An onion's flowering process ruins the quality of a bulb onion. In northern Colorado, onions grown for winter storage will begin to form bulbs as summer days get longer, usually in July. When shopping for seed, look for long-day varieties such as Copra, and Early Yellow Globe for yellow onions. Try Mambo and Southport Red Globe for red onions, and Bedfordshire Champion and Snow White for white onions. Good, mild onions for short-term storage include Ailsa Craig Exhibition and Walla Walla Sweet.
Onions grow best in fertile soil that drains well.
But, they also grow in sandy or clay soils that have been amended with organic material. Onions only require ample nitrogen until mid-July. Onion seed should be planted as soon as the soil can be worked, depending on spring weather. For the Denver metro area, mid- to late March is best, but onions can be planted until late April, depending on the variety. Remember that hard freezes can damage young seedlings.