How to Cure Onions
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How to cure onions means drying harvested bulbs in a warm, dry, well-ventilated place until the necks are dry and the outer scales are papery, and then storing them cool with airflow around each bulb. The Iowa State, UMass and Nebraska extension pages we opened describe the same basic sequence. They differ on several numbers, including the curing time, the storage range and how far to trim the neck, so this guide gives each figure next to its source.
Quick answer
- Harvest when most of the tops have fallen over and begun to dry, and pull the bulbs with the tops attached.
- Cure the bulbs in a warm, dry, well-ventilated place, spread in a single layer, for two to four weeks.
- Treat the onions as cured when the neck is tight and the outer scales rustle when handled.
- Store them cool and dry with airflow, at a temperature the sources put between 32 and 40 degrees Fahrenheit.
When to harvest onions
Harvest timing comes first. The Iowa State FAQ says onions “should be harvested when most of the tops have fallen over and begun to dry”, and it tells gardeners to “Carefully pull or dig the bulbs with the tops attached.” The Nebraska article gives a fraction to watch for. Its advice is to harvest once about two-thirds of the tops have fallen over and dried, and the necks have started to dry. It also warns about waiting too long, saying “don’t wait more than one or two weeks to harvest” once the plants reach that stage.
The UMass fact sheet takes a storage view of the same question. It says the optimum time for storage life “occurs while the onion foliage is still partially (30-40%) erect,” which is earlier than the point when every top is down. It adds that “excessively field-drying onions increases the risk of bald onions in storage.” That is the main disagreement among the three pages about timing, and it is covered again in the section on what the sources do not say. If you are still choosing what goes in the same bed, see vegetables that grow well together in raised beds.
Lift and handle without washing
Careful handling matters from the first pull. The Nebraska article says, “Careful handling is essential during harvest to avoid bruising, which makes the bulbs susceptible to storage rot.” It describes two ways to lift the crop. 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.” The same article gives a firm rule about water: “Do not wash them!”
The Nebraska article says to “Gently brush remaining soil off the bulbs with a soft brush or gloved hand.” The UMass page adds a reason to avoid nicks: “Bruises provide direct entry points for diseases to get started.”
Set up the curing spot
The Iowa State FAQ asks for a particular kind of place. It says to “dry or cure the onions in a warm, dry, well-ventilated location, such as a shed or garage.” It then gives a layout: “Spread out the onions in a single layer on a clean, dry surface.” The Nebraska article makes the same point with a practical frame. It says to “Place the onions in single layers, in large, flat trays”, and it describes a simple build: “Simple trays can be made with lathe strips, leaving one half inch between strips to allow for adequate air circulation.”
Weather decides where the onions can cure. The Nebraska article says that when rain is predicted, onions “can be cured in a warm, dry, well-ventilated location out of direct sun.” The UMass page warns against field curing when rain is in the forecast, and it describes the trade-off with the sun. It says “sunshine and temperatures in the 80s will enhance the bronze color in the skins, but extremely hot sun and temperatures in the 90’s can cause sunscald.” The Nebraska article also gives the temperature range it wants for curing: “they should be kept warm, 75-90°F, in a well-ventilated place for 2 to 4 weeks.”
Know when the onions are cured
The UMass fact sheet gives the clearest test. It says onions “are considered cured when the neck is tight and the outer scales are dry and make a rustling sound when handled.” It also gives a weight figure that a grower can use to judge progress: “This condition is reached when onions have lost 3-5% of their weight.” A home gardener is unlikely to weigh every bulb, so the neck and the rustle are the practical checks.
The timing figures differ. The Iowa State FAQ says to “Cure the onions for 2 to 3 weeks until the onion tops and necks are thoroughly dry and the outer bulb scales begin to rustle.” The UMass page says that “Adequate curing in the field or in open sheds may require 2 to 4 weeks, depending on the weather”, and the Nebraska article gives the same two-to-four-week window. The safe reading is that the time depends on weather and on the spot, so the neck and scale checks should decide when to move the onions into storage. UMass is blunt about the cost of skipping this step: “If not adequately cured, onions are likely to decay in storage.”
Trim the tops and check the necks
Trimming happens after curing, not during. The Iowa State FAQ says “After the onions are properly cured, cut off the tops about 1 inch above the bulbs.” It adds a practical check during the trim: “As the onions are topped, discard any that show signs of decay.” Nebraska gives the same length in a different way, saying 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.”
The UMass fact sheet asks for a longer neck. Its harvest tips say “Leave 2-3 inches of neck on the bulb”, and the reason it gives is that this “increases the distance from the cut surface to the bulb for these pathogens to travel.” That is one of the clearest differences in the three sources, and a gardener who wants to follow one guide should keep the trim length consistent with that guide from start to finish. The Iowa State FAQ also offers an alternative for the tops: leave them untrimmed and braid the dry foliage together.
Store with airflow and steady cool
Storage needs a container that lets air through. The Iowa State FAQ lists options: “Place the cured onions in a mesh bag, old nylon stocking, wire basket, or crate.” It adds, “It’s important that the storage container allow air to circulate through the onions.” A cool basement, cellar or garage is a common choice, and the same page names those three places as possible storage spots.
The temperature and humidity figures are where the pages overlap most closely, but they still differ. The Iowa State FAQ gives “Storage temperatures should be 32 to 40 degrees Fahrenheit” and says “The relative humidity should be 65 to 70 percent.” The Nebraska article narrows the range, saying onions “should be kept at a temperature between 32-36°F, but do not let them freeze”, and it gives a humidity ceiling of “Ideal humidity is 60% or less for longest storage.” The UMass page gives a single target of 32 degrees Fahrenheit with 65 to 70 percent relative humidity, and it asks for slow cooling: “it is important to bring them down to this temperature slowly.” It also warns against cooling bulbs to well below the average daily temperature, because “they will draw moisture from the warmer air, which can lead to disease.”
Storage life depends on the variety. The Iowa State FAQ notes that “good keepers, such as ‘Copra’ and ‘Stuttgarter,'” can be stored for several months, while “Poor keepers, such as ‘Walla Walla’ and ‘Candy,'” last only a few weeks. The Nebraska article says the storage life “is determined by cultivar and your storage conditions”, and it suggests checking the variety description for its storage potential when you buy sets or transplants.
Common storage problems to avoid
- Warm storage. Nebraska Extension says onions “will start to sprout if storage temperatures are 40°F or higher.”
- Bruised bulbs. The Nebraska article says onions that are bruised “should not be stored,” and that they should be used “as soon as possible” for fresh eating or cooking.
- Thick-necked bulbs. The Iowa State FAQ says to “Use the thick-necked bulbs as soon as possible as they don’t store well.”
- Light after curing. The UMass page says “exposure to light after curing will induce greening of the outer scales.”
- Storing with other produce. Nebraska advises, “Do not store other fruits or vegetables with onions and garlic”, because “Apple and potatoes will absorb onion flavor.”
Each problem traces back to a rule in the sources. Keep the onions out of warm storage, sort out the bruised and thick-necked bulbs early, and keep them away from light and from fruit and potatoes. The Nebraska article also says that “Not curing stems and trimming too close to the bulb will reduce storage potential and increase the likelihood of storage rots”, which ties the trim step back to storage.
Compare the figures side by side
The table below puts the figures from the three pages in the same columns. Where a page does not give a figure, the cell says so.
| Source | Curing time | Curing temperature | Storage temperature | Storage humidity | Neck trim |
|---|---|---|---|---|---|
| Iowa State University Extension FAQ | 2 to 3 weeks | Not stated; warm, dry, well-ventilated location | 32 to 40 degrees Fahrenheit | 65 to 70 percent | About 1 inch above the bulb |
| UMass Extension fact sheet | 2 to 4 weeks, depending on weather | 75 to 90 degrees Fahrenheit for best skin color | 32 degrees Fahrenheit, reached slowly | 65 to 70 percent | Leave 2 to 3 inches of neck |
| Nebraska Extension article, August 2022 | 2 to 4 weeks | 75 to 90 degrees Fahrenheit | 32 to 36 degrees Fahrenheit, do not freeze | 60 percent or less | At least 1 inch of stem |
Read the table as a set of ranges rather than one schedule. The three pages agree on warmth, airflow and a dry, cool place for storage. They disagree on trim length, on the storage range and on the harvest stage. Choose one page’s figures and follow them through the whole process, so that the trim, the cure and the storage all match. If you plan the next crop for the same bed, see vegetables to plant in raised beds.
How we chose
We began with the search question, how to cure onions, and looked for university or extension pages that a reader could open and read. We considered several pages, and we kept three that describe home curing and storage directly: the Iowa State University Extension Yard and Garden FAQ on harvesting, drying and storing onions, the UMass Extension vegetable fact sheet on harvesting and curing onions, and the Nebraska Extension article on harvesting and curing onions, dated August 7, 2022. We opened each page with curl on 2026-10-23 and confirmed that its text covers curing. We also opened an Oregon State storage guideline for onions and did not use it, because its figures are written for storage facilities rather than home gardens. Commercial sites appeared in the results too, and we did not use them for any rule or figure. We kept a rule only when one of the three pages states it, and we checked each quoted phrase against the text we downloaded.
What we could not verify
The three pages do not agree on several figures, and this guide does not choose the correct one for you. The pages also do not say how to cure onions in a humid climate, and they do not say how a home basement with a heater compares with a cellar. This guide makes no product claim.
What the sources do not say
- The sources do not agree on the trim length. Iowa State and Nebraska say about 1 inch, while UMass says to leave 2 to 3 inches of neck.
- The sources do not agree on the harvest stage. Iowa State and Nebraska describe tops that have fallen over, while UMass says storage life is best when the foliage is still partially erect.
- The Iowa State FAQ does not give a curing temperature. It asks for a warm, dry, well-ventilated place and stops there.
- The Nebraska and UMass pages do not give the same humidity target for storage. Nebraska says 60 percent or less, and UMass says 65 to 70 percent.
- The sources do not give a storage life for a named variety in a home setting, beyond the Iowa State examples, which are described as good and poor keepers.
Frequently asked questions
How can you tell when onions are ready to harvest?
The Iowa State FAQ says to harvest once most of the tops have fallen over and begun to dry. The Nebraska article describes two-thirds of the tops down, with the necks starting to dry. Once onions reach that stage, the same article says not to wait more than one or two weeks.
How long should onions cure?
Iowa State says 2 to 3 weeks. UMass and Nebraska say 2 to 4 weeks, depending on weather and conditions. Check the neck and the outer scales, since the UMass page says cured onions have a tight neck and scales that rustle when handled.
Should onions be washed before curing?
No, according to the Nebraska article, which says “Do not wash them!” Gently brush the remaining soil away with a soft brush or a gloved hand instead.
Can onions be cured in the sun?
Nebraska advises curing out of direct sun when rain is predicted. UMass says sun and temperatures in the 80s help the skin color, but extremely hot sun can cause sunscald, and it says to get the onions out of the sun after curing because light induces greening. A shaded, well-ventilated spot covers both points.
What temperature should stored onions stay at?
The sources give different ranges. Nebraska says 32 to 36 degrees Fahrenheit and do not let them freeze. Iowa State says 32 to 40 degrees Fahrenheit. UMass gives 32 degrees Fahrenheit as its target and says the bulbs should be brought down to that temperature slowly. If you store onions in an unheated garage, the Iowa State FAQ says to move them before temperatures drop below 32 degrees Fahrenheit.
Sources
- https://yardandgarden.extension.iastate.edu/faq/how-do-you-harvest-dry-and-store-onions (opened 2026-10-23)
- https://www.umass.edu/agriculture-food-environment/vegetable/fact-sheets/harvesting-curing-onions (opened 2026-10-23)
- https://lancaster.unl.edu/harvesting-and-curing-onions (opened 2026-10-23)
