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A sweet onion is a variety of onion that is not pungent. Their mildness is attributable to their low sulfur content and high water content when compared to other onion varieties.
Origins in the United States 
United States sweet onions originated in several places during the early twentieth century.
Vidalia onions were first grown near Vidalia, Georgia in the early 1930s. Today the name refers to onions grown in a 20-county production region in the state of Georgia as defined by both Georgia state and federal law (CFR).
The Walla Walla sweet onion is named for Walla Walla county in Washington where it is grown. Its development began around 1900 when Peter Pieri, a French soldier who settled in the area, brought a sweet onion seed from the island of Corsica with him to the Walla Walla Valley. This sweet onion was developed by selecting and reseeding onions from each year's crop that possessed sweetness, jumbo size, and round shape.
Other U.S. varieties 
- Imperial Valley Sweets come from the Imperial Valley in far southern California. This is one of the leading growing areas for sweet onions, although they are available only from late April through June.
- The Carzalia Sweet onion is a variety of sweet onion grown by Carzalia Valley Produce in Columbus, New Mexico.
- The Sunbrero (Texas) Sweet Onion is grown in Texas and distributed by Sweet Onion Trading Company, Melbourne, Florida.
- The Sweetie Sweet is a variety of sweet onion grown in the Mason Valley in Yerington, Nevada. The Sweetie Sweet onion can be found in marketplaces throughout the September through the end of January.
- The Glennville sweet onion is grown in Tattnall County, in Glennville, Georgia.
- Maui onions are one of the smaller varieties of onions grown on the Hawaiian island of Maui. They are trademarked "Kula-grown" onions.
- Pecos onions are sweet onions grown in the Pecos Valley in the state of Texas.
- Texas 1015 Super Sweet onions are named for their optimum planting date, October 15. They were developed by Texas A&M University.
European onions 
In Europe, the Oignon doux des Cévennes from Cévennes, South East France has PDO status.