As the onion approaches harvest time, the lower leaves of the onion plant begin to wither and dry out, and then the stems fall over. This means that the bulb bulbs have fully developed and are almost ready to harvest.
Once the tips of most onions have fallen over, stop watering and leave the onions in the ground for about a week. To harvest onions, gently pull on the stems (but if they don't come out easily, dig them out with a shovel).
In the low Arizona desert, onions are typically harvested in early May. Already at the end of April or until the end of May, look for signs of readiness to harvest.