Sweet onions (Allium cepa) aren't as pungent as other onions due to their low sulfur content. They are thin skinned and juicy due to their high water content, and this makes them fragile and more perishable than other onions. All onions are either short day, intermediate day or long day. Short-day onions start forming bulbs when the day length reaches 10 to 12 hours and are best when planted in the south for the largest bulbs. Long-day onions start forming bulbs when the day length reaches 14 to 16 hours and are best planted in the north. Intermediate-day onions form bulbs at day lengths of 12 to 14 hours and grow best in the central U.S. Onions are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 10.
The "Yellow Granex" onion is a short-day variety with tan skin and pale yellow to white flesh. It has a flattened globe shape. The flavor of the onion changes slightly according to the conditions in which it's grown, and therefore onions grown in different locales are given different names to differentiate them. The "Vidalia," for example, is a patented variety grown only in a 20-county region of Georgia, an area of lower elevation and the mild temperatures of USDA zone 8. The "Maui Sweet" is grown in the red, volcanic soils of Maui at high elevations and under balmy conditions. The "Noonday" is grown within 10 miles of Noonday, Texas, a small town south of Tyler, in USDA zone 8. Each of these locations imparts a subtle flavor to the onion.
Texas Super Sweet
The "Texas Super Sweet" (or "Texas Supersweet") onion also goes by the name "TX 1015Y." The 1015 relates to the recommended planting date of October 15. This is a short-day type, with yellow skin and white flesh. Developed by Dr. Leonard Pike at Texas A&M University, this onion is mild enough that it rarely causes the tears associated with some onions. The bulbs grow to about 4 inches across and become sweeter as the bulb matures.
The "Walla Walla" onion is large and has yellow skins and white flesh. It is named for the town and county of Walla Walla in Washington. It is a long-day type and is best grown in northern locations. The original Walla Walla seed was reportedly brought to Washington by a French soldier named Peter Pieri from the French island of Corsica in the 1800s. This onion is well adapted to the higher altitude and colder winters found in this region, often down to 20 degrees Fahrenheit.
Other Sweet Onions
Certain regions grow their own particular kinds of onions. From California comes the "Sweet Imperial," grown in California's Imperial Valley, and from New Mexico comes the "Nu-Mex Sweet" and "Carzalia." The "Nu-Mex" variety has several sub-varieties, such as "NuMex Sweetpak" and "NuMex Arthur," all releases from New Mexico State University's onion breeding program. From the country of Chile hails the "Oso Sweet," an onion that can be enjoyed in the winter months when other sweet onions aren't available.