From Wikipedia:

“A Vidalia onion is a sweet onion of certain varieties, grown in a production area defined by Georgia law and by the United States Code of Federal Regulations (CFR). The varieties include the hybrid yellow granex, varieties of granex parentage, and other similar varieties recommended by the Vidalia Onion Committee and approved by the U.S. Secretary of Agriculture. The onions are named Vidalia because of where they are grown, Vidalia, Georgia (growing there started in the early 1930s). The different varieties are unusually sweet, due to the low amount of sulfur in the soil in which the onions are grown. The Vidalia onion was named Georgia’s official state vegetable in 1990.”