How to Plant Onions: A Complete Growing Guide

Table of Contents

Plant onion sets or transplants as soon as your soil reaches about 50°F in spring, placing them 1–2 inches deep and 3–4 inches apart in rows spaced 12–18 inches apart. For seeds, sow them at the recommended depth indoors several weeks before your last frost date. That’s the short version. The longer version below covers every decision that separates a bag of small, disappointing bulbs from a pantry full of keepers.

Here’s what you need to get started:

Grab your planting material: sets or transplants for beginners; seeds if you want variety control
Check soil temperature: aim for at least 50°F before planting sets or transplants
Prepare your bed: loose, well-drained soil at pH 6.0–6.8
Plant at the right depth: sets and transplants 1–2 inches deep; seeds 1/4–1/2 inch deep
Space correctly: 3–4 inches apart in rows 12–18 inches apart, per The Old Farmer’s Almanac
Water in immediately after planting

The single most important decision you’ll make is choosing the right daylength variety for your region. Get that wrong and your onions will bulb too early, stay small, or never bulb at all, regardless of how well you do everything else.

Key Takeaways

Picking the right daylength variety for your USDA zone is the single most important decision in growing onions, because a mismatched variety will produce small bulbs regardless of how well you manage soil, water, and fertilizer.

Point	Details
Match variety to daylength	Short-day for zones 7–10, intermediate for zones 6–7, long-day for zones 3–6.
Plant at the right depth	Sets and transplants go 1–2 inches deep; seeds go 1/4–1/2 inch deep, 3–4 inches apart.
Start early for bigger bulbs	Each leaf before the bulbing trigger becomes one bulb layer; more weeks of growth means a larger onion.
Side-dress nitrogen before bulbing	Apply a nitrogen-rich fertilizer 1–2 weeks before bulb enlargement begins, then stop once necks soften.
Cure fully before storing	Dry bulbs in a ventilated space for 2–4 weeks until necks are papery before trimming and storing.

When should you plant onions?
How do you choose the right onion variety?
Onion sets, transplants, or seeds: which should you use?
What soil and site conditions do onions need?
How to plant onions step by step
How do you water and fertilize onions through the season?
What pests and diseases should you watch for?
How do you know when onions are ready to harvest?
Can you grow onions in containers?
Expert tips that actually make a difference
A gardener’s honest take on growing onions
Sources

When should you plant onions?

Timing onions is about two triggers: soil temperature and daylength. Most gardeners focus on the last frost date, which matters, but soil temperature is the more reliable signal. University of Maryland Extension recommends planting sets in spring as soon as the soil is workable, which typically coincides with that 50°F threshold. Onions tolerate light frost once established, so you can push them out earlier than most warm-season crops.

Daylength is the other half of the equation. Onions start forming bulbs when day length hits a specific threshold, and that threshold varies by variety type. Plant a long-day variety in Georgia and it may never bulb properly. Plant a short-day variety in Minnesota and it will bulb before it has grown enough leaves to make a decent-sized onion.

The table below gives practical planting windows by region and variety type.

Region / USDA Zones	Daylength Type	Planting Window (Sets/Transplants)	Seed Start Indoors
Deep South (Zones 7–10)	Short-day	October–February	August–November
Mid-South / Transition (Zones 6–7)	Intermediate	February–March	December–January
Northern US (Zones 3–6)	Long-day	March–May (after last frost risk)	January–February
Pacific Northwest (Zones 7–8)	Long-day or Intermediate	February–April	December–January

For succession planting, sow a second round of seeds or sets 2–3 weeks after the first. This staggers your harvest and keeps fresh green onions coming through summer without a single glut.

For more detailed regional planting calendars, the Lushygardens planting timing guide breaks down timing by vegetable and zone.

How do you choose the right onion variety?

The daylength category is the non-negotiable filter. Everything else, flavor, size, color, storage life, comes after you’ve confirmed the variety will actually bulb in your location. Martha Stewart’s horticulturist guidance puts it plainly: short-day varieties suit southern zones, long-day varieties suit northern zones, and intermediate (day-neutral) types work across a wider range.

Short-day onions (zones 7–10) begin bulbing when days reach about 10–12 hours. They tend to be sweeter and milder but have shorter storage lives. Classic examples include Granex-type yellows (the parent of Vidalia-style onions) and red varieties popular in the South.

Long-day onions (zones 3–6) need 14–16 hours of daylight to trigger bulbing, which is why they’re planted in early spring in the North. They typically produce larger, more pungent bulbs with better storage life. Yellow storage types in this category can last 6–8 months in a cool, dry space.

Intermediate or day-neutral varieties bulb at 12–14 hours and give growers in the middle zones (and much of the Pacific Northwest) more flexibility. Candy and Super Star are well-known intermediate types.

A few use-case pointers:

For long storage: choose a yellow or white long-day type with a firm, dry neck
For sweet salad onions: short-day or intermediate types harvested fresh
For green onions and scallions: any variety works when harvested young; bunching types like Evergreen Hardy White are bred specifically for this use
For shallots: these are multiplier onions that split into clusters; plant sets in fall (warm zones) or early spring (cool zones)

When buying sets or transplants, look for a label that specifies the daylength category. If the label just says “yellow onion” with no daylength information, ask the nursery or skip it.

Onion sets, transplants, or seeds: which should you use?

Each method has a real tradeoff, and the right choice depends on your season length, your patience, and how specific you want to get about variety.

Onion sets are small, dried bulbs grown the previous season and sold in mesh bags at garden centers. They’re the easiest starting point: push them into prepared soil, pointed end up, and they’re off. Sets establish quickly in cool soil, tolerate a bit of neglect, and shorten time to harvest. The downside is limited variety selection. You’ll usually find yellow, red, and white, but not much beyond that. Sets also carry a slightly higher risk of bolting (sending up a flower stalk) if they were stored improperly or are too large.

Transplants are seedlings started indoors and sold as bundles of pencil-thin plants. They give you a head start similar to sets but with better variety options and a lower bolt risk. Many gardeners find transplants the sweet spot between ease and control.

Direct seeding takes the most time but opens up the full range of onion varieties. Gardening Know How recommends starting seeds indoors 8–12 weeks before your last frost date, keeping the soil at 65–75°F for germination, and hardening seedlings off for 7–10 days before transplanting. It’s more work, but if you want a specific heirloom or a variety not sold as sets in your area, seeds are the only path.

Multiplier onions and shallots are a separate category worth a mention. They’re planted as sets in fall (in warm zones) or early spring, multiply underground into clusters, and are harvested as the tops die back. They’re low-maintenance and productive in small spaces.

What soil and site conditions do onions need?

Onions are not forgiving of poor drainage or compacted soil. A bulb forming underground needs loose, friable soil to expand without restriction. Clay-heavy beds produce misshapen, small bulbs even when fertility is good.

Ohio State University Extension (CFAES) recommends sandy-loam to silty-loam soils with a pH of 6.0–6.8. Below 6.0, nutrient availability drops and onions struggle to take up the nitrogen they need for leaf growth. A soil test before planting is the highest-leverage action you can take: it tells you exactly what to add and prevents over-fertilizing, which pushes leaf growth at the expense of bulb development.

Before planting, work 2–3 inches of compost into the top 6–8 inches of soil. This improves drainage in clay soils, adds moisture retention in sandy ones, and feeds the soil biology that makes nutrients available to roots. For a full walkthrough on bed preparation, the beginner vegetable garden setup guide at Lushygardens covers the steps in detail.

A quick pre-planting soil checklist:

Test pH and adjust with lime (to raise) or sulfur (to lower) at least a few weeks before planting
Work in compost to a depth of 6–8 inches
Rake the bed smooth and remove rocks or clods that would obstruct bulb development
Avoid planting where onions, garlic, or leeks grew in the past 3 years (rotation reduces disease pressure)

Onions need full sun, at least 6–8 hours daily. Partial shade reduces leaf growth, and fewer leaves means smaller bulbs. Row orientation matters less than sun exposure, but north-south rows maximize light interception in most US gardens.

How to plant onions step by step

Planting sets

Rake the prepared bed smooth and open a furrow 1–2 inches deep.
Place sets pointed end up, 3–4 inches apart along the row. Rows should be 12–18 inches apart.
Cover with soil so the tip of the set is just at or slightly below the surface. Burying sets too deep invites rot; too shallow and they dry out before rooting.
Firm the soil gently around each set with your fingers.
Water in thoroughly so the soil settles around the sets.

Planting transplants

Open a furrow or individual holes deep enough to bury the roots and the bottom inch of the stem.
Set transplants 3–4 inches apart, firming soil around the base so the plant stands upright.
Water immediately. Transplants wilt quickly after planting; consistent moisture for the first week is critical.

Starting seeds indoors

Fill trays or pots with a lightweight seed-starting mix.
Sow seeds 1/4–1/2 inch deep, about 1 inch apart.
Keep the mix at 65–75°F until germination (usually 7–10 days).
Once seedlings reach pencil thickness, thin to 1 inch apart.
Harden off for 7–10 days before transplanting outdoors.

For more on outdoor direct sowing, the beginner’s guide to planting seeds outdoors at Lushygardens covers the process from soil prep through germination.

Thinning

Thin seedlings in stages. Pull or snip every other plant when they reach 4–6 inches tall, using the thinnings as green onions. For full-size bulbs, University of Maryland Extension recommends a final spacing of 4 inches for standard bulbs and 8 inches for very large bulbs. Scallions can stay at 2 inches apart since you’re harvesting them before they bulb.

Pro Tip: When planting sets, resist the urge to push them deep for stability. The pointed tip should sit just at soil level. Deep planting delays emergence and creates a moist pocket around the neck that encourages rot, especially in heavy soils.

How do you water and fertilize onions through the season?

Onions need steady moisture during bulb development but hate waterlogged soil. The practical test: press your finger an inch into the soil. If it feels dry, water. If it feels damp, wait. During the active bulbing phase (roughly 60–90 days after planting, depending on variety), consistent moisture directly affects final bulb size.

Drip irrigation or soaker hoses work better than overhead watering for onions because wet foliage promotes fungal disease. If you use overhead sprinklers, water in the morning so leaves dry before evening.

For fertilizing, the sequence matters:

Before planting: incorporate a balanced granular fertilizer or compost into the bed based on your soil test results
At transplanting: a dilute liquid starter fertilizer helps transplants establish roots quickly
Side-dressing: apply a nitrogen-rich fertilizer (such as blood meal or a granular 10-0-0 or similar) 1–2 weeks before bulb enlargement begins, and again as needed if leaves look pale, per CFAES guidance
Stop fertilizing once the necks begin to soften and tops start to fall over. Late nitrogen pushes leaf growth at the expense of curing and storage quality.

Mulching with 2–3 inches of straw or shredded leaves suppresses weeds (which compete aggressively with shallow-rooted onions), retains moisture, and keeps soil temperature stable. Apply mulch after planting but keep it pulled back slightly from the base of each plant to avoid trapping moisture against the neck.

Yellowing lower leaves mid-season are normal as the plant matures. Yellowing of upper leaves combined with stunted growth usually points to nitrogen deficiency or waterlogging. Tip burn (brown leaf tips) often signals inconsistent watering or low calcium in the soil.

What pests and diseases should you watch for?

Onions have fewer pest problems than many vegetables, but a handful of issues can wipe out a crop if you miss them early.

Common pests:

Onion maggots are the larvae of a small fly that lays eggs at the base of plants in late spring. The larvae tunnel into bulbs and stems, causing plants to wilt and collapse. Row covers placed at planting are the most effective prevention. Crop rotation (avoid planting alliums in the same spot for 3 years) reduces overwintering populations.
Thrips are tiny, pale insects that rasp leaf surfaces, leaving silver streaking. They’re worst during hot, dry spells. Reflective mulch and a strong spray of water can knock populations down. Severe infestations may warrant an insecticidal soap application.
Slugs feed on young seedlings, especially in wet springs. Hand-picking at night, iron phosphate baits, and reducing surface debris all help.

Common diseases:

Downy mildew appears as pale, fuzzy patches on leaves in cool, wet weather. Improve air circulation by thinning plants and avoid overhead watering.
Neck rot (caused by Botrytis fungi) develops during curing if bulbs are harvested with wet necks or stored before fully dry. Cure thoroughly before storage.
Basal rot causes the base of the bulb to turn soft and brown. It’s soil-borne and worsened by poor drainage. Rotation and raised beds are the main defenses.

For a broader approach to garden pest management, Lushygardens’s organic pest management guide covers identification and low-toxicity controls across common vegetable crops.

Monitoring is straightforward: walk your beds twice a week during the growing season and check the base of plants and the undersides of leaves. Catching problems at the first sign of damage is far easier than managing a full infestation.

How do you know when onions are ready to harvest?

The signal is unmistakable once you know what to look for: the tops fall over and begin to yellow and dry. This usually happens in late summer for spring-planted onions. Don’t rush it. Let at least half the tops fall naturally before you harvest.

For green onions and scallions, harvest anytime after they reach pencil thickness. Pull them whole or snip leaves from the outside, leaving the center to keep growing.

For bulb onions, once most tops have fallen, bend over any remaining upright tops by hand to speed the process. Wait a few more days, then lift bulbs gently with a garden fork. Avoid bruising them.

Curing is not optional for long storage. Spread bulbs in a single layer in a warm, dry, well-ventilated spot out of direct rain. A covered porch, a shed with good airflow, or a screen rack all work. Cure for 2–4 weeks until the necks are completely dry and papery and the outer skins rustle when touched. Trim roots and cut tops to about an inch above the neck only after curing is complete.

Long-day storage types can last 6–8 months under these conditions. Sweet short-day varieties store for only 1–2 months and are better eaten fresh or refrigerated.

Common storage mistakes: packing bulbs in sealed containers (they need airflow), storing before the neck is fully dry, or keeping them in a warm kitchen. Mesh bags, old pantyhose, or slatted crates all allow the air circulation bulbs need.

Can you grow onions in containers?

Yes, and it works well if you match the container to the type of onion you’re growing. Scallions and bunching varieties are the easiest container crop: they have shallow roots, mature quickly, and can be succession-planted every 2–3 weeks for a continuous harvest.

Martha Stewart’s horticulturist sources recommend containers at least 10–12 inches deep for bunching onions, with deeper pots (at least 12–15 inches) for bulbing varieties. Drainage holes are non-negotiable. Onions sitting in waterlogged potting mix will rot.

Key container tips:

Soil mix: use a quality potting mix blended with compost and a small amount of coarse sand or perlite for drainage. Garden soil alone compacts too much in pots.
Fertilizing: container plants need more frequent feeding than in-ground onions because nutrients leach out with each watering. A dilute liquid fertilizer every 2–3 weeks works well during active growth.
Watering: check containers daily in warm weather. They dry out much faster than garden beds.
Varieties: bunching types, shallots, and intermediate-day varieties tend to perform best in pots. Large long-day storage onions need more root room than most containers provide.
Succession planting: sow a new pot of scallion seeds every 3 weeks from early spring through midsummer for a near-continuous harvest.

For more on container setups and potting mixes, the growing vegetables in pots guide at Lushygardens goes deeper on drainage, mix ratios, and pot selection.

Expert tips that actually make a difference

The most useful insight from university extension research is also the least obvious: every leaf an onion plant produces before the daylength bulbing trigger becomes one layer in the final bulb. More leaves equal more layers equal a bigger onion. Lancaster County Cooperative Extension makes this explicit, and it reframes the entire planting strategy.

The practical implication: starting early is not just about getting ahead of the season. It’s about giving your onion plant as many weeks as possible to build leaves before the daylength trigger fires. Every week of early growth you sacrifice to a late start is a layer of bulb you’ll never get back.

This is why sets and transplants outperform direct-seeded onions for most home gardeners: they arrive with a head start on leaf count. If you do start from seed, begin indoors as early as January or February in northern zones to maximize that pre-bulbing growth window.

Practical troubleshooting from extension recommendations:

Small bulbs despite good care: the most common cause is wrong daylength variety for your zone, or a late start that left too few leaves before bulbing triggered
Bolting (flower stalks): usually caused by large sets, temperature fluctuations, or stress; remove the flower stalk immediately and use those onions fresh since they won’t store
Soft necks at harvest: indicates the plant was still actively growing when pulled; wait for more tops to fall before harvesting
Pale, slow growth: check nitrogen levels and soil pH; onions are heavy nitrogen feeders and pH below 6.0 locks up nutrients

For localized planting dates, pest pressure alerts, and variety recommendations specific to your county, your state’s cooperative extension service is the most reliable resource. Search “[your state] + cooperative extension + onion planting” to find the relevant fact sheet.

A gardener’s honest take on growing onions

Onions reward patience and punish guessing. The first season you grow them, you’ll probably make at least one of the classic mistakes: wrong variety for your zone, planting too late, or pulling bulbs before they’ve cured. That’s fine. Most gardeners do.

What changes in the second season is that you know your soil, you know your timing, and you’ve seen what happens when the tops fall over naturally versus when you rush it. Sets are a great starting point because they take the seed-starting complexity off the table entirely. But once you’ve grown a successful crop, trying a few varieties from seed opens up a world of flavor and size options that sets simply can’t match.

The learning curve here is genuinely short. One season of paying attention to daylength, planting depth, and curing time is usually enough to go from a handful of small, soft bulbs to a pantry full of keepers. Give it two seasons and you’ll have a system that works for your specific garden.

Sources

These are the sources cited throughout this guide, plus a few places to go for localized timing and variety lists:

I’m Eleanor, a seasoned gardener with over three decades of experience tending to Mother Nature’s creations. Through Lushy Gardens, I aim to share my wealth of knowledge and help fellow plant enthusiasts uncover the wonders of gardening. Let’s dive into this journey together, one leaf at a time.