It's especially important for onions to get water just after planting. A well-hardened transplant can survive almost two weeks in dry soil. But, in the long run, early dryness will hurt the crop. The bulbs just won't measure up at the end of the season.
If soil is allowed to dry out during bulb formation, the onion may split and form two bulbs. It helps to apply mulch when the tops are 10 to 12 inches tall, because the mulch helps retain moisture.
Because their roots are so shallow, onions dry out faster than many other crops during a drought. When that happens, the onions often mature early, and that doesn't help the size of the bulbs. Watchword: Don't let onions dry out.
Many people in the West and Southwest must irrigate their home gardens all season long, while gardeners in other parts of the country may face a dry spell of several weeks during the season. Where water is in good supply, people turn to sprinklers and furrow irrigation to keep gardens supplied with water. In areas where water isn't plentiful, moisture-saving drip or trickle irrigation can be a life-saver.
When to Water
If you use a hose or sprinkler to water your garden, remember it's best to water early in the morning rather than during the heat of the day. Too much water is lost to evaporation if you water when the sun is high.
Watering onions and other plants from above in the evening can leave plants with wet foliage overnight. Often that can be an invitation to trouble, because with moisture remaining on the leaves, disease can spread rapidly.
Onions do need a lot of water, but the soil shouldn't be soggy all the time. "Just enough" water is better than "too much." Ideally, you want to provide a thorough soaking to a depth of six inches once a week rather than just a light sprinkling each day.
Here are some guidelines for watering: