edible gardening
How to grow onions — sets vs seed, day length
Grow onions from sets, seed or transplants: pick long-day vs short-day for your latitude, spacing, when to stop watering, the harvest and cure signal.
How to grow onions — sets vs seed, long vs short day
Onions look simple but quietly fail more often than any other staple, almost always for one reason: the wrong day-length type for the gardener's latitude. Get that one decision right and the rest — planting, weeding, watering, curing — is straightforward. This guide covers sets versus seed versus transplants, the day-length map, spacing, the stop-watering rule, and curing for storage, for US and UK gardens. Onions share a bed plan and a curing routine with their close allium relative — see how to grow garlic — and rotate well after a potato crop.
Onion timing for your location: Add your zip code or postcode to Growli and it sets your spring planting window against your local frost date — plus the stop-watering date and harvest signal later in the season.
The one decision that matters most: day length
Onions switch from growing leaves to forming a bulb when the day reaches a trigger length. Choosing the wrong type for your latitude is the number-one reason home onions stay tiny.
| Type | Bulbs at | Best latitude | Where in US/UK |
|---|---|---|---|
| Short-day | 10-12 hours daylight | ~25-35°N | US South, below latitude 36°N (TX, FL, GA, AZ low desert) |
| Intermediate / day-neutral | 12-14 hours | ~32-42°N | US central band; modern hybrids tolerate a wide range |
| Long-day | 14-16 hours | ~37-47°N | US North, plus all of the UK |
The practical rule: the 36th parallel splits the US. North of it, grow long-day onions; south of it, grow short-day. The whole UK is far north — UK gardeners grow long-day onions. Day-neutral hybrids are the safe choice if you garden in the transitional middle band or aren't sure.
Why latitude matters: onions need as much leafy top growth as possible before bulbing starts, because each leaf feeds a bulb scale. The right day-length type ensures the plant builds enough leaf before the photoperiod tips it into bulb mode.
Sets vs seed vs transplants
- Sets (small immature bulbs) — easiest and fastest, most beginner-friendly, less weather-sensitive. Slightly higher risk of "bolting" (running to flower). The standard home method.
- Seed — cheapest, widest variety choice, best storage potential, but needs an early indoor start (8-10 weeks before last frost) because onions are slow.
- Transplants (pencil-thick seedlings, often mail-ordered in bundles) — best of both: variety choice of seed, head start of sets, lower bolting risk. The top choice for big storage onions, especially short-day in the US South.
Choose a variety
US varieties
- Walla Walla (short-day/intermediate) — large, sweet, mild; not a keeper, eat fresh.
- Texas Sweet / Texas Grano types (short-day) — the classic Southern sweet onion.
- Patterson (long-day) — the benchmark long-day storage onion; stores into winter.
- Yellow / Red Candy Apple (day-neutral) — reliable across the transitional band.
UK varieties (RHS Award of Garden Merit)
The UK grows long-day onions; these hold the RHS AGM:
- Sturon AGM — round, straw-coloured, reliable spring set, good keeper.
- Stuttgarter Giant AGM — flat yellow bulbs, heavy yields, excellent storage.
- Red Baron AGM — round red onion, strong flavour, stores well into winter.
- Senshyu (Senshyu Yellow) — a Japanese overwintering type for autumn planting (mid-September to early November) and an early-summer harvest the next year.
Soil and site
- Full sun — 6+ hours direct; onions are poor competitors and sulk in shade.
- Rich, free-draining soil with compost worked in. Avoid fresh manure (forked bulbs, rot).
- pH 6.0-7.0 — check with the soil pH guide; acidic soil stunts onions.
- Rotation: don't follow any allium (garlic, leek, shallot) for 3 years. Companion planting note: onions interplanted with carrots help confuse carrot root fly and onion fly.
Planting
| Method | When | Depth | Spacing |
|---|---|---|---|
| Sets (spring) | Early spring, soil workable | Tip just showing | 4-6 in apart, rows 12 in |
| Seed (indoors) | 8-10 wk before last frost | 1/4 in | Thin / transplant to 4 in |
| Transplants | Early spring (US South: fall for short-day) | 1 in | 4 in apart, rows 12 in |
| Overwinter sets (UK) | Mid-Sept to early Nov | Tip showing | 4-6 in apart |
Closer spacing (4 in) gives more, smaller onions; wider spacing (6 in) gives fewer, larger bulbs. Plant sets shallow — burying them deep slows bulbing.
Care
- Weeding is the whole game. Onions have shallow, sparse roots and lose to weeds badly. Weed by hand or shallow-hoe carefully — deep hoeing nicks the bulbs and invites rot.
- Watering: keep evenly moist (about 1 inch per week) through the leaf-building phase and early bulbing. Then stop watering once the bulbs have swollen and the necks start to soften — excess water at maturity causes rot in store and prevents the necks drying down.
- Feeding: a balanced feed early supports leaf growth (more leaves before bulbing = bigger bulbs). Stop nitrogen once bulbing begins; late nitrogen grows thick necks that don't store.
- Remove flower stalks if any appear (bolting) — bolted onions won't store; use them fresh first.
Harvest and curing
The signal: the tops yellow, soften at the neck, and flop over naturally. When about half to three-quarters of the tops have bent over, stop watering entirely and let them sit a week or two more. Do not bend the tops over by hand — that old advice can introduce rot.
- Lift on a dry day with a fork; loosen, don't yank.
- Cure for 2-3 weeks somewhere warm, dry, and well-ventilated, out of direct sun and rain — a shed, greenhouse bench, or covered porch. Spread in a single layer or hang in bunches.
- Onions are cured when the necks are completely dry and tight and the outer skins rustle.
- Store cured onions somewhere cool (35-50°F / 2-10°C), dark, dry, and airy — netted bags or braided. Use any thick-necked or bruised bulbs first; they won't keep. Sweet onions (Walla Walla, Texas Sweet) store only a few weeks regardless — eat them first.
Safety: onions are toxic to dogs and cats
Hard flag. All alliums — onion, garlic, leek, chive — are toxic to dogs and cats and listed as toxic by the ASPCA. Onions contain thiosulfate and N-propyl disulfide, which destroy red blood cells and cause Heinz-body hemolytic anaemia. Cats are the most susceptible species; dogs are also at clear risk. Concentrated forms (onion powder, dehydrated flakes, soup mix) are the most dangerous, but raw and cooked onion and the green tops all count.
Signs can take several days to appear: weakness, pale gums, lethargy, rapid breathing, dark or red urine, collapse. Keep onion sets, trimmings, kitchen scraps, and curing bulbs away from pets, and never feed onion (or onion-containing food) to a dog or cat. Call ASPCA Poison Control at (888) 426-4435 if ingestion is suspected.
Related articles
- How to grow garlic — its allium cousin, planted in fall
- When to plant garlic — the fall allium calendar
- How to grow chives — the perennial allium for the kitchen
- Companion planting — onions and carrots confuse each other's pests
- Soil pH guide — keep the onion bed near neutral
- Frost date calculator — set your spring planting window
- How to start a vegetable garden — beginner overview with onions in the rotation
Reviewed and updated by the Growli editorial team. For questions about anything here, open Growli and ask — or email hello@getgrowli.app.
Frequently asked questions
How do you grow onions for beginners?
Buy onion sets matched to your latitude — long-day if you garden north of latitude 36°N (US North, all of the UK), short-day if south of it. Plant them in early spring in full sun and rich free-draining soil, tip just showing, 4-6 inches apart. Keep the bed scrupulously weed-free, water evenly until the bulbs swell, then stop. Harvest when the tops flop over naturally and cure for 2-3 weeks before storing cool and dry.
What is the difference between long-day and short-day onions?
Long-day onions form bulbs when days reach 14-16 hours and suit northern latitudes (about 37-47°N) — all of the UK and the US North. Short-day onions bulb at 10-12 hours and suit the US South, below latitude 36°N. Day-neutral hybrids bulb across a wide range and are the safe pick for the transitional middle band. Choosing the wrong type for your latitude is the main reason home onions stay tiny.
Should I grow onions from sets or seed?
Sets (small immature bulbs) are the easiest and fastest, the best beginner option, but have a slightly higher bolting risk. Seed is cheapest with the widest variety choice and best storage potential, but needs an early indoor start because onions are slow. Transplants (bundled seedlings) combine the variety choice of seed with the head start of sets and are ideal for big storage onions.
Why do my onions stay small?
Usually the wrong day-length type for your latitude (a short-day onion grown in the North, or vice versa) so the plant bulbs before it builds enough leaf. Other causes: too little sun, weed competition (onions have weak shallow roots), planting sets too deep, or too little water during leaf growth. Each leaf an onion grows before bulbing becomes a bulb scale, so maximise leaf growth early.
When do you stop watering onions?
Stop watering once the bulbs have swollen and the necks begin to soften — usually when about half the tops have started to flop over. Watering at maturity keeps the necks from drying down and causes rot in storage. Let the plants sit dry for a week or two after the tops flop, then lift on a dry day.
How do you know when onions are ready to harvest and how do you cure them?
Onions are ready when the tops yellow, soften at the neck, and flop over naturally (don't bend them by hand). Lift on a dry day with a fork, then cure for 2-3 weeks somewhere warm, dry, and airy until the necks are completely dry and the skins rustle. Store cured onions cool, dark, and dry; use thick-necked or bruised ones first.
Are onions toxic to dogs and cats?
Yes. The ASPCA lists onions (and all alliums) as toxic to dogs and cats. They contain thiosulfate compounds that destroy red blood cells and cause Heinz-body hemolytic anaemia. Cats are most susceptible. Raw, cooked, and powdered onion all count, and signs (weakness, pale gums, dark urine) can take days to appear. Keep onions and scraps away from pets and call ASPCA Poison Control at (888) 426-4435 if ingested.
Can you grow onions in containers?
Yes — use a container at least 8-10 inches deep with drainage holes and fill with quality potting mix. Space sets 3-4 inches apart for medium bulbs. Containers dry out fast, so water evenly through leaf growth, then ease off as bulbs mature. Choose a compact long-day or day-neutral variety, and give the container the sunniest spot you have.
How does Growli help with growing onions?
Add your location to Growli and it recommends long-day, short-day, or day-neutral onions for your latitude, sets your spring planting window against your local frost date, and reminds you of the stop-watering date and the tops-flopping harvest signal. Photograph leaf problems and Growli flags allium issues like onion fly, downy mildew, or rot.