Sweet onions, prized for their mild flavor and low pungency, are a favorite among gardeners and home cooks alike. The characteristic sweetness is a direct result of the bulb’s low sulfur content, which prevents the formation of volatile sulfur compounds that cause sharpness. Successfully cultivating these low-pungency onions requires careful attention to variety selection and consistent environmental management, especially concerning day length and nutrition.
Selecting the Right Variety Based on Location
The most important factor determining success when growing sweet onions is selecting a variety that matches the local day length. Onions are classified into three types based on the number of daylight hours required to trigger bulbing. Planting the wrong type will result in the plant failing to form a large bulb.
Short-day onions require only 10 to 12 hours of daylight to begin bulbing and are best suited for the Southern United States, generally in latitudes below 35 degrees. These onions, such as the famous Vidalia or Texas 1015, are typically planted in the fall or winter for a spring harvest.
In contrast, long-day onions need 14 to 16 hours of daylight to bulb and are the correct choice for Northern regions above 42 degrees latitude, where they are planted in spring for a summer harvest. Intermediate-day onions, sometimes called day-neutral, are suitable for the central latitudes between the two extremes, generally requiring 12 to 14 hours of daylight. Each leaf on the plant corresponds to one ring on the mature bulb, so ample leaf growth is necessary for a large harvest.
Preparing the Soil and Planting
Sweet onions thrive in a loose, well-draining soil environment. Compacted or heavy clay soil can inhibit bulb expansion and should be amended with organic matter to improve aeration. Onions prefer a slightly acidic to neutral pH range, ideally between 6.0 and 7.0.
The best method for planting sweet onions is using transplants, which are small plants started from seed, rather than sets (tiny bulbs). Transplants should be planted shallowly, burying the roots and only about one inch of the lower stem. Planting too deep can hinder the proper formation of the bulb.
Spacing is important for achieving maximum bulb size, with transplants typically placed four to six inches apart within the row. An initial application of a balanced fertilizer should be worked into the soil before planting to support early growth.
Nurturing Bulbs for Maximum Sweetness
Achieving maximum sweetness and size depends on consistent water management and a specific fertilization schedule. Onions are shallow-rooted plants that require a steady supply of moisture to prevent stress. Water stress can increase the concentration of pungent sulfur compounds, reducing the desired mild flavor.
The plants need roughly one inch of water per week, delivered consistently, especially as the bulbs begin to enlarge. Nitrogen is the main nutrient required early in the season to encourage the production of large, healthy green tops. Fertilization should be done every two to three weeks until the bulbing process begins.
Once the onions start to push the soil away and the ground around the neck cracks, nitrogen application must stop. Continuing to fertilize with nitrogen after bulbing starts will delay maturity and can result in softer bulbs that do not store well.
Harvesting, Curing, and Storage
Sweet onions signal readiness for harvest when the green tops naturally begin to weaken and flop over. This indicates the plant has finished transferring energy from the leaves down to the bulb. Once about half of the tops have fallen, the remaining plants should be gently bent over to ensure uniform maturity.
The bulbs should be carefully lifted from the soil on a sunny day and left in the garden for a few days to begin curing. Curing involves allowing the neck tissue to dry completely, which seals the bulb and prepares it for storage. The onions are then moved to a warm, airy, and shaded location to finish drying for one to two weeks, until the outer skins are papery and the necks are brittle.
Sweet onions have a significantly shorter storage life compared to pungent varieties due to their high water content and low sulfur levels. The compounds that cause pungency are also responsible for preservation. Cured sweet onions should be used within one to two months, as they are prone to rot and spoilage if kept longer.