Harvesting and Curing Onions

Knowing when your onions are ready for harvest and how to prepare them for storage are important steps toward having a successful crop.

A common practice passed down among generations of gardeners is that bending over the onion leaves, while they are still green and growing, will prevent the leaves from growing so much and “send more energy to the bulbs” ultimately resulting in larger onions. It’s also thought this practice will make the onions mature faster. Unfortunately, neither is true; this practice only succeeds in reducing bulb growth since the leaves, which manufacture carbohydrates stored in the bulbs and increase bulb size, are killed. Don’t do that!

When to Harvest
Onions should be harvested when about two-thirds of the tops have fallen over and dried naturally and the “necks” of the onions have started to dry. The neck is the base of the leafy stem where it joins the bulb. When onions are young and growing, their necks will be green and fleshy or succulent. As onions mature, the necks will begin to dry out, shrivel and eventually become papery.

Research has shown optimum flavor and sweetness is achieved when onions are harvest at 80% foliage die back. However, once your onions reach this stage, don’t wait more than one or two weeks to harvest; the bulbs may begin to rot, or grow again and go to seed.

Harvesting
Careful handling is essential during harvest to avoid bruising, which makes the bulbs susceptible to storage rot. Onions can either be lifted by gently pulling them by their foliage or by pushing them up with a garden fork inserted below the bulbs.

Gently brush remaining soil off the bulbs with a soft brush or gloved hand. Do not wash them!

Curing
After harvest, onions must be “cured” if they will be stored for any length of time. Curing can be done either in the field or in a protected location away from rain. Whether cured in the field or other location, they should be kept warm, 75-90°F, in a well-ventilated place for 2 to 4 weeks. Poorly cured onions will not store well and will be prone to storage rots

Field cure onions after harvesting by placing them in rows with the leaves partially covering the bulbs. This helps prevent sunburn or greening. Leave the onions in the field until the outer leaves and neck are completely dry and papery. After curing, dried leaves may be left on and braided into strings, or cut off leaving at least 1-inch of the stem for improved storage life.

If rain is predicted during the harvest period, onions can be cured in a warm, dry, well-ventilated location out of direct sun. After harvesting and removing excess soil, trim away the foliage, leaving a 1-inch section of stem at the neck. Place the onions in single layers, in large, flat trays. Simple trays can be made with lathe strips, leaving one half inch between strips to allow for adequate air circulation. An old screen door works well, too.

Leave the onions on the trays until the outer leaves and neck are dry and papery. Allow a few outer papery leaves to remain during storage.

Storage
The ultimate storage life of onions is determined by cultivar and your storage conditions. Check the cultivar description when purchasing sets or transplants for the onion’s storage potential – short, medium or long.

Only firm bulbs should be kept for storage. Onions that are bruised or have a thick neck should not be stored, but used for fresh eating or cooking as soon as possible.

Do not store other fruits or vegetables with onions and garlic. Apple and potatoes will absorb onion flavor.

Onions in storage should be kept at a temperature between 32-36°F, but do not let them freeze. They will start to sprout if storage temperatures are 40°F or higher. Ideal humidity is 60% or less for longest storage.

1. Not curing stems and trimming too close to the bulb will reduce storage potential and increase the likelihood of storage rots. "Onion Harvest" by Tobyotter is licensed under CC BY 2.0.
2. If storage temperature is too warm, bulbs will sprout and begin to grow. Image from Pixabay.com.