Onions are ready to be harvested when most of the tops have fallen over and begun to dry, according to Iowa State University Extension and Outreach horticulturists. Once onions are harvested, they should dry in a warm, dry, well-ventilated location such as a garage. Onions should be spread out in a single layer and cured for two to three weeks until the onion tops and necks are dry and outer bulb scales begin to rustle. The cured onions should be kept in a mesh bag or crate that allows air to circulate through the onions. Also, they should be stored at temperatures between 32 and 40 degrees Fahrenheit.