These golden onions produce fruit up to 1 lb (16 oz.) and their great flavor lasts longer than most other varieties. Yellow Sweet Spanish onions are gardeners’ favorites because they grow quickly without much effort. You will be able to harvest your onions less than four months after planting. Onions may need lots of moisture at the beginning, but less when the bulbs are approaching full size. When the tops begin to fall over naturally, it's time to pull the onions up.