Growing Onions: Everything You Need to Know to Actually Get a Bulb

Onions are one of the most unforgiving crops in the vegetable garden. Not because they are delicate. Not because they are disease-prone. But because there is one decision you make before you ever put a plant in the ground that determines, with near-total certainty, whether you get a harvest at all. Get it wrong and you will have the most vigorous, healthy-looking onion tops you have ever grown. You just will not have any onions.

That decision is day-length type. And the majority of home gardeners -- including plenty of experienced ones -- get it wrong every year.

Onions do not bulb based on calendar date, soil temperature, or how much you have been feeding them. They bulb in response to daylight hours. Each variety category has a specific hour threshold: reach it and the plant starts forming a bulb; fall short of it and the plant just keeps growing leaves forever. A long-day variety planted in Georgia will produce a spectacular green plant and absolutely nothing to eat. A short-day variety planted in Minnesota will bulb within weeks of planting -- when the plant is the size of a pencil -- and hand you a marble.

The fix is simple once you know the rule. The rest of this guide is everything else: soil, timing, planting methods, watering, varieties by zone, common failures, and how to store your harvest so it actually lasts.

Let's make this the year your onions come in right.

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Quick Answer: Onion Growing at a Glance

USDA Zones: 3 through 10 (with the correct day-length type for your latitude)

Day-length types: Short-day (zones 7-10), Intermediate-day (zones 5-7), Long-day (zones 3-6)

Sun: Full sun minimum -- 6 hours daily, 8+ preferred

Soil pH: 6.0-7.0

Spacing: 4-6 inches between plants, 12-18 inches between rows

Water: 1 inch per week consistently during active growth; stop completely when 50% of tops fall

Fertilizer: Nitrogen every 2-3 weeks during leaf growth; stop all feeding the moment bulbing begins

Planting method: Transplants recommended; sets are acceptable for green onions only

Best storage variety: Copra (long-day, 6-9 months); Patterson (long-day, 6-8 months)

Best sweet variety: Vidalia/Yellow Granex (short-day); Walla Walla (long-day)

Harvest signal: 50% of tops have fallen over naturally

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The Day-Length Problem (Why Your Onion Grew Tops and Nothing Else)

This is where I need to spend some time, because no amount of perfect care rescues you from the wrong day-length type.

Onion varieties fall into three categories based on their bulbing trigger. Short-day varieties begin bulbing when days reach 10-12 hours. Intermediate-day varieties need 12-14 hours. Long-day varieties need 14-16 hours. These thresholds correspond directly to latitude, because latitude determines how long your days get at the summer solstice.

Here is what this means in practice. In the northern US, summer days stretch to 15-16 hours. Long-day varieties need that extended photoperiod to trigger. Plant a short-day variety in Minneapolis and you hand it a 10-12 hour day in early spring -- it bulbs immediately, while the plant is still tiny, giving you a marble-sized bulb. Meanwhile, a long-day variety in Houston is waiting for a 14-16 hour day that never comes at that latitude; the plant grows leaves all season and delivers nothing.

The relationship is absolute. There is no fertilizer fix. There is no timing workaround. This is plant biology.

Matching your variety to your latitude is straightforward:

• 25-35N latitude (Deep South, Gulf Coast): Short-day varieties
• 32-42N latitude (Mid-South, transition zone, mid-Atlantic): Intermediate-day varieties
• 37-47N latitude (Northern US, upper Midwest, New England): Long-day varieties

If you land in an overlap zone -- say 32-35N -- go intermediate. They tolerate the widest range of day lengths and are the safest bet when you are on a boundary.

One more thing: most garden center sets are labeled only as "yellow," "red," or "white." No day-length information. A national distributor ships the same long-day sets to a garden center in Louisiana and one in Montana. The Louisiana gardener gets a pretty green plant and nothing else. This is why specialty transplant suppliers -- who ship zone-matched varieties on a zone-appropriate schedule -- are so much more reliable than the bags of sets at the hardware store.

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Best Onion Varieties by Zone

Variety selection is the second decision that locks in your outcome. Once you know your day-length type, choose from the options below based on whether you want a fresh-eating sweet onion, something that will sit in storage through February, or a striking red for salads.

Southern Zones (7-10): Short-Day Varieties and Fall Planting

Southern onions follow a counter-intuitive calendar. You plant in fall -- October through January depending on your zone -- and harvest in spring. The plants put on leaf mass through the cool winter months, then bulb as days lengthen in March and April. By summer, it is over. This is the opposite of what most northern gardeners picture when they think of growing onions.

Texas 1015 is the workhorse of the short-day world. Bred specifically for Texas and the Gulf states, it produces large, sweet, mild bulbs that have made it a staple in zones 8-9. The 1015 in the name refers to the planting date: October 15th. That should tell you something about how seriously Texans take planting timing.

Vidalia / Yellow Granex is the most famous American onion for a reason. Grown in the sandy, low-sulfur soils of coastal Georgia, it delivers exceptional sweetness -- but here is what the famous name does not tell you: the sweetness comes as much from the soil as from the variety. Onion pungency is directly tied to soil sulfur content. Low-sulfur, sandy soil produces milder onions regardless of variety. Plant Vidalia types in heavy, high-organic soil and you will get a decent onion. Plant them in sandy, low-sulfur soil and you get something worth talking about.

Red Burgundy is the best red option for short-day zones -- mild enough for raw eating in salads, good color, and well-adapted to the Southeast. Georgia Boy and Southern Belle Red round out the selection for gardeners who want variety.

Storage reality for all short-day types: plan on 1-2 months maximum. High sugar and water content -- the properties that make them sweet -- are the same properties that make them rot faster. Grow them for fresh eating, not for the pantry.

Zone 7 sits in the transition zone. Short-day varieties fall-planted here often work, but intermediate-day varieties planted in early spring are frequently more reliable. If you are in zone 7, you have options in both directions -- which makes the intermediate section below worth reading too.

Middle Zones (5-7): Intermediate-Day Varieties

Zone 5 through 7 is where gardeners have the most flexibility. Long-day and intermediate-day varieties both have a shot here. In my opinion, intermediate-day varieties are almost always the smarter choice for this range -- they perform across a wider latitude band and deliver better fresh flavor without sacrificing all storage life.

Candy is the variety I recommend first to any gardener in this zone. It is adaptable across the widest latitude range of any intermediate variety, produces large, sweet bulbs, and holds 3-4 months in storage. That is a useful combination. If you are in zone 5-7 and you cannot figure out what to grow, grow Candy.

Superstar is an All-America Selections winner with impressive size and mild white flesh -- excellent for onion rings. Red Candy Apple fills the red slot at the intermediate level: sweet enough for raw eating, with good color and 2-3 months of storage. Talon is worth attention if storage is a priority -- it holds 4-5 months, which is notable for an intermediate variety.

Zone 6 is the heart of intermediate territory. All of the varieties above perform reliably. Plant in March as soon as soil is workable.

Zone 7 gardeners can take a creative approach: plant intermediate-day varieties in early spring for a summer harvest, and plant short-day varieties in fall for a late-spring harvest. Stagger both and you get onions across two distinct windows. That is the kind of efficiency that makes a small garden punch above its weight.

Northern Zones (3-6): Long-Day Varieties

Long-day onions are the storage champions. The same northern latitudes that give them the 14-16 hour days they need also give growers the option of proper curing conditions and cool-enough storage spaces to keep onions for the better part of a year.

Copra is the storage king. Full stop. It holds 6-9 months under proper conditions, with thick skin, dense flesh, and the kind of concentrated flavor that improves as it sits. If you grow onions in zones 3-6 and you are not growing Copra, you are leaving months of usable storage on the table.

Patterson is a close second for storage -- 6-8 months, thick skin, very reliable performance. Grow both Copra and Patterson and you have a storage program.

For sweet onions in northern zones, Walla Walla is the name. The famous Washington sweet onion, large and mild, great raw. Like its southern counterparts, it pays for its sweetness in storage life -- 1-2 months. Plan to eat it, not store it.

Ailsa Craig is an exhibition variety that can reach two pounds or more per bulb. Spectacular for the shock value alone. Flavor is mild, storage is poor, but if you want to hand someone a single onion that weighs as much as a softball, this is your variety.

Red Wing is the best-storing red long-day variety -- 4-6 months, which is significantly better than most reds. Norstar earns a specific mention for zones 3-4, where cold hardiness matters: it is pungent, reliable, and one of the toughest long-day varieties available.

Zones 3-4 gardeners should prioritize transplants over sets. The growing season is short enough that losing three to four weeks to set establishment can meaningfully reduce bulb size. Starting with transplants -- or better yet, starting seeds indoors 8-10 weeks before the last frost date -- gives you every possible day.

Quick Reference Table: Top Picks by Zone Group

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When and How to Plant

Timing by Zone

Onion timing is driven by one objective: maximize the number of leaves before bulbing begins. Every leaf equals one ring in the bulb. An onion planted four weeks late might produce 6-8 leaves instead of 10-13. That is roughly 40% less bulb volume. No amount of fertilizer compensates for lost time.

In zones 3-5, plant as soon as soil is workable -- late April through early May at the northern edge. Onions are cold-hardy. They tolerate frost and light freezes. Do not wait for warm soil the way you would for tomatoes. Get them in early.

In zones 6-7, target March through April for spring planting. Zone 7 gardeners also have the fall planting window for short-day or intermediate varieties.

In zones 8-10, plant in fall and winter: October through January depending on how far south you are. The plants grow through winter, bulk up before the heat arrives, and harvest in spring.

Planting Methods: Sets, Transplants, or Seed

There are three ways to start onions. They are not equally good.

Sets (small dried bulbs sold in mesh bags) are the most popular method because they are the easiest to find and fastest to plant. They are also the most bolt-prone. A set has already been through one growth cycle. That history makes it significantly more likely to flower prematurely -- and a bolted onion develops a hard, woody core running through the center that stores poorly and tastes worse. If you use sets, choose the smallest ones available, under 3/4 inch diameter. Larger sets bolt at higher rates. And consider using sets only for green onions, where bolting does not matter because you are harvesting before bulbing anyway.

Transplants are the recommended method. Less bolt-prone than sets, far better variety selection, and available from specialty suppliers who match the variety to your zone and ship on the right schedule. Dixondale Farms is the major US supplier for home gardeners. Their transplants arrive bareroot, bundled, and ready to plant within a few days of receipt. The variety selection alone justifies the modest cost increase over hardware store sets.

Seed is the cheapest per plant and offers the widest possible variety selection. It is also the most demanding method: start indoors 8-10 weeks before the last frost date, manage tiny seedlings through the slow germination period (7-14 days), and maintain an indoor growing setup until transplant time. For experienced gardeners who want specific varieties or are growing at scale, seed is the clear winner. For everyone else, start with transplants.

Planting Depth and Spacing

Sets go in pointed-end up, 1 inch deep. Transplants go in just deep enough to cover the roots, 1-2 inches. Direct-seeded onions start at 1/4 inch deep and thin to final spacing.

Space 4-6 inches within the row, 12-18 inches between rows. Closer spacing produces smaller bulbs -- useful if you want green onions or small storage onions. Wider spacing at 6 inches gives each plant room to develop a full, properly sized bulb.

Site and Soil

Full sun is non-negotiable. Six hours minimum, eight or more preferred. In the Pacific Northwest, chase every hour of sun you can get.

Onion bulbs expand laterally -- they swell outward in the soil, not downward. Compacted soil physically restricts this expansion and produces undersized, misshapen bulbs. Work the soil to 6-8 inches deep before planting. Incorporate 2-4 inches of compost. The target texture is loose and crumbly -- if you squeeze a handful and it holds a tight clump, keep working it.

Target soil pH of 6.0-7.0. Below 5.5, phosphorus becomes unavailable to the plant. Above 7.5, iron and manganese deficiency appear. A soil test from your county cooperative extension -- typically $10-25 -- tells you exactly where you stand and what amendments you need. It is the most cost-effective step in onion growing.

One soil note that surprises most gardeners: soil sulfur content directly affects onion flavor. Low-sulfur, sandy soils produce milder, sweeter onions. High-sulfur, high-organic soils produce more pungent cooking onions. The famous sweetness of Vidalia onions is not just genetics -- it is the low-sulfur, sandy soils of coastal Georgia. If you want the mildest possible fresh-eating onion, grow sweet varieties in sandy, low-sulfur soil and avoid sulfur-containing fertilizers.

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Fertilizing Onions: Two Phases, One Hard Stop

Onion fertilization has a structure to it that most gardening advice does not explain clearly enough. There are two distinct phases, and the transition between them is not gradual -- it is an immediate stop.

Phase One: Leaf Growth

From planting through the onset of bulbing, fertilize every 2-3 weeks with a nitrogen-rich product. This is the growth phase, and nitrogen drives leaf production. Since each leaf becomes one ring in the bulb, the goal is straightforward: grow as many leaves as possible before the day-length trigger fires.

Options include calcium ammonium nitrate (21-0-0), blood meal (organic, approximately 12-0-0), or fish emulsion (5-1-1, which also provides trace nutrients). A balanced 10-10-10 works if soil phosphorus and potassium are already adequate.

At planting, incorporate a high-phosphorus starter -- bone meal (3-15-0) at 2-3 pounds per 100 square feet is an excellent organic option. Onions have shallow, sparse root systems and are poor foragers. Phosphorus does not move through soil with water, so surface applications alone are not effective. Work it into the top 6-8 inches before planting.

Phase Two: The Hard Stop

When the base of the plant visibly swells beyond the diameter of the neck, bulbing has begun. Stop all nitrogen fertilization immediately. No tapering. No "just one more light feeding." Stop.

Nitrogen applied during bulbing promotes soft, lush growth in tissue that needs to harden and dry. The result is a thick, fleshy neck that refuses to cure properly. Botrytis -- neck rot -- enters through wet neck tissue. Bulbs with thick necks from late-season nitrogen are the ones that rot in storage within weeks. The irony is painful: the grower who fertilized more gets worse results.

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Watering: One Inch a Week and One Hard Stop

Onion watering has the same two-phase logic as fertilization -- consistent moisture during growth, then a decisive stop at the right moment.

During Active Growth

Provide 1 inch of water per week from irrigation and rainfall combined, delivered in 1-2 deep waterings rather than daily shallow ones. The goal is to moisten the top 6-8 inches of soil consistently. Erratic watering -- drought periods followed by heavy irrigation -- causes split bulbs, where the outer rings crack as inner tissue expands rapidly. It also causes secondary growth, where the plant essentially restarts and produces a deformed double bulb. Those days of drought are simply lost; the bulb cannot compensate later.

Drip irrigation or soaker hoses are the clear best choice for onions. They deliver water directly to the root zone, keep foliage dry (which prevents downy mildew and other foliar diseases), and make the pre-harvest stop-watering transition easy to execute. If you have to choose between overhead watering (which knocks thrips off leaves) and drip irrigation (which prevents fungal disease), choose drip and manage thrips separately with spinosad.

Mulch extends the benefit of every watering. Apply 1-2 inches of straw between rows after plants are established -- about two to three weeks after planting. Keep mulch away from the onion neck itself. Straw mulch reduces the need for irrigation by roughly 25-30% and buffers the moisture fluctuations that cause splitting. In hot, dry zones (8-10, arid West), mulch is essential rather than optional; unmulched soil in those climates loses moisture two to three times faster.

The Stop Signal

When approximately 50% of onion tops have fallen over naturally, stop all watering. This is not a suggestion -- it is the trigger point. The plant is entering dormancy and the bulb is complete. Continued irrigation at this stage keeps the neck tissue wet, which is precisely the condition that causes storage rot. If it rains during this window, harvest promptly rather than leaving bulbs in saturated soil.

Do not bend tops over manually to speed the process. This is a commonly repeated piece of garden advice, and it is wrong. Manually lodging tops damages the neck tissue and creates direct entry points for Botrytis. The tops fall when the plant is ready. Wait for it.

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Pests and Diseases Worth Knowing About

Onions have fewer problems than most garden crops. But the problems they do have can wipe out an entire planting if you are not paying attention.

Onion Maggot

The most damaging onion pest in northern gardens -- zones 3-7. The adult looks like a small housefly. It lays eggs at the base of onion plants in spring. Larvae tunnel into the bulb and destroy it from the base up. You will know you have them when plants suddenly wilt and yellow; pull the plant and you find the bulb is tunneled and rotting.

Floating row covers applied immediately after planting are the single most effective defense. They physically block the egg-laying flies while allowing light, air, and water through. Crop rotation -- no alliums in the same bed for three or more years -- limits population buildup in the soil. Remove all onion debris after harvest; pupae overwinter near old plantings.

If you have an active infestation, remove affected plants immediately to slow spread. Beneficial nematodes (Steinernema feltiae) applied to soil can parasitize larvae. Spinosad is the preferred organic treatment for heavy pressure.

Thrips

The most widespread onion pest, present in every growing zone. Onion thrips are tiny -- 1/25 inch -- and often invisible until you look closely between leaf bases. The diagnostic sign is silvery-white streaking on the leaves, which is where they have rasped the surface and fed on plant juices. Light infestations are cosmetic. Heavy infestations reduce bulb size by up to 50% because damaged leaves cannot photosynthesize efficiently. Each damaged leaf means a thinner ring in the bulb.

Spinosad is the preferred organic treatment and is effective against thrips when applied directly to leaf surfaces. Remove weeds around onion beds -- thrips harbor in grassy weeds and migrate to onions. Monitor weekly from late spring through harvest by pulling apart leaf bases.

Downy Mildew

The most common onion disease in humid climates, particularly zones 5-7 in the Southeast and Mid-Atlantic during wet springs. Look for pale yellow patches on leaves with gray-purple fuzzy growth visible in morning humidity. It spreads rapidly in cool (50-75F), rainy conditions.

Prevention is the only effective strategy. Proper spacing -- 4-6 inches between plants, 12-18 inches between rows -- allows leaves to dry quickly. Drip irrigation eliminates leaf wetness entirely. Copper-based fungicides applied preventatively before symptoms appear provide organic protection.

Neck Rot (Botrytis)

The primary storage disease, and unlike most of the problems above, you often do not know you have it until you are pulling onions out of storage three weeks after harvest and finding gray mold at the neck. Botrytis enters through neck tissue that is not fully dry after curing. Thick necks from late-season nitrogen fertilization are the most susceptible. Proper curing -- 2-3 weeks in a warm (75-85F), dry, ventilated space -- is the only effective prevention.

White Rot

Worth a specific mention not because it is common, but because of its consequences. White rot produces white fluffy mycelium with tiny black dots at the base of the bulb, and it renders soil unusable for alliums for 15-20 years. Buy from reputable suppliers, do not move soil from suspect areas, and if you confirm white rot in a bed, stop planting alliums there. There is no recovery path other than time.

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Harvesting and Storage: The Part Most Gardeners Get Wrong

You can grow a textbook onion crop and still end up with nothing three weeks after harvest. Storage failure is overwhelmingly a curing problem.

When to Harvest

Harvest when approximately 50% of the onion tops have fallen over naturally. The neck softens, the tops bend at a distinct kink point, and they lay flat on the soil. This is the universal signal that bulbing is complete and the plant is entering dormancy.

Do not wait until every top is brown and dead. By that point, the outer scales may have begun to deteriorate. And do not force tops down manually -- you already know why.

Choose a dry period for harvest. Use a garden fork inserted 3-4 inches from the bulb to lever the soil upward; do not pull straight up by the tops. The neck needs to stay attached and intact for curing. Leave the tops on.

The Curing Process

Cure in a warm (75-85F), dry, well-ventilated area for a minimum of 2-3 weeks. A covered porch, carport, or garage with a fan are all good options. Spread onions in a single layer so bulbs do not touch. Drape on wire mesh or old window screens so air circulates underneath.

Do not cure in direct sun -- you get sunscald, not proper drying. Do not cure in a closed, humid space -- you get mold, not drying.

Curing is complete when all three of these are true: the neck is completely dry and collapses easily when squeezed an inch above the bulb; the outer skins are papery and rustle when handled; and the roots snap off cleanly. Anything less and the onion is not ready for long-term storage. Once curing is complete, trim dried tops to 1 inch above the bulb and trim roots close to the basal plate.

Storage Conditions

The ideal storage environment is 32-40F with 65-70% humidity and moderate airflow. Temperature is the most critical factor. Above 40F, onions break dormancy and begin sprouting. Below 32F, cell damage occurs.

Mesh bags are the best storage container -- maximum airflow, easy inspection, and they hang from hooks to save space. Pantyhose (the old-timer technique: drop in a bulb, tie a knot, repeat, hang vertically) works on the same principle. Never use sealed plastic bags or closed containers. Trapped moisture guarantees rot.

Storage Life by Variety

Storage duration is genetic. Sweet varieties store poorly because the properties that make them sweet -- high water, high sugar content -- are exactly what rot organisms prefer.

Long-storage varieties (6-9 months): Copra is the benchmark. Dense flesh, thick skin, and it actually improves in flavor as it sits. Patterson follows close behind at 6-8 months. Red Wing holds 4-6 months and is the best option in the red category. Norstar is comparable for those in zones 3-4.

Short-storage varieties (1-2 months): Vidalia/Yellow Granex, Texas 1015, Walla Walla, and Ailsa Craig all fall here. These are fresh-eating onions. Plan accordingly -- eat them, give them away, pickle them, or make large batches of French onion soup. Do not count on them sitting in a mesh bag through December.

The practical approach for most gardens: grow both. Plant a sweet variety for immediate gratification in the weeks after harvest, and a long-day storage variety like Copra for everything after that.

Inspect stored onions every 2-3 weeks. A single soft, rotting bulb spreads to adjacent bulbs within days. Catch it early, remove it, and save the rest.

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The Top Mistakes That Kill Onion Harvests

If you read nothing else in this guide, read this section. These are ranked by how often they cause complete harvest failure.

Mistake #1: Wrong Day-Length Type for Your Latitude

We opened the guide with this and it stays at number one because nothing else comes close in terms of frequency and severity. Lush tops and no bulb is almost always this. Tiny premature marble-sized bulbs in the North is always this.

The fix: match your variety to your latitude. Use transplants from a specialty supplier rather than unlabeled sets from a garden center. If you are in an overlap zone, go intermediate. This one decision matters more than everything else combined.

Mistake #2: Planting Too Late

Every leaf equals one bulb ring. An onion planted four weeks late might produce 40% less bulb volume regardless of how well you manage everything else. Onions are cold-hardy -- they tolerate frost. Plant them when the soil is workable, not when the weather is warm.

Mistake #3: Fertilizing During Bulbing

Nitrogen after bulbing begins produces soft tissue that cannot cure properly. The resulting thick necks invite Botrytis and the whole batch rots in storage. The fix: watch for the base of the plant to swell, and stop all nitrogen the moment you see it. No exceptions.

Mistake #4: Using Large Sets

Sets are more bolt-prone than transplants to begin with. Large sets -- over 3/4 inch diameter -- are the most bolt-prone of all. A bolted onion has a hard, woody flower stalk through the center and will not store. Use transplants for bulb onions. If you insist on sets, choose the smallest ones in the bag.

Mistake #5: Compacting Soil Around Bulbs

Onion bulbs swell outward, not downward. Compacted soil physically restricts this expansion and produces undersized or misshapen bulbs. Never walk on the growing bed. Never hill up soil against onion bulbs (this is correct for potatoes -- it is wrong for onions). As bulbs swell, they will naturally push partially above the soil surface. Leave them alone.

Mistake #6: Inconsistent Watering

The drought-flood cycle that splits bulbs and causes secondary double-bulb formation is entirely preventable. One inch per week, delivered consistently with drip or soaker irrigation, and mulch between rows to buffer fluctuations. The consistent growers get smooth, full bulbs. The inconsistent ones get cracked and deformed ones.

Mistake #7: Skipping or Rushing the Cure

Two to three weeks. That is what curing takes. Not five days. Not a week in the sun. The neck must be completely dry and papery, the outer skins must rustle, and the roots must snap. Shortcut this and you lose your harvest to neck rot in storage.

Mistake #8: Poor Weed Management

Onions are among the worst weed competitors in the vegetable garden. Their narrow, upright leaves cast almost no shade. Their roots are shallow. Research shows weed competition during the first 6-8 weeks can reduce bulb yield by 50% or more. Weed weekly for the first two months. Mulch between rows. Hand-pull near plants rather than hoeing, which damages shallow onion roots.

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Frequently Asked Questions

What is the difference between short-day, intermediate-day, and long-day onions?

The difference is the number of daylight hours required to trigger bulbing. Short-day onions bulb at 10-12 hours, intermediate at 12-14 hours, and long-day at 14-16 hours. These correspond to latitude: southern latitudes never provide 14-16 hours, so long-day varieties fail there. Northern latitudes exceed 12 hours very early in spring, so short-day varieties bulb prematurely. Match your variety to your latitude and you are most of the way to a successful harvest.

Can I grow sweet onions in the North?

Yes, but not all sweet varieties. Walla Walla is the go-to sweet long-day onion for northern zones -- it produces large, mild, sweet bulbs and is the Pacific Northwest's answer to Vidalia. Ailsa Craig is another northern sweet variety with exceptional size. Just plan to eat them within 1-2 months of harvest. The sweetness that makes them appealing is the same property that shortens their storage life.

Should I use sets or transplants?

Transplants for bulb onions. Sets for green onions. Sets are more bolt-prone, have poor variety selection, and the day-length type is often unknown. Transplants from a specialty supplier -- Dixondale Farms is the major US source -- arrive zone-matched, variety-labeled, and significantly less bolt-prone. The cost difference is modest. The outcome difference is substantial.

Why are my onion tops falling over before the bulbs seem big enough?

Check your variety selection first. If you planted the wrong day-length type, the plant triggered bulbing early when it was still small. The tops fall because bulbing is complete, but the bulb is undersized because the plant did not have enough leaf mass before bulbing began. The fix is correct variety selection for your latitude next season. If your variety was correct, look at your planting date -- late planting reduces leaf mass and therefore bulb size.

How do I know when curing is done?

Squeeze the neck one inch above the bulb. It should collapse easily and feel completely dry -- like paper. If it gives any resistance or feels thick and moist, keep curing. The outer skins should rustle like paper when handled, and the roots should snap off cleanly. All three conditions, not just one. Rushing this step causes neck rot in storage. Two to three weeks minimum in a warm, dry, ventilated location.

Can I grow onions in containers?

Yes, with limitations. Onion bulbs need room to expand laterally, so containers need to be at least 12 inches deep and wide. Bunching onions and green onions work better in containers than bulb onions because they do not need the lateral expansion room. For bulb onions in pots, use a well-draining mix, maintain consistent moisture (containers dry out faster than ground beds), and fertilize more frequently since nutrients leach with each watering. Day-length rules still apply in containers -- the plant does not know it is in a pot.

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The Bottom Line

Onions are not difficult. They are specific.

Get the day-length type right for your latitude and you have solved the problem that defeats most home growers. Plant early, use transplants from a zone-matched supplier, give them full sun in loose soil, fertilize hard during leaf growth and stop completely at bulbing, water consistently at 1 inch per week and cut off completely when tops fall, and cure for a full 2-3 weeks before storage.

Do those things and you will have onions through the following spring -- storage varieties like Copra and Patterson routinely deliver 6-9 months under proper conditions. Fresh-eating sweet varieties like Walla Walla and Vidalia/Yellow Granex will be the best onions you have ever eaten, because you harvested them at peak ripeness instead of three weeks early for shipping durability.

Start with the day-length question. Everything else follows.

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Research for this guide was drawn from cooperative extension publications and university agriculture resources, including Texas A&M AgriLife Extension (Texas 1015 and short-day variety trials), University of Georgia Cooperative Extension (Vidalia and Yellow Granex production), Dixondale Farms zone-matched transplant guides, and USDA zone-to-latitude mapping for day-length type selection. Variety storage data sourced from commercial cultivar trial records for Copra, Patterson, Red Wing, Walla Walla, and Ailsa Craig.