Lift onions once most of the tops have flopped over and the necks have gone soft, which in Utah usually means late summer into early fall. Then cure them. Spread the bulbs in a warm, dry, shaded spot with moving air until the necks are tight and the skins feel papery. Only cured bulbs belong in storage, and even then the variety sets the limit. A sweet onion fades early, while a pungent one can carry a household into March.
When do you lift onions in Utah?
The plant gives the signal. Once the bulb stops growing, the tops weaken, bend at the neck and fall over. Stop watering when that starts and let the soil dry out. Then pick a dry day, loosen the soil beside the row with a garden fork and pull the bulbs by hand. Prying under a bulb with the fork tines nicks the skin, and a nicked bulb is the first one to rot.
Timing depends on when the bulbs formed, and day length controls that. Salt Lake City sits at 40.78 degrees north and gets 14 h 56 of daylight on the longest day of the year. Clemson Cooperative Extension puts the bulbing trigger for short-day varieties at about 12 hours, so they start bulbing early and stay small this far north. Long-day varieties wait for 14 to 16 hours, and the Salt Lake Valley clears that bar. Gardeners well north or south of the city should look up their own latitude. The far south of the state has a shorter longest day, so check your own latitude before choosing a long-day onion. The The Best Onion Varieties for Utah guide sorts the choices by day length.
Check each bulb as it comes out of the ground. Any that were cut by the fork, bruised or chewed by pests should go into the kitchen pile and be eaten first. What Eats Onion in Utah, and When explains which damage to expect and when it shows up. For the broader growing picture, start with Onions in Utah.
How do you cure onions before storing them?
Curing dries the outer skins into a papery wrapper and seals the neck, which is the opening where rot gets in. Leave the tops on while the bulbs cure. Lay them in a single layer on screens, wire racks or slatted crates, somewhere warm, dry and shaded with air moving past. A covered porch, a shed with the door propped open or a garage with the big door up all work. Keep the bulbs out of rain and out of midday sun, because direct sun can scald the exposed skin.
Late-summer air in Utah is usually dry, so curing often goes quickly. A stretch of wet weather or sprinkler overspray slows it, so move the bulbs under cover if the forecast turns. How long it takes depends on the weather and the size of the bulbs, so go by the onion rather than the calendar.
The test is simple. Squeeze the neck. A cured neck is tight and dry, the outer skins rustle, and the roots are wiry. If the neck is still thick, soft or damp, keep curing. A bulb that never tightens, usually one with a thick, green neck, will not store, so cook it first. Once a bulb passes the squeeze test, trim the roots and cut the tops down to a short stub. Brush off loose soil with your hand. Do not wash the bulbs, because water on the skins undoes the curing.
Keeping them, and for how long
What temperature and humidity do stored onions need?
Clemson Cooperative Extension’s Home & Garden Information Center says: “Ideal conditions are between 45 °F and 55 °F, and 50 to 60 percent humidity.” Onions want a cool place that is also dry. That is the opposite of carrots, beets and other root crops, which keep best in cold, damp air. One cellar corner cannot hold both well. If you grow both, give the onions their own spot.
Storage times assume those ideal conditions, and they are maximums. A bulb that was cured well, stored dry and kept at a steady temperature can reach the top of its range. A bulb that sat a few degrees too warm, or in damp air, will sprout or soften sooner. Check the stored onions every couple of weeks and pull any that feel soft or smell sour, because one rotting bulb can spoil its neighbors.
Use mesh bags, open crates or baskets, and avoid sealed plastic. Air has to move around the bulbs. Do not store them beside potatoes, which give off moisture.
Which onions keep longest?
Variety matters as much as the storage room. University of Missouri Extension (Planting, growing and harvesting onions, G6355) states it plainly: “As a rule, ‘sweet’ onions do not store as well as the more pungent types.” The bite of an onion comes from volatile sulfur compounds, and those same compounds help protect the bulb in storage. A sweet variety has fewer of them. That makes it better raw and worse in January.
Day length lines up with this trade-off. Short-day types run sweeter and keep poorly. Long-day types are more pungent and keep longer. That is why a southern garden gives onions to eat soon, and a northern garden gives onions that can feed you into March. Salt Lake City’s 14 h 56 longest day puts it on the long-day side. Grow sweet onions for summer salads and burgers, and plan on eating them first. Do not count on one to last the winter. For storing, choose pungent long-day varieties. The The Best Onion Varieties for Utah guide covers which are which.
Where in the house that actually is
The target is 45 °F to 55 °F with dry air, and most houses do not have a room that fits exactly. The fridge runs well below that range. Cold, damp air in a crisper drawer pushes onions to soften and can make them sprout, so the fridge is only good for a cut onion. A heated living space is too warm, and onions kept there break dormancy and sprout early. A garage swings with the weather. In a Utah winter it can drop below freezing, and a frozen onion turns to mush when it thaws. In late summer it can bake.
| Spot | Against the 45 °F to 55 °F target | Against dry air |
|---|---|---|
| Refrigerator | Far too cold | Too damp |
| Heated living space or kitchen | Too warm | Usually dry enough |
| Garage | Swings hot and cold | Varies with weather |
| Cool interior room, closet or basement room away from the furnace | Closest match | Often fine in a dry climate, check it |
The room that comes closest is usually a cool, dry interior space. That could be a basement room kept away from the furnace and the damp outside walls, an unheated spare bedroom or a closet on an interior wall. Utah’s dry climate helps with humidity, but a basement can still run damp, especially near a laundry or a floor drain.
Put a thermometer in the spot for a week before you commit the crop. A min/max model shows the overnight low and the afternoon high, which a single reading will miss. Add a cheap hygrometer if you can. If the readings stay between 45 °F and 55 °F with humidity in the 50 to 60 percent range, you have found the spot. If the room is a little warm, expect a shorter storage season and eat the onions earlier. Skipping this check can cost you the harvest, because sprouting and rot usually show up only after the bulbs are stored.
Common questions
Can I store whole onions in the refrigerator?
Not for the long haul. A refrigerator is far colder than the 45 °F to 55 °F storage range, and the air in it is damp. Whole onions may soften or sprout there. Keep the fridge for cut onions in a sealed container, and store whole cured bulbs in a cool, dry room.
Should I wash onions before I store them?
No. Brush off loose soil once the bulbs are cured and trimmed. Water on the skins keeps the neck damp and invites rot. If a bulb is very dirty, use it first and wash it when you cook it.
What if the neck is still thick after curing?
Keep curing it a while longer in a warm, dry, airy spot. If the neck never tightens, the bulb will not keep, and a thick, green neck is where rot starts. Cook that bulb first. Check again with the squeeze test, and expect to set aside several bulbs in a wet season.
Why did my sweet onions rot while the pungent ones kept?
Sweet onions have fewer of the volatile sulfur compounds that protect a bulb in storage, so they do not keep as well. Missouri Extension’s rule of thumb is that sweet onions store less well than pungent types. Eat sweet varieties soon after curing, and save the pungent long-day types for winter.