The many varieties of onions grown commercially fall into three general classes:
Globe onions are the most common group and are considered primarily cooking onions. There are many varieties, mostly with yellow skins, but also some white and red-skinned types. Globe onions are predominantly round to oval, and have rather pungent flavor. They are available in quantity during the late summer, fall, and winter. Most fall in the medium size range but in some cases the smaller onions are packed and sold separately.
Granex-Grano onions are available during the spring and summer, coming from the warmer growing areas. Most are yellow-skinned; a few are white. The shape tends to be less round and less symmetrical than the globes, ranging from somewhat flattened to top shaped. Rather mild in flavor they are considered ideal for slicing and eating raw and good for cooking. In size they range from medium to large.
Spanish onions resemble globe onions in shape but they are much larger. Most varieties are yellow, but some are white-skinned. They are mild in flavor, often called “sweet Spanish,” and are ideal for slicing or for salads. Sometimes the medium sizes are packed separately from the large ones (3 inches or more in diameter). Spanish type onions are generally available in moderate supply during fall and winter.
Major onion growing areas are California, New York, Texas, Michigan, Colorado, Oregon, and Idaho.
When buying look for Hard or firm onions which are dry and have small necks. They should be covered with papery outer scales and reasonably free from green sunburn spots, and other blemishes.
Avoid onions with wet or very soft necks, which usually are immature or affected by decay. Also avoid onions with thick, hollow, woody centers in the neck or with fresh sprouts.