Onions are shallow-rooted with 90% of the roots located in the top 12 inches of the soil. Because of the shallow root system, deficient irrigation can trigger early bulb initiation, resulting in smaller sized onions and reduced yield. Intervals between irrigations will depend upon the soil type, stage of crop development, weather conditions, pest pressure, and the irrigation system. Light, frequent irrigations should be used when the plants are small to minimize leaching of nitrogen from the root zone. Increase the amount of water applied as plants and roots increase in size. During the summer, onions may use 0.15 to 0.25 inches of water per day, and thus, may require irrigation every 5 to 10 days. Irrigation during July and August should thoroughly wet the soil 20 to 24 inches deep. In most years, seeded onions should be irrigated 10 to 15 times during the growing season, applying 1.5 to 3 inches of water each time.

The critical period for irrigation is from the plant establishment through bulb expansion stage. Soil type usually does not affect the amount of total water needed during a growing season, but does dictate the frequency of the water application. Lighter soils need more frequent water applications, but a less amount per application. Heavier soils need less frequent irrigation and a greater amount of water applied per irrigation set. It is important to maintain moisture near the soil surface for good root generation. Research has shown that onion roots generate at the stem plate only when moisture is present. Proper moisture management is also important for general root health, bulb growth, and vigor. Watering should be terminated after the bulbs have reached full size and tops have begun to senesce (at least 2 weeks prior to lifting).

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