Onions do not adhere to a single growing season; when they grow depends entirely on the specific variety and the local climate. Although they prefer cooler temperatures for most of their life cycle, the formation of the underground bulb is determined by the number of daylight hours, not the temperature. Successful cultivation relies on selecting a variety suited to the region’s latitude and planting it so the vegetative growth phase aligns correctly with the seasonal increase in daylight. The growing cycle is a carefully timed process, often beginning with planting in one season and concluding with harvest in a different one.
Daylight Requirements Dictate Onion Variety
The factor controlling when an onion switches from producing leaves to forming a bulb is photoperiodism, the plant’s physiological response to day length. Onion varieties are classified into three main categories based on the minimum daylight hours required to trigger bulbing. If plants do not receive the correct amount of light for their type, they will fail to form a mature bulb.
The correct selection is important because ample leaf growth must occur before the required daylight hours are reached.
Onion Variety Classifications
Short-Day varieties are triggered to bulb when daylight reaches 10 to 12 hours. They are successful in Southern regions with milder winters. Intermediate-Day types require 12 to 14 hours of daylight and are well-suited for middle-tier latitudes. Long-Day onions require 14 to 16 hours of daylight to transition to bulb enlargement, making them the appropriate choice for Northern regions where summer days offer extended light duration.
Mapping Varieties to Planting Seasons
The onion must establish a robust leaf system before the lengthening days trigger bulbing. Short-Day onions are typically planted in the autumn months, often between October and December, in the Southern United States (below the 35th parallel). This fall planting allows the young plants to develop foliage throughout the mild winter, leading to a harvest in the late spring or early summer.
Long-Day varieties are best suited for planting in the early spring in Northern climates (above the 42nd parallel). Planting occurs as soon as the soil is workable, allowing the onion to develop a large top before the peak of summer daylight triggers bulbing. Harvest for these spring-planted onions usually takes place in the late summer or early fall.
Intermediate-Day varieties thrive in the transitional zone between the northern and southern extremes (approximately between the 35th and 42nd parallels). They can be planted in late winter or early spring. Their bulbing is triggered by the 12 to 14 hours of daylight that arrives in late spring to early summer.
Starting Methods and Growth Timeline
The total time an onion takes to grow to maturity can vary significantly, ranging from 90 to 180 days depending on the starting material and variety. Gardeners typically choose one of three starting methods: seeds, transplants, or sets, each with a different timeline and set of trade-offs.
Seeds
Growing onions from seeds offers the widest selection of varieties and is the most economical method, but it is also the slowest. Seeds are usually started indoors in late winter, 8 to 10 weeks before the last expected frost, to give them a head start. This method requires the longest duration, needing about four months from seed to full bulb maturity.
Transplants
Transplants are small, live seedlings, usually purchased in bunches, and offer a mid-length approach. This is the most common method for home gardeners because it bypasses the delicate germination stage and significantly reduces the time to harvest compared to seeds. They are planted directly into the garden in early spring and generally mature in about 90 to 120 days.
Sets
Sets are small, immature onion bulbs from the previous year’s growth and provide the quickest route to a harvest, maturing in as little as 10 to 14 weeks. Sets are the easiest for beginners but carry a higher risk of bolting, where the plant prematurely sends up a flower stalk in response to temperature fluctuations. They are best used for an early crop or quick consumption, as the resulting bulbs are often smaller and do not store as well as those grown from seeds or transplants.
Harvesting and Post-Harvest Curing
The physical signal that an onion is ready for harvest occurs when the green foliage naturally begins to yellow and fall over, signifying that vegetative growth has stopped. This collapse indicates that the bulb has reached its maximum size and the plant is preparing for dormancy. Once about half to three-quarters of the onion tops have fallen over, the gardener can pull the bulbs from the ground.
After harvesting, the bulbs must undergo curing to prepare them for long-term storage. Curing involves drying the outer skin and the neck of the onion to seal the bulb, which prevents moisture loss and infection. This step is accomplished by spreading the onions in a single layer in a warm, dry, and well-ventilated area, such as a covered porch or shed, away from direct sunlight.
The curing process typically takes two to four weeks, during which the necks will wither and the outer skins will dry into their characteristic papery layers. Once fully cured, the roots and dried tops can be trimmed, leaving about an inch of stem. Proper curing allows the harvest to be preserved for many months in a cool, dry, and dark location.