Walla Walla Sweet Onions were a strain of mild onion that was brought from the Mediterranean island of Corsica to the Walla Walla valley in the late 1800s by Peter Pieri, a French soldier. Once they made it to Walla Walla, the onions found enthusiastic support from Italian immigrant farmers of the region who appreciated their exceptionally mild flavor and winter hardiness (the first onion crops are planted in September and harvested the following summer). The onions also benefit from the low-sulfur soil of the area, which helps to keep the onions' pyruvic acid level low, lending them even sweeter taste.
The popularity of the onion has grown to big business, with more than 1,200 acres of the specialty crop being planted annually in the Walla Walla Valley.
In order to protect their unique sweet onion variety, packers applied for and won the Walla Walla Sweet Onion Marketing Order in 1995. The order protects Walla Walla Sweet Onions as a unique variety and federally protects the growing area of the Walla Walla Valley in southeast Washington and northeastern Oregon. Sweet onions can be grown just about anywhere, but they can't be called Walla Walla Sweet Onions unless they bear the "Walla Walla Genuine" label.
We are at the end of the fresh harvest Vidalias that run about April though June. Vidalias were the first sweet onions to be promoted and distributed nationally. They first appeared in 1931 when Georgia farmer Mose Coleman discovered that the onions he planted were not hot, as he expected, but actually sweet. Came into its own in the early 1940s, when the F1 Hybrid Yellow Granex was developed. Named for a town in Georgia and grown in 20 specific counties mandated by a Federal Marketing Order.
Also we are at the end of the Imperial Sweet Onion from the desert of ElCentro CA. Grown below sea level in the rich, loamy, desert soil of Southern California's Imperial Valley, where ideal growing conditions produce large, mild, juicy onions. The high moisture and sugar content is the result of the location where they are produced.
In order for an onion to be called a Sweet Imperial, it must be yellow, globe-shaped, and a minimum of 2 1/2 inches in diameter - and be very sweet.
The late Dr. Henry Jones is credited with being the first person to hybridize the flat Bermuda and the top-shaped grano into a new onion he called the Granex. It was a deeper shaped onion, with sweetness, and a longer shelf-life in markets.
When Dr. Jones retired from the USDA, he joined the research staff of a seed company in the Imperial Valley and continued to develop new varieties, many of which are grown today to produce the Sweet Imperial onion.
Not to worry if you miss the freshly harvested sweet onions, storage onions last well into fall to give nearly year round availability.
Other sweet onion growing areas include Texas and Chili.