Harvest green onions when they are big enough to use—commonly when the stems are ½ to 1 inch in diameter. Green onions mature about 7 to 8 weeks after sowing but can be used much sooner.
Harvest full-sized bulb onions for drying and storage when half to three-quarters of the strappy green foliage has turned yellow and fallen over. Bulb onions for storage mature about 90 to 110 days after sowing.
Harvest full-sized sweet-tasting slicing onions—also known as Bermuda or Spanish onions—when half to three-quarters of the strappy green foliage has turned yellow and fallen over.
Slicing onions are thinner-skinned than storage onions and do not store well. (Sweet slicing onion varieties include ‘Vidalia’, ‘Walla Walla’, and ‘Texas Supersweet’.) Like storage onions, slicing onions reach full size 90 to 100 days after sowing.
The stalks and grassy tops of bulbless green onions—also called bunching onions, scallions, or spring onions—are commonly eaten raw in salads, dips, and stir-fries or cooked in soups.
Bulb onions—also called main crop onions–are commonly sliced or chopped and served raw in salads or on hamburgers or sandwiches or cooked in soups and stews. Main crop onions are more pungent than sweet slicing onions.
Main crop onions for storage should be cured before they are stored. Start the curing process a week before harvest by withholding water and partially severing the roots with a spade. (This will allow their thick, moist necks to dry preventing rot.)
Loosen the soil around each bulb with a hand or garden fork before you lift; damaged onions are quick to rot.
After you fully lift the bulbs, let the bulbs dry for a few days on top of the ground—cover the bulbs with foliage to prevent sunburn–or spread them out in a shaded, warm, dry, well-ventilated place. Dry bulbs until their skins are papery—usually 2 to 10 days outdoors and about 2 weeks indoors in a well-ventilated spot.
When the neck of a cured onion is tight and the outer scales are dry, cut the tops back with a garden pruner to about one inch and clip the shriveled roots. Drying seals the top of the onion and keeps the bulb from forming a seed stalk. (Don’t allow bulbs to form seed stalks; that will leave them woody and unsuitable for storage and eating.)
Store bulb onions in a cool (35°-40°F/2°-4°C), dry, well-ventilated place. Store bulb onions in a hanging mesh bag if possible. Slicing onions should be used in a few weeks. Cured bulb onion will store for 5 to 8 months. If you harvest bulb onions while some of the tops are still green, use them right away—they will not keep.
Store green onions in a perforated plastic bag in the vegetable crisper of the refrigerator. Green onions will keep for 3 to 4 weeks.