Onions have a relatively high water demand and the yields and quality respond to soil moisture levels in the upper 12” of the soil. When irrigating your onions, you need to provide enough water for the onion to use by replacing the amount that is lost to evapotranspiration and the amount the onion is taking up to grow. Evapotranspiration (ET) is basically the reverse of rainfall. Rainfall adds moisture to your crop, ET estimates the amount of water that is lost from a crop from evaporation from the soil and transpiration from the plant leaves. More than half of the annual rainfall is consumed by ET.  The amount of ET will vary greatly from onion crops grown in the south during the winter months and crops grown in the northern states during the summer months. As temperatures increase, the rate of evapotranspiration increases. Factors that affect evapotranspiration include the plant’s growth stage or level of maturity, percentage of soil cover, solar radiation, humidity, temperature, and wind and type of plant.

Water use by onion plants increases as the plants grow, with maximum water use during the bulb formation stage in the middle of the season. Since onions have a shallow root system the amount of ET near the top 12” of the soil plays an important role in how much water needs to be replaced. A general rule of thumb is the onions will require 1” of water every 4 days during the vegetative crop development stage (approximately the first 45-60 days). Increase the watering to 2” every 4 days for the midseason bulb formation stage (approximately the 2nd 45-60 days) and then decrease back to 1.5” every 4 days during the late-season maturation stage (15-30 days) Irrigation should be discontinued one to two weeks before harvest or when the tops start leaning over.

Adjust the amount of irrigation by the amount of rainfall.

Individual water applications should not exceed the water holding capacity of the soil, as this will lead to water loss through leaching and run-off. When using drip or furrow irrigation, each application should result in water reaching the plants furthest from the drip line or furrow.

With drip irrigation, pulsing water in short, frequent applications can establish a larger wetted sphere of soil than the use of fewer, longer applications. Shorten the duration of applications if the soil below the root zone becomes excessively wet. Lengthen durations if the wetting front does not reach the bases of plants furthest from the drip line. Because drip systems apply water directly to the root-zone, less water is lost to evaporation from the soil surface. Drip tape or tubing is available with emitters at various spacings and flow rates. It is important to select a combination of drip-line spacing (lines per bed), emitter spacing, and flow rate that will apply water at an appropriate rate for the type of soil in the field. On heavier soils (Figure A), water tends to move more horizontally, and lower flow rates and wider drip-line spacing can achieve uniform application across the bed with fewer losses to leaching. In lighter soils (Figure B), water tends to move downward in the soil therefore a closer line spacing and faster rate of application are needed to have the water move horizontally to reach the outer rows and uniformly wet the root zone across the bed.

Following these irrigation guidelines will increase the size and quality of your onions.