In the beginning, onions have very small roots and tend to dry out quickly, but once the bulb is matured, stop watering, and let the skins form. If onions are handled and stored properly, disease should not be a major concern.
Onions for use in the green stage are harvested as soon as they reach edible size.

Onions that are to be stored should be harvested when most of the tops have broken over.
The onions should be placed on the ground in the sun with the tops covering the bulbs to prevent sunscald. The length of time required for the tops to dry depends on the weather and may be anywhere from 3 to 10 days. After the tops are fairly well dried down, they are cut off. Leave about 1 inch of top attached to the bulb. Onions that are to be stored for winter use must be thoroughly cured. The curing means that the onions should be held in a place that is well ventilated with good air circulation for about 3 to 4 weeks. The onions should not be placed where they can freeze. The storage area should be well ventilated and dry.