For spring-seed onions, harvest starts near the end of August and continues through early October, with the main harvest season being in September. The average yield of onions in Utah is approximately 1,200-1,500 bags/acre (600-750 cwt/A) with higher yields reaching 2,000 bags/acre (1000 cwt/A).
Onion harvest occurs from August to early October.
Research has shown that the optimum harvest time is when onion foliage is still partially erect (Fig. 7.2), and long before maximum yield is attained (when tops are completely down and dry). Yields can increase 30-40% between the stage when tops begin to go down, and the leaves are fully down and dry. It may be tempting to leave onions to cure in the field as long as possible before lifting, but this will reduce the time available for drying. If it rains after onions have been lifted, bulbs may not dry out in the lower daytime temperatures of the early fall. Lifting and curing onions too late into the fall can also expose them to freezing temperatures.
Once harvested, onions need several weeks of warm temperatures in storage to complete the curing process. If bulbs are left too long in the field, quality will be sacrificed when they are brought out of storage due to rots and other storage problems. The optimum time for harvest, therefore, is a balance between highest yields and storage quality