Excess nitrogen late in the growing season delays maturity and causes doubling. During the growing season make sure onions have plenty of moisture, especially after bulbs begin enlarging to assure large bulbs and better yields. A week before harvest, discontinue watering to encourage the onion to form protective scales. Cultivate shallowly as often as needed to control weeds, but do not hill up soil around the plants as this can encourage stem rot. Once onion roots have begun to extend into the row middles, discontinue cultivation.
Late cultivation can cause root pruning, leading to water stress and invasion by diseases.
About a month before harvest, start working the soil gradually away from the bulbs. By about 7 to 10 days before harvest time, the bulb should be one-third above ground to hasten bulb and neck drying. Diseases are seldom a problem on onions in the home garden. Good cultural practices will help prevent disease. Never plant onions in the same area more often than once every four years.