Yellow onions are the all-round storage onion: the one that keeps for months in a cool cupboard and goes into almost every cooked dish. Grown from seed they store better and grow bigger than sets, but they demand patience — the indoor start is long, and the bulbs take 90–120 days to mature.

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

Yellow 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 Yellow Onions seeds indoors

Start Yellow 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. Sow yellow onion seed indoors 8–10 weeks before your last frost — among the earliest sowings of the year. Zones 3–5: late-winter sowing under lights for summer bulbs. Zones 6–7: the same lead time, with harvest in mid to late summer. Zones 8–9: autumn sowing for winter growth often suits yellow storage types. Zones 10–13, which are frost-free: grow in the cooler months and choose varieties matched to your day length, since onions bulb in response to day length. Your local extension service can confirm which types suit your latitude.

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 Yellow Onions seeds indoors

Yellow 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. Yellow 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 Yellow Onions once soil is workable. Check old packets with the seed-viability checker.

Light for seedlings

Light separates stocky seedlings from leggy ones. Give Yellow 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 Yellow 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. Yellow 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 Yellow 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 Low light; tops untrimmed Trim to 4 inches; lights 2–4 inches above.
Poor germination Stale seed or dry compost Use fresh seed each year; keep compost moist for 7–14 days.
Damping off Wet compost, still air Bottom-water and ventilate.
Small bulbs at harvest Started too late or planted too close Sow 8–10 weeks before last frost; thin to 4–6 inches.

Thin, floppy seedlings

Low light; tops untrimmed is the usual culprit. Trim to 4 inches; lights 2–4 inches above. If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Poor germination

Stale seed or dry compost is the usual culprit. Use fresh seed each year; keep compost moist for 7–14 days. If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Damping off

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

Frequently asked questions

How long do yellow onions take from seed?

About 90–120 days to mature bulbs, plus 8–10 weeks indoors. Start early and be patient.

Do yellow onions store the longest?

Yes — of the three common types, yellow storage onions keep best, often for months in a cool, dry, dark place.

Should I trim onion seedling tops?

Yes, back to about 4 inches when they reach 5–6 inches tall. It keeps them sturdy and encourages thicker stems.

Why are my onion seedlings so thin?

Onion seedlings are naturally grass-like. Strong light and regular trimming are what thicken them up.

Can I grow yellow onions from sets instead?

Yes, sets are easier — but seed gives you storage varieties and bigger bulbs, and costs far less per plant.

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