White onions are the sharp, crisp onion of Mexican cooking and fresh salsas — milder than yellow when young, with a clean bite. Like all onions from seed, they reward an early indoor start: sown in late winter under lights, they grow into sturdy transplants that bulb up reliably in summer.

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

White 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 White Onions seeds indoors

Start White 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 white onion seed indoors 8–10 weeks before your last frost. In Zones 3–5 this is a late-winter job under lights; in Zones 6–7 the same timing gives summer bulbs; in Zones 8–9, autumn sowing for a winter crop often works better. In frost-free Zones 10–13, grow through the cooler months and pick varieties bred for your day length — onions only bulb properly when day length matches the variety, so check the packet or ask your local extension service.

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 White Onions seeds indoors

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

Light for seedlings

Light separates stocky seedlings from leggy ones. Give White 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 White 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. White 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 White 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
Floppy seedlings Thin tops; low light Trim to 4 inches and move lights closer.
Poor germination Old seed Buy fresh seed yearly; test with the seed-viability checker approach of a damp-paper-towel test.
Damping off Soggy compost Water from below; ventilate.
Bulbs not sizing up Late start or crowding Start 8–10 weeks early; thin to 4–6 inches.

Floppy seedlings

Thin tops; low light is the usual culprit. Trim to 4 inches and move lights closer. If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Poor germination

Old seed is the usual culprit. Buy fresh seed yearly; test with the seed-viability checker approach of a damp-paper-towel test. If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Damping off

Soggy compost is the usual culprit. Water from below; ventilate. If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Frequently asked questions

How long do white onions take from seed?

About 90–120 days to mature bulbs, after an 8–10-week indoor start. It is a long-season crop, so start early.

Should I trim onion seedlings?

Yes — when tops reach 5–6 inches, trim them back to about 4 inches. It keeps seedlings stocky and upright.

Do white onions store well?

Less well than yellow storage onions — white onions are best used fresh within a few weeks of harvest.

Why is my onion seed not germinating?

Onion seed goes stale quickly. Buy fresh seed each year and keep the compost evenly moist for the 7–14-day germination window.

Can I direct sow white onions?

Yes, but germination is slower and weed competition is fierce. Indoor-started transplants establish faster and bulb more reliably.

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