Red onions earn their place for colour and mild sweetness: sliced raw into salads, pickled, or grilled, they do work white and yellow onions cannot. Growing them from seed — rather than sets — gives you far more variety choice and bigger bulbs, but onion seed needs the longest indoor head start of any vegetable on this list.

They are best started indoors, under cover, and planted out once the weather is right.

Red Onions at a glance

Growing factor What this crop needs
Days to germination 7–14
Sowing method both
Days to maturity 90–120
Soil pH 6.0–7.0
Spacing 4–6 inches
Sun full sun
Cold tolerance hardy

When to start Red Onions seeds indoors

Start Red Onions indoors 8–10 weeks before your last frost. Sow too early and you get leggy, root-bound plants; sow too late and you lose weeks of the season. Onion seed needs 8–10 weeks indoors before your last frost, which makes it one of the earliest sowings of the year. In Zones 3–5, sow in late winter under lights; in Zones 6–7 the same lead time applies, and bulbs mature in summer. In Zones 8–9, onions are often an autumn-sown, winter-grown crop. In frost-free Zones 10–13, choose varieties suited to your day length and grow through the cooler months. Onion varieties are bred for different day lengths, so check the packet — or ask your local extension service — for types that bulb up where you live.

Zones 3–5: count back 8–10 weeks from your last frost and sow under lights. Zones 6–7: the same lead time. Zones 8–9: the window opens earlier, and autumn sowing is often possible. Zones 10–13 are frost-free, so sow in the cooler months and let heat set the calendar. Use the frost-date calculator and the seed-starting calculator for your dates, and check your local extension service when in doubt.

What you need

You do not need much: fine, sterile seed-starting mix (not garden soil), cell trays or small pots, a tray to catch water, a spray bottle or fine watering can, labels and a marker, and a clear lid or cling film to hold humidity until germination. A grow light is the one item that transforms results — a sunny windowsill can work, but lights give stocky seedlings instead of leggy ones. The grow-light calculator helps size a light for your setup, and the seed-spacing calculator shows how many plants fit your beds at 4–6-inch spacing. Check old packets with the seed-viability checker.

Step by step: starting Red Onions seeds indoors

Red Onions can be direct-sown or started indoors — both work. Indoor starting wins when you want an earlier crop, even germination, or protection from slugs and cold soil.

  1. Fill your containers. Use cell trays, small pots or soil blocks of fine, sterile seed-starting mix — not garden soil, which is too heavy and can carry damping-off fungi. Dampen the mix before sowing.
  2. Sow at the right depth. Sow seed ¼ inch deep. Red Onions germinate in 7–14 days, and depth matters — too deep and seed rots before it emerges.
  3. Label everything. Write the variety and date on every tray. In three weeks, when the trays look identical, you will be glad you did.
  4. Cover and keep moist. Cover with a clear lid or cling film to hold humidity. Ventilate daily once sprouts appear, and remove the cover within a day or two.
  5. Give light immediately. The moment seedlings emerge, move them under a grow light 2–4 inches above, or to the sunniest windowsill. Seedlings left in dim light go leggy within days.
  6. Thin to one per cell. Snip weaker seedlings at soil level rather than pulling them, which disturbs the keeper's roots. Final spacing outdoors is 4–6 inches.
  7. Feed lightly. Once true leaves appear, feed at quarter-strength once a week. Seed compost has almost no nutrients, and hungry seedlings turn yellow and stall.

You can also direct sow Red Onions once soil is workable. Check old packets with the seed-viability checker.

Light for seedlings

Light separates stocky seedlings from leggy ones. Give Red Onions seedlings 14–16 hours of light a day, with the source just 2–4 inches above the plants — raise it as they grow. A south-facing windowsill can work in spring, but turn trays daily or seedlings lean towards the glass, and on cloudy weeks even a good window is not enough. Our sunlight calculator tells you how much direct sun a spot gets, and the grow-light calculator helps size a light. If seedlings stretch, the light is not close or bright enough.

Watering seedlings

Seedlings want steady moisture, not wet feet. Water from below where possible — stand the tray in a shallow dish for ten minutes and let the compost wick it up — because overhead watering knocks tiny seedlings flat and keeps stems wet, which invites damping off. Check trays daily, since seed compost dries fast under lights, and Red Onions have moderate water needs — keep the compost evenly moist but never soggy. Mist the surface lightly after sowing until germination, then switch to bottom watering. As transplanting approaches, let compost dry slightly more between waterings to toughen seedlings up.

Hardening off

Hardening off is the week that decides whether seedlings thrive or sulk. Over 7–10 days before transplanting, put trays outside for a few hours a day in a sheltered spot, increasing time, sun and wind exposure each day, and bring them back in at night. Start with shade and still air; end with a full day outside if temperatures allow. Seedlings that go straight from a warm room to open ground get sunscald, windburn or a cold check that sets them back for weeks. Red Onions are hardy — it tolerates light frost, so it can go outside earlier than tender crops. Do not rush this step, and bring trays in if frost threatens.

Transplanting outdoors

Transplant on a cloudy day or in the evening, when the sun will not immediately stress seedlings. Water the trays an hour before, dig holes at the final spacing of 4–6 inches, and set each seedling at the same depth it was growing. Firm soil gently around the roots, water each plant in well, and mulch around (not against) the stems. Good companions for Red Onions include marigolds, basil, onions, lettuce. Watch for aphids, slugs, cutworms in the first weeks — the pest-identifier helper can help name anything you find. Water daily for the first week while roots establish.

Common problems and fixes

Problem Likely cause Fix
Thin, floppy seedlings Normal for onions; too little light Trim tops to 4 inches and give strong light.
Seeds not germinating Old seed or dry compost Use fresh seed — onion seed loses viability fast — and keep compost moist.
Seedlings damping off Wet compost Bottom-water and improve airflow.
Small bulbs Planted too late or too close Start 8–10 weeks early and thin to 4–6 inches.

Thin, floppy seedlings

Normal for onions; too little light is the usual culprit. Trim tops to 4 inches and give strong light. If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Seeds not germinating

Old seed or dry compost is the usual culprit. Use fresh seed — onion seed loses viability fast — and keep compost moist. If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Seedlings damping off

Wet compost is the usual culprit. Bottom-water and improve airflow. If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Frequently asked questions

Is it better to grow red onions from seed or sets?

Seed gives more varieties and often bigger bulbs; sets are easier and faster. Seed is worth it if you want specific varieties or storage onions.

Why are my onion seedlings flopping over?

Normal at first — onion seedlings are thin and grass-like. Trim the tops to about 4 inches to keep them upright and encourage thicker necks.

How long do red onions take from seed?

About 90–120 days to mature bulbs, plus the 8–10-week indoor start — plan for a long season.

Do onions need a lot of space?

No — 4–6 inches apart is enough, which makes them a good crop for small beds and between slower vegetables.

When do I stop watering onions?

As bulbs swell and tops start to fall over, ease off watering so bulbs cure well for storage.

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