The Art of Harvesting Onions

Friday, July 23, 2010 by Barbara Pleasant - Categories: storing onions
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It was late January when I started my onions from seed, and last week we tenderly carried the main crop from garden to deck. Oh, happy day! A few shallots aren't quite ready, but having a fine assortment of onions curing in warm shade will make any gardener's day. But let's back up a bit to consider the fine points of how to harvest onions and prepare them for storage, because how you handle your mature onion bulbs will greatly affect how long they will keep. Some onions store better than others, but all will keep better if they are attentively harvested and cured.

When to Harvest Onions

With any bulb-forming onion, the general rule of thumb is to wait for half of the plants to fall over, and then harvest the entire planting. This is fine if you have dry weather, but under moist conditions it's better to feel the necks of your plants for a soft spot a few inches above the onion bulb. This soft spot is a sign that the bulb has finished growing, and you can feel it a day or two before the top of the plant falls over. When rain is predicted, it's best to pull every plant that has gone soft at the neck, because moisture spells trouble for mature onions.

Shallots are an exception to the fall-over rule, because they often splay out over the ground as they divide into separate bulbs, but the plants may still be growing. This rather unattractive posture should therefore be tolerated until shallots' green leaves have withered back about halfway. Then they are ready to harvest and cure.

As for bulb onions, so-called short-day varieties will begin forming bulbs when days are only 12 hours long, while long-day onions form bulbs after days are about 14 hours long. However, bulbing can come on early or late depending on whether spring and summer have been warm or cool. Above-average temperatures that help onions accumulate "thermal time" can make the plants form bulbs ahead of schedule, whereas lower temperatures can lead to slight delays.

Onion plants with lots of big, healthy leaves will produce large bulbs, while those that bolt, or develop seed stalks, will produce small, lopsided bulbs. I rarely have a bolting problem when I grow onions from seed, but it's a common syndrome with onions grown from sets, or from oversized seedlings that are set out too early. Sets are a great way to grow green onions (scallions), but for a bolt-free bulb onion patch you'll need to use seedlings smaller than a pencil, set out after nighttime temperatures have warmed to above 45°F/°C.

How to Harvest Onions

My best advice on harvesting onions is to treat them like eggs. Gently pull up on the plant's neck, and shake off excess soil. Allow no unnecessary bruising, including what you might cause by brushing off dirt. It's okay to flick away clods of soil and outer wrapper leaves that are obviously rotten, but postpone more serious cleaning until after the onions have air-dried in a warm, shady place for a couple of days.

Leaving the tops and outer wrapper scales intact when harvesting onions buys the bulbs time for the important task ahead – drying (or curing) until several layers of dry outer scales come together to close up the neck at the top of the bulb. From garden to long-term storage, the process of curing onions takes about a month.

Curing Your Onions

After a few days resting in a warm, dry place, I like to clean newly harvested onion bulbs to help them cure faster. Leaving as many outer scales intact as possible, I trim off leaves that have gone slimy or brown, and cut the green ones back to about 12 inches (30 cm). I lightly rub off soil and place the bulbs on an open shelf outdoors, protected from rain.

About two weeks later, the bulbs are ready to groom a second time and move indoors for a bit more drying time. This round, I use scissors to clip off roots and all but two inches of the withering tops. Then I gently wipe the curing onions with a damp cloth to clean them, and arrange them in a single layer indoors. By this time they hardly smell at all, so the final curing usually takes places on the cool stones of my wood stove hearth. After two weeks there, the onions are fully cured, ready to be relieved of their little stubs and shifted to the basement.

How to Store Onions

Sweet, juicy onions (including most short-day onions) deteriorate quickly, so they should be stored in the refrigerator. Other onions can be store in any cool, dry place. If you don't have a cool basement, look for alternative spots like downstairs closets or under your bed.

Given the same storage conditions, some onions will break dormancy long before others. When this happens, the onions soften and a green shoot may show at the top. One of my favorite onions, 'Long Red of Tropea', will break dormancy in December no matter how well it's cured, but then it's not a storage onion. In my experience, varieties described in seed catalogs as “hard storage” onions will last until February, though they usually get eaten by then. Dense little cipollinis and shallots stay in good condition all the way through winter, so they take over in the kitchen when the bulb onions are gone. All in all, I can't think of a better use of six months of gardening time than growing and curing a generous supply of interesting and delicious gourmet onions that will last until spring.

By Barbara Pleasant