How to Grow Onions That Bulb, Cure, and Store Well

Bulb onions don’t wait for a certain number of weeks and then decide to swell. Each cultivar begins bulbing when days reach a particular length. Short day onions usually respond at about 10 to 12 hours of daylight, intermediate day onions at roughly 12 to 14 hours, and long day onions at about 14 hours or more. Those ranges overlap because cultivars differ, so the variety description matters more than the color of the bulb.

Short day onions are generally suited to southern gardens, where they can grow leaves through the mild part of the year and begin bulbing as days lengthen in late winter or spring. Long day onions usually fit northern gardens because summer days become long enough to trigger them after the plants have had time to develop. Intermediate day onions often work through the middle latitudes, although local recommendations are more reliable than a national map near the edges of those regions.

That timing affects bulb size. Once bulbing starts, the plant shifts away from making new leaves. An onion that receives its bulbing signal while it’s still small doesn’t have enough leaf area to build a large bulb. Before ordering seed or transplants, look for a named cultivar, its day length class, and a recommendation for your state or nearby states. A bag of red, white, or yellow sets with no cultivar name gives you much less certainty.

Also check what kind of onion you’re buying. Bunching onions are meant to make tender stems and leaves, not large dry bulbs. Sweet bulb onions can grow impressively but often store for only a short time. If winter storage is the goal, choose a cultivar specifically described as a good keeper.

Seed offers the widest cultivar choice, which makes it easier to match day length and storage quality. In northern areas, onion seed is often started indoors about 10 to 12 weeks before outdoor planting because direct seeded plants may not grow enough leaves before long summer days trigger bulbing. Direct sowing can work where the season and locally recommended planting date allow it. Most home garden guidance places seed about ¼ to ½ inch deep.

Transplants remove much of that early wait. They’re often the most dependable route to full bulbs when spring is short, as long as the seller identifies the cultivar and day length class. Choose firm, green plants rather than oversized plants with swollen bases. A large, overmature transplant is more likely to produce a flower stalk after cold exposure, and a bolted onion won’t make a good storage bulb.

Sets are small bulbs produced the previous season. They’re easy to handle and establish quickly, which makes them especially useful for green onions. For mature bulbs, sort sets before planting. Illinois Extension recommends using sets smaller than a dime for dry onions because larger sets are more likely to bolt. If the package doesn’t name the cultivar or day length class, treat that uncertainty as a reason to grow them for green onions rather than committing the whole bed to storage bulbs.

Planting dates can’t be reduced to a single national frost rule. Across much of the North, onions go out as soon as the soil is workable in spring because established plants tolerate light frost. In the Southeast, short day onions are commonly transplanted in fall or winter. Western coastal and overwintering systems have their own cultivar and timing requirements. Use the planting window from your local Extension office or state vegetable guide, then work backward if you’re starting seed indoors.

A common final spacing for bulb onions is 3 to 4 inches between plants. Four to 5 inches can support larger bulbs where the cultivar, fertility, and season allow them, while closer spacing tends to produce smaller bulbs. Raised bed rows can be closer than field rows if you can reach the plants without stepping into the bed, but crowding within the row still affects the bulbs.

You can use close planting deliberately. Set plants about 2 inches apart, harvest every other onion while it’s young, and leave the remaining plants with roughly 4 inches of room. This works only if the thinning happens before the bulbs begin pressing against one another. Green onions can be planted much closer because they’re harvested before bulb size matters.

Most Extension directions place sets about 1 inch deep and transplants shallowly, usually around 1 to 2 inches depending on the transplant and local soil. Keep the growing point above the soil and don’t hill soil around onions meant for dry storage. Deep planting or hilling can leave a thick, damp neck that cures poorly.

A raised bed doesn’t need a rigid square foot formula for onions. Measure the actual spacing between plants and decide whether your goal is tender green onions, medium kitchen bulbs, or the largest bulbs your cultivar can make. That decision is more useful than filling every open spot simply because onion leaves look narrow.

Onions grow best in full sun and soil that drains well while holding steady moisture. Minnesota Extension gives a soil pH range of 6.0 to 7.0 as a useful target. If the bed has never been tested, a soil test is more useful than choosing fertilizer by habit. It can show whether phosphorus or potassium is actually needed and provide a nitrogen recommendation suited to your soil.

Mixing finished compost or well rotted manure into the bed can improve soil structure. Don’t use fresh manure around a food crop. It can introduce food safety concerns, encourage weeds, and supply nutrients at the wrong time.

Onions need available nitrogen while they’re building leaves, and each healthy leaf contributes to a ring in the finished bulb. That doesn’t mean more nitrogen is always better. Excess nitrogen late in the season can delay maturity and leave thick necks, soft bulbs, or poor storage quality. Follow the rate and timing on a current soil test or local Extension recommendation. Don’t copy a fertilizer rate from another state without checking the product analysis and regional guidance.

Weeds matter more in an onion bed than their size suggests. Onion leaves cast little shade, and the root system doesn’t compete strongly. Pull weeds while they’re small or slice them off with shallow cultivation. Deep hoeing can cut onion roots or wound bulbs, so this isn’t a bed to turn aggressively once plants are established. A light layer of clean mulch can reduce new weeds after the onions are growing well, but don’t pack wet material around the necks.

Onions have shallow roots, so a bed can look moist below the surface while the active root zone is drying. Check the soil several inches down instead of judging by the top crust. Water deeply enough to moisten the root zone, then check again before watering on a fixed schedule. Sandy beds usually need smaller, more frequent applications than heavier soil, while a poorly drained bed may stay wet long enough to damage roots.

Consistent moisture matters from establishment through bulb expansion. Repeated dry spells during that period reduce bulb size, but constantly saturated soil isn’t an improvement. Drip irrigation or a soaker hose can wet the soil without keeping foliage damp, provided you occasionally check how far the water has moved through the bed.

As the bulbs reach full size and tops begin to soften and fall naturally, reduce or stop irrigation according to the weather and your local guidance. Continuing heavy irrigation into maturity can delay curing and increase storage problems. Don’t bend or step on green tops to speed the process. Illinois Extension warns that forced top breaking stops bulb development and can leave onions that don’t cure properly.

Silvery white streaks or speckling on the inner leaves can point to onion thrips, especially during hot, dry weather. Severe feeding can dry leaves early and reduce bulb size, but drought stress and disease can also brown leaf tips. Open the folded leaves near the neck and look for the tiny, slender insects before choosing a treatment. If control is warranted, use only a product labeled for thrips on edible onions in your state, and follow its harvest interval.

A plant that yellows, wilts, and pulls up with damaged roots or tunneling near the base may have onion maggots. Once larvae are feeding inside the plant, there isn’t a useful home garden rescue treatment. Remove badly damaged plants and their roots. If the problem has occurred before, rotate onions and related alliums to another bed and clean up bulbs after harvest. A floating row cover can exclude the adult flies, but don’t put it over soil that grew onions or another susceptible crop the previous year because emerging flies may already be underneath it.

White rot deserves a different response. The fungus is named Stromatinia cepivora, although older references may call it Sclerotium cepivorum. Affected onions may be stunted or yellow, with white cottony growth and tiny black survival structures called sclerotia around the bulb or roots. Remove infected plants and don’t compost them. Buy disease free sets or seedlings, and clean soil from tools and shoes before leaving the bed. Sclerotia can survive in soil for more than 20 years, so a short crop rotation won’t make an infested bed safe for onions, garlic, leeks, shallots, or other alliums. Get the diagnosis confirmed locally before deciding how the bed will be used.

Not every small bulb is diseased. Before reaching for a spray, check the cultivar’s day length, planting date, spacing, shade, water history, and whether the plant bolted. Those causes are more likely when the whole planting is uniformly undersized.

Green onions can be pulled whenever they reach a useful size. Storage onions need to finish naturally. Extension recommendations commonly place harvest when about half to most of the tops have fallen and begun drying. Choose a dry stretch if possible, loosen the soil, and lift the bulbs without cutting or bruising them. Don’t wash bulbs intended for storage.

Cure onions in a warm, dry, shaded place with good airflow. Spread them in a single layer rather than piling them into a bucket. Curing usually takes about two to three weeks, although humid weather can make it take longer. An onion is ready when the outer skins rustle, the roots are dry, and the neck feels tight and papery all the way through. Thick necked, bruised, sunscalded, or damaged bulbs should go to the kitchen rather than storage.

Once the crop is cured, trim the tops about an inch above the bulb unless you’re braiding fully dry foliage. Use a mesh bag, wire basket, or shallow crate that lets air move around the onions. Iowa State recommends 32°F to 40°F and 65 to 70 percent relative humidity for long storage, but many homes can’t hold that exact range. The practical goal is a cool, moderately dry place with airflow that stays above freezing. Check the crop regularly and remove any bulb that softens or sprouts.

Storage life isn’t proof that you cured correctly. Cultivar still sets the ceiling. Pungent storage types may keep for months under good conditions, while sweet onions often last only weeks. Keep those groups separate and use the short keepers first.