The common onion, Allium cepa, is a widely cultivated cool-season vegetable prized for its bulbs. Successful bulb formation is strictly regulated by precise timing, making the correct planting window the most important factor in onion gardening. This timing depends on regional daylight hours and the type of planting material used.
Understanding Regional Day Length Requirements
Bulb development in onions is governed by photoperiodism, the plant’s response to the relative length of day and night. Onions are categorized into three main types based on the daylight hours required to trigger bulbing. Planting the wrong type for a region will result in small, underdeveloped bulbs.
Short-day onions are adapted for southern latitudes, requiring 10 to 12 hours of daylight to begin forming a bulb. Long-day varieties need 14 to 16 hours of daylight and are suited for northern regions where summer days are longer. Intermediate or day-neutral onions require 12 to 14 hours of daylight, making them suitable for mid-latitude areas.
Timing Based on Planting Material
The material used to start the crop—seeds, sets, or transplants—directly influences the necessary start date. Starting onions from seeds requires the longest lead time but is the most economical method. Seeds must be started indoors 8 to 10 weeks before the last expected spring frost to ensure sufficient leaf growth before transplanting.
Onion sets are small, immature bulbs from the previous season that were forced into dormancy. They are the fastest starting material, bypassing the need for indoor preparation. Sets can be planted directly into the garden as soon as the soil is workable and the danger of a hard freeze has passed. However, sets are more prone to bolting, the premature production of a flower stalk, especially if the bulbs are too large at planting.
Transplants, sold as small bundles of grass-like seedlings, offer a middle ground between seeds and sets. These small plants have already been started by a grower, giving them a significant head start. Transplants can be planted outdoors around the same time as sets, typically two to four weeks before the last expected spring frost. This method reduces the risk of bolting compared to sets and eliminates the long indoor commitment of starting from seed.
Optimal Seasonal Planting Schedules
The calendar date for planting depends on the regional day-length requirement and the local frost-free date. In northern regions, which grow long-day onions, the main planting window is early spring. Transplants and sets should be placed in the ground as soon as the soil can be worked, often two to four weeks before the last anticipated frost. This early planting allows the foliage to maximize growth during the cool spring before the long summer days trigger bulbing.
In southern regions, which grow short-day onions, the planting schedule is shifted significantly earlier. To maximize size, short-day varieties are typically planted in late fall or early winter for an early spring harvest. This allows the plants to establish large tops during the cooler months before the shorter days of early spring signal them to bulb. If a southern gardener waits until spring to plant, the bulbing signal will arrive too soon, resulting in small bulbs.
Fall planting is an option in milder climates for overwintering onions, providing an exceptionally early spring harvest. Seeds for overwintering are typically direct-sown outdoors six to eight weeks before the first hard fall frost. The goal is to establish small plants that will go dormant during the winter and resume growth immediately in the spring.
Duration Until Harvest
Once planted, onions require a long growing period to reach full maturity, typically taking between 90 and 180 days from planting to harvest. Sets are generally the quickest to mature. This long duration underscores why early planting is important, especially in regions with shorter growing seasons.
The end of the growing cycle is determined by visual cues from the plant itself, not a specific date. Onions are ready for harvest when the leaves begin to yellow and soften at the neck, eventually falling over. This “toppling” signals that the plant is shifting its energy entirely from leaf production to bulb maturation. After the tops have fallen, the bulbs should be lifted and allowed to cure for several weeks in a warm, well-ventilated area before being stored.