Growing onions is generally approachable for most gardeners, but it requires consistent attention to specific environmental factors throughout its long growing cycle. Onions demand full sun exposure and well-drained soil with a pH level ideally between 6.0 and 6.8. If these requirements are met, the difficulty shifts from initial planting to sustained maintenance and proper variety selection.
Choosing Your Starting Method and Variety
The initial decision of how to start your crop significantly impacts the ease and success of the harvest. Onions can be grown from seeds, small dormant bulbs called sets, or young plants known as transplants. Growing from seed offers the widest selection of varieties and is the most economical method. However, it is also the slowest and most demanding, often requiring an indoor start 10 to 15 weeks before the last frost.
Onion sets, which are small bulbs grown the previous year, are typically the simplest method for beginners because they establish quickly. However, sets are limited in variety. If the bulbs are too large, they can be prone to bolting (prematurely producing a flower stalk), which halts bulb development. Transplants, which are bare-root seedlings, offer a middle ground, providing a substantial head start over seeds and generally leading to the best yields.
The most important factor influencing success is selecting a variety appropriate for your latitude, as onions are highly sensitive to day-length (photoperiodism). Long-day varieties require 14 to 16 hours of daylight to trigger bulbing, making them suitable for northern regions. Short-day onions only need 10 to 12 hours of light and thrive in the southern United States.
Intermediate-day varieties require 12 to 14 hours of daylight and are best suited for middle latitudes. Planting a short-day variety too far north causes it to bulb too early, resulting in small onions. Planting a long-day type too far south means it will never receive enough daylight hours to form a full bulb. This varietal choice is the most common reason for beginner failure.
Essential Care During the Growing Season
The labor involved in growing onions centers on managing their environment, particularly controlling competition and providing steady nutrition. Onions possess a shallow root system, meaning they compete poorly with weeds for water and nutrients. Constant, shallow weeding is necessary throughout the growing season to prevent competition. Care must be taken not to cultivate too deeply, which can damage the roots.
The plants are heavy feeders, requiring a consistent supply of nitrogen early on to support the growth of foliage, as each leaf layer contributes to a ring in the final bulb. Gardeners should apply a nitrogen-heavy fertilizer every few weeks after planting and water it in thoroughly. This feeding should continue until the bulb begins to swell and push the surrounding soil away.
Consistent moisture is necessary, as the shallow roots can quickly dry out. Onions require about one inch of water per week, especially during the bulb development stage. Watering should remain consistent, as moisture stress can cause split or double bulbs.
Bolting can occur if the plants are exposed to temperature extremes, such as a prolonged cold snap followed by warm weather. This tricks the biennial plant into thinking it has completed a full growing cycle. Monitoring for pests like onion thrips is important; these tiny insects appear as small yellow or dark specks that can cause the foliage to turn gray. Proper plant spacing helps with air circulation, which reduces fungal issues like purple blotch and blight that thrive in damp conditions.
Harvesting and Curing for Long-Term Storage
The final stage involves a specific harvesting sequence necessary for successful storage. Onions signal readiness when their leafy tops naturally begin to soften, yellow, and fall over, typically when 50 to 80 percent of the tops have collapsed. Watering should be stopped completely once the tops flop over, allowing the soil to dry out and the outer skins to firm up.
Carefully lifting the bulbs from the soil prevents bruising, which can lead to rot during storage. After harvesting, the onions must be cured. Curing is a process where the neck and outer layers dry completely to seal the bulb. This is performed by spreading the onions in a single layer in a warm, well-ventilated, and shaded area for two to three weeks.
Curing is complete when the necks are thoroughly dry and papery, and the outer skin rustles to the touch. This drying seals the neck, preventing moisture and pathogens from entering the bulb. Once cured, the roots are trimmed and the dry tops are cut back to about one inch above the bulb. They are then ready for storage in a cool, dark, and dry location, preserving the harvest for months.