Since bulb onions mature in response to changing amounts of daylight there are varieties that work best depending on what part of the county you live. Short-day varieties grow best in the South. Intermediate-day varieties are grown best in the northern Arizona to Washington D.C. area, and Long-day varieties are best suited for the North. However, there are exceptions to the rules: The Intermediate-day varieties can be grown as early onions in the North....which is what we did.
The best soil for these guys is slightly acidic and well-drained. When the tops start to lay over (by themselves) as in the 2nd photo, then you know they are ready to harvest. Then to cure the onions you just place them in a warm, dry place and away from direct sunlight. Short-day varieties cure for a few days before you clip off the tops and roots. Intermediate-day and Long-day varieties cure for up to three weeks. Most properly cured onions should store up to 8 months in a cool basement or root cellar. Using a mesh bag or other "breathable" material to store them is advisable. Be warned that sweet onions do have a shorter storage life than most.
A bit of trivia to leave you with: Did you know the sweet onion became the Texas state vegetable in 1997?