Water regularly during dry weather to prevent splitting. Weed consistently, as onions are very intolerant of weeds. Remove any seed pods that appear at the top of the plants. Snap them off as soon as they appear to prevent small, tough bulbs. A thin layer of sand around the bulbs will discourage onion maggots from laying eggs there.
Onion thrips cause blotches on onion leaves, and can be controlled with insecticidal soap.
Aphids can be washed off with a water spray. Harvest as scallions during the summer by cutting off just the tops, or as onions when they develop. In August or September, when the tops start to turn yellow, knock the tops over with a rake. When the tops turn brown it's time to pull them up. Leaving the onions in the ground after the tops turn brown will usually cause them to rot.