargement.
The plant provides clear visual cues when it is ready for harvest, typically when the tops begin to yellow, weaken at the neck, and fall over naturally. This signals that the plant has matured the underground bulb and ceased putting energy into foliage. Once approximately half to two-thirds of the tops have fallen, carefully lift the onions from the soil, ideally on a dry, sunny day.
The bulbs require a curing process for long-term storage, involving drying them in a warm, well-ventilated, and shaded area for one to two weeks. Curing allows the outer skin layers to dry and tighten, sealing the bulb and preventing decay. Once the necks are completely dry, the roots and dried tops can be trimmed, and the onions moved to a cool, dark storage location.