Mature onions are harvested when their green tops start to droop, though they can be harvested earlier than this. When onions are harvested early, they can be used as scallions. The gardener can also check to see if the onion's skin is papery.
If more than half of the green tops have begun to droop, the gardener can push the tops the rest of the way down. Ideally, the onion should be lifted out of the soil with a garden fork rather than pulled. It should be left to dry for a couple of days, then stored in a root cellar along with its green tops.