Onions need a long season to size up, and their seed is tiny and slow to get going, so starting indoors is the standard route for seed-grown onions. An indoor start gives you sturdy young plants with a head start on bulb formation, instead of thin seedlings still struggling when summer arrives.
Starting onions indoors is the standard approach for this crop, and this guide follows it step by step. Germination takes 7–14 days, maturity 90–120 days, spacing is 4–6 inches and soil pH 6.0–7.0.
Onions at a glance
| Growing factor | What onions need |
|---|---|
| Days to germination | 7–14 |
| Sowing method | both |
| Days to maturity | 90–120 |
| Soil pH | 6.0–7.0 |
| Spacing | 4–6 inches apart |
| Sun | full sun |
| Cold tolerance | hardy |
When to start onion seeds indoors
Sow indoors 8–10 weeks before your last frost date. In Zones 3–5 that usually means late winter, in Zones 6–8 late winter, and in Zones 9–10 late winter. Zones 11–13 are frost-free, so sow in the cooler months and check your local extension service for the best local sowing window.
Work backwards from your own last frost date: find it with our frost-date calculator, then count back the weeks above. If you are unsure, check your local extension service, and use the seed-starting calculator to work out your sowing date.
What you need
You do not need expensive kit: clean trays, cells or small pots, a bag of seed-starting mix (not garden soil, which is too heavy and can carry disease), seed, labels and a pen, and a watering can with a fine rose or a spray bottle. A heat mat helps if your room is cool, and a clear lid keeps the mix moist until seedlings appear — take it off the moment they do, or damp air invites disease. Our grow-light calculator checks your light setup, and the seed-viability checker tells you whether last year's seed is still worth sowing.
Step by step: starting onion seeds indoors
- Fill your containers. Fill clean trays or pots with damp seed-starting mix, firm it gently and level the surface — it should feel like a wrung-out sponge.
- Sow the seed. Sow onions seed at about twice the seed's own width deep, spacing seeds so seedlings do not crowd each other, and sow a few extra to allow for failures.
- Label everything. Label each tray with the crop and sowing date before you water — unlabeled trays are the commonest seed-starting mistake.
- Water gently. Water from below by standing the tray in a shallow dish, or mist lightly; heavy watering from above washes seed out of place.
- Keep it warm and covered. Cover with a lid or cling film, keep somewhere warm, check daily, and remove the cover the moment seedlings appear.
- Give light immediately. Move the tray to your brightest spot or under grow lights as soon as seedlings emerge. Onion seedlings benefit from a haircut: when the tops reach a few inches tall, trim them back by about a third with clean scissors — it keeps them stocky and stops them flopping over each other.
- Pot on when ready. When seedlings have two or three true leaves, pot the strongest ones into individual pots, handling them by a leaf, never by the stem.
Light for seedlings
Light decides whether your onions seedlings are stocky or stretched, and it matters from the day they emerge. Onions need full sun: indoors, that means your brightest spot — a south-facing windowsill — or a grow light hung just a few inches above the plants. A light too far away is almost as bad as no light at all, so turn trays daily so they do not lean. Pale, tall seedlings need more light, not more feed. Our grow-light calculator checks your setup.
Watering seedlings
Water needs for onions are moderate: seedlings need steady moisture, never sodden and never bone dry. Water from below where possible, so the surface stays drier and disease has less chance to take hold — a constantly wet tray is the main cause of damping off, which fells seedlings at soil level overnight. Water in the morning and let the top of the mix dry slightly between waterings once seedlings have true leaves.
Hardening off
Hardening off is the week-long process of getting indoor seedlings used to outdoor life — skipping it is the fastest way to lose a tray of good plants. Start about a week before planting out: put seedlings in a sheltered, shady spot outside for a few hours on day one, then bring them in at night. Lengthen the time outside and the sun each day, leaving them out overnight only in the last day or two, when nights are mild, and bring them in if frost, wind or heavy rain is forecast. By the end of the week they should stand in full sun and a breeze without wilting.
Transplanting outdoors
Onion seedlings are hardy and can go out before the last frost has fully passed, once the soil can be worked. Plant them at the spacing in the table below, just deep enough to cover the white base, and water them in well.
Good neighbours for onions include marigolds, basil, onions. Set transplants out on a cloudy day or in the evening, water each one in well, and keep the soil moist for the first week while roots establish — a transplant that dries out early rarely recovers its vigour. Mulch once plants are settled, and check spacing with our plant-spacing calculator.
Common problems and fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Damping off — seedlings collapse at soil level | Mix too wet, poor air flow | Sow thinner, water from below, remove covers once seedlings emerge |
| Leggy, pale seedlings | Not enough light | Move lights closer, give more hours of bright light, keep seedlings cool |
| Slow or patchy germination | Mix too cool or too dry | Keep the tray warm and moist, check seed viability |
| Aphids damage | Pest attack on young plants | Inspect leaf undersides weekly, wash off or hand-pick, encourage beneficial insects |
| Slugs damage | Pest attack on young plants | Inspect plants early, remove affected leaves, keep plants well watered and unstressed |
Damping off
Damping off makes healthy-looking seedlings keel over at soil level overnight. It thrives in wet, still air, so prevention is everything — clean trays, fresh mix, thin sowing, watering from below and good air movement. A damped-off seedling cannot be saved, so remove it at once.
Aphids and Slugs
aphids, slugs, cutworms are the pests most often reported on onions. Check young plants weekly, especially leaf undersides, and act at the first sign of damage. A strong jet of water dislodges many small pests, hand-picking deals with larger ones, and healthy, well-watered plants resist attack far better than stressed ones. Our pest-identifier helper can help identify what you are seeing.
Frequently asked questions
How long do onion seeds take to germinate?
Onions seed takes 7–14 days to germinate in warm, moist mix. Cooler conditions slow it down, and old or badly stored seed may fail — test it first with our seed-viability checker.
How far apart should I space onions?
Space onions 4–6 inches apart at planting-out time. That spacing keeps air moving and gives each plant room to reach full size — crowding is a common cause of small harvests and more disease.
What soil pH do onions need?
Onions grow best at a soil pH of 6.0–7.0. Test your soil before amending it, adjust over a season rather than guessing, and dig in compost to improve the soil whatever its pH.
Should I trim my onion seedlings?
Yes — trimming the tops back when they get long and floppy keeps seedlings stocky and upright, and it does not harm bulb formation. Use clean scissors and trim by about a third.