Onions are ready to harvest when about half the plants have tops that have fallen over. They can sunburn if left in the ground. The secret to onions keeping well is to allow the tops to dry completely before storage, Upham says. Move them to a shaded, well-ventilated area after harvest. After tops are completely dry, store in a cool, dry place. Large-necked onions take more time to dry than small-necked onions such as Bermuda types. Use an open, mesh bag rather than a plastic bag for storage, he says.