Onions have a shallow, fibrous root system, making consistent moisture necessary for successful growth. Most roots are concentrated in the top 12 inches of soil, meaning they are highly susceptible to drying out. Providing the right amount of water at the correct time is the single most important factor after planting to ensure a good harvest. Maintaining soil moisture levels throughout development directly impacts the final size and quality of the bulb.
Initial Needs and Early Growth
During the initial growth phase (transplants, sets, and early vegetative growth), the goal is to keep the upper few inches of soil consistently moist but never waterlogged. This is when the plant establishes its top growth and develops the root structure that supports later bulbing. The shallow root system means the plants dry out faster than deeper-rooted crops.
The soil should feel like a wrung-out sponge, indicating uniform moisture without saturation. In hot or sandy conditions, this may require light, frequent watering, possibly daily or every other day, to prevent the top layer from drying out. Water stress during the four- to eight-leaf stage can negatively affect the final bulb structure by reducing the likelihood of a single, uniform center. Early stage watering should focus on the top few inches where the young roots are concentrated.
Watering During Bulb Formation
The watering strategy must shift once the onion begins the bulb formation stage, often signaled by the neck thickening and the development of its full complement of leaves. This bulbing stage is the most water-intensive period, as the plant dedicates energy to lateral expansion. Consistent, deep irrigation is required to support this rapid growth and maximize yield potential.
The standard recommendation is one inch of water per week, either through rain or supplemental irrigation. Some commercial recommendations suggest up to two inches every four days during peak summer heat. This water should soak the soil deeply, encouraging mature roots to extend further down. Inconsistent watering during this period is risky and can cause the bulb to split, crack, or bolt (flower prematurely), which severely reduces storage quality and size.
Recognizing Water Stress
Gardeners can diagnose water stress by observing the foliage and checking the soil. Underwatering manifests as wilting leaves during the day, progressing to yellowing tips and stunted growth if prolonged. The soil around the plant will be dry to the touch when checked up to the first knuckle.
Conversely, overwatering is indicated by pale, yellowing leaves that wilt despite the soil being saturated. Chronically wet soil starves the roots of oxygen and creates conditions favorable for fungal diseases like root rot, causing the neck to feel soft and mushy. To prevent overwatering, ensure the onions are planted in well-draining soil, allowing excess moisture to pass through the root zone.
Preparing for Harvest (The Drying Phase)
As the onion approaches maturity (usually 2 to 3 weeks before harvest), a final change in watering is necessary to initiate the curing process. Watering should cease entirely once the tops begin to yellow, brown, and fall over. This cessation of moisture is a deliberate action to dry out the neck tissue and outer skins, signaling that the plant is diverting its final energy reserves into the bulb.
Stopping irrigation is important for long-term storage, as a dry neck prevents moisture from entering the bulb, reducing the risk of rot and disease. The goal is to allow the outer layers to develop a dry, papery texture that protects the bulb. Continuing to water once the tops have flopped leads to thick, soft necks that are difficult to cure and result in poor storage life.