Why Are My Onions Rotting in Storage?

You grew a beautiful onion crop, pulled it, stashed it away, and weeks later opened the bag to soft, smelly, rotting bulbs. It is a disheartening but very common problem, and it almost always traces back to what happened at and just after harvest rather than to the storage shelf itself. The leading causes are inadequate curing and neck rot, a fungal disease that infects through the neck and shows up only in storage. Add in harvesting too early or too late, choosing poor storage varieties, and the wrong storage conditions, and you have the full list of reasons onions fail to keep. The good news is that storage rot is highly preventable once you understand the chain of events. This guide explains how to diagnose why your onions rotted and exactly how to grow, cure, and store them so they last for months.

Diagnose by When and How the Rot Appears

The timing and look of the rot point to the cause. Bulbs that soften from the top/neck down, turning the neck spongy with a gray fuzzy mold and sunken brown tissue, are showing classic neck rot, which appears weeks into storage. Bulbs that rot generally, get slimy, and smell bad soon after storage usually went in poorly cured or bruised, letting bacteria and fungi take hold. Thick-necked bulbs that never sealed rot quickly because the open neck is a wide doorway for decay. Bulbs with rot starting at the base/root plate may have basal rot or white rot from the soil. Noting where the rot begins, and how soon, tells you whether to fix curing, harvest timing, disease management, or variety choice.

Cause 1: Inadequate Curing

Diagnose: Curing is the single most important step for storage onions, and skipping or shortcutting it is the most common reason they rot. Curing dries the outer skins into a protective wrapper and, crucially, dries and seals the neck so moisture and pathogens cannot enter. Onions stored with green, thick, or still-moist necks rot fast because that open neck wicks in decay. If your bulbs went into storage soon after pulling, or with necks that still felt soft or damp, under-curing is your answer.

Fix: Cure thoroughly. After harvest, spread the onions in a single layer in a warm, dry, airy, shaded spot out of direct sun, such as a well-ventilated garage, shed, or covered porch, with good air movement around them. Leave the tops on during curing. Let them dry for about 2 to 4 weeks until the necks are completely tight and dry, the outer skins are papery and rustle, and the tops have fully withered. Only then trim the tops to about an inch and the roots close to the base. Do not wash the bulbs; brush off loose soil. Properly cured onions with tight, dry necks resist rot for months.

Cause 2: Neck Rot

Diagnose: Neck rot, caused by Botrytis fungi, is the classic storage disease that ruins onions weeks after they look perfectly fine. The fungus infects the leaves and neck in the field, often late in the season, but stays hidden until storage, when affected bulbs soften at the neck, develop sunken brownish tissue, and grow a characteristic gray fuzzy mold between the scales, sometimes with small black resting bodies. Because it is invisible at harvest, you only discover it when bulbs collapse in storage. Wet seasons, lush late growth from heavy nitrogen, and poor curing all favor it.

Fix: Prevention is the only real control, since you cannot treat an infected bulb in storage. Cure thoroughly so the neck dries and seals, which limits the fungus’s entry. Avoid heavy late-season nitrogen, which produces thick, succulent necks that cure poorly and invite infection. Practice crop rotation, avoiding planting onions and their relatives in the same bed for several years, and start with clean seed or sets. At harvest, handle bulbs gently to avoid bruising, discard any with thick or soft necks for immediate use rather than storage, and inspect stored onions regularly, removing any that begin to soften before they spread. Good airflow in storage further discourages the mold.

Cause 3: Wrong Harvest Timing and Over-Watering

Diagnose: Pull onions at the wrong moment and they store badly. Harvested too early, before the bulbs mature and the tops signal readiness, they have thin, poorly developed skins and high moisture, so they shrivel or rot. Left too long in wet soil after the tops die, the bulbs can begin to rot in the ground or split, and the necks may regrow. Over-watering and over-fertilizing late in the season produce soft, thick-necked, watery bulbs that simply will not keep. If your onions had lush green tops at harvest or very thick necks, timing and irrigation are likely factors.

Fix: Time the harvest by the tops. When about half to most of the foliage has naturally flopped over and begun to yellow and dry, the bulbs are mature; do not bend the tops down artificially, just let them fall. Stop watering once the tops start to topple so the bulbs can begin drying in the ground, and ease off nitrogen as bulbs size up rather than pushing late growth. Harvest on a dry day, ideally after a few rain-free days, lifting the bulbs gently and leaving the tops attached for curing. Choosing the right window and tapering water and feed gives you firm, tight-necked bulbs built to last.

Cause 4: Poor Storage Varieties and Conditions

Diagnose: Even perfectly cured onions rot if they are the wrong type or kept in the wrong place. Sweet onions (like Vidalia types) and many short-day onions have high water and sugar content and are bred for fresh eating, not storage, so they spoil within weeks no matter what you do. And any onion kept warm, damp, or poorly ventilated will rot, while cold and humid spots invite mold. If your keepers were sweet varieties or stored in a sealed bag, a damp basement, or near the warmth of appliances, that explains the loss.

Fix: Match variety and conditions to long storage. For keeping, grow pungent, firm-fleshed long-day storage varieties such as Copra, Patterson, Yellow Globe, or similar, which are bred to last for months, and eat sweet onions first. Store cured onions in a cool, dry, dark, well-ventilated place ideally around 32 to 40 F with moderate humidity, in mesh bags, slatted crates, braided ropes, or old pantyhose with knots between bulbs, never sealed plastic. Keep them away from potatoes, which release moisture and gases that hasten onion spoilage. Inspect monthly and pull any softening bulbs immediately. For a related root-storage problem, see our guide on why garlic rots.

Other Storage Rots Worth Knowing

While poor curing and neck rot account for most storage losses, a few soil-borne diseases can show up later too, and recognizing them guides your prevention. Fusarium basal rot attacks the root plate at the bottom of the bulb, causing the base to turn brown, soften, and rot upward, often with a pinkish mold on the roots; it comes from infected soil and is worse in warm conditions. White rot, caused by a long-lived soil fungus, rots the base and roots and produces a fluffy white mold with tiny black sclerotia, and once it is in your soil it persists for many years. Bacterial soft rot turns affected scales into a watery, foul-smelling mush, usually entering through bruises, sunscald, or that open neck on a poorly cured bulb.

The defenses against these overlap heavily with good general practice. Rotate onions and their relatives out of the same bed for at least three to four years so soil pathogens decline, and never replant into a bed that has shown white rot. Plant in well-drained soil, avoid injuring bulbs at harvest, and protect curing onions from sun so they do not scald. Start with clean, certified seed or sets rather than questionable bulbs, and discard any diseased onions well away from the garden rather than composting them. Because these rots enter through wounds and the neck, the same gentle handling and thorough curing that stop neck rot also limit them, which is why a careful harvest-and-cure routine protects against nearly everything.

From Garden to Pantry: A Storage-Success Routine

Long-keeping onions are the product of good decisions made well before storage. Choose true storage varieties, ease off water and nitrogen as bulbs mature, and harvest when about half the tops have naturally fallen and yellowed, on a dry day. Cure for 2 to 4 weeks in a warm, airy, shaded spot until necks are fully tight and dry and skins rustle, then trim and gently brush clean without washing. Sort out any thick-necked, bruised, or soft bulbs to eat right away, and store only the best in cool, dry, dark, ventilated containers, away from potatoes. Inspect regularly and remove any onion that begins to soften so one bad bulb does not spoil the lot. Follow this chain and a good crop of storage onions will carry you through much of the winter. For more vegetable how-tos, visit the Plants & Flowers hub.