Iceberg is the fussiest lettuce to grow well: it needs a long, cool run to form a dense, crisp head, and a few hot days can stop heading altogether. Starting indoors is the best way to give it that long cool season — sow early, transplant into cool spring soil, and you get heads instead of loose leaves.

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

Iceberg Lettuce at a glance

Growing factor What this crop needs
Days to germination 7–10
Sowing method both
Days to maturity 70–85
Soil pH 6.0–7.0
Spacing 12–18 inches
Sun full sun
Cold tolerance hardy

When to start Iceberg Lettuce seeds indoors

Start Iceberg Lettuce indoors 4–6 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. Iceberg needs more time and cooler weather than any other lettuce, so indoor starting is almost essential outside frost-free zones. In Zones 3–5 start 4–6 weeks before your last frost; in Zones 6–7 the same lead time works, with a late-summer sowing for autumn heads. In Zones 8–9, grow iceberg in autumn and winter. In frost-free Zones 10–13, sow in the coolest months and give afternoon shade. Warm soil above 24°C (75°F) sharply reduces germination.

Zones 3–5: count back 4–6 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 12–18-inch spacing. Check old packets with the seed-viability checker.

Step by step: starting Iceberg Lettuce seeds indoors

Iceberg Lettuce 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 barely covered — lettuce seed needs light to germinate. Iceberg Lettuce germinates in 7–10 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 12–18 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 Iceberg Lettuce once soil is workable. Check old packets with the seed-viability checker.

Light for seedlings

Light separates stocky seedlings from leggy ones. Give Iceberg Lettuce 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 Iceberg Lettuce has 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. Iceberg Lettuce is 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 12–18 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 Iceberg Lettuce include marigolds, basil, onions, lettuce. Watch for aphids, slugs, thrips 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
No head forming Heat or crowding Give 12–18 inches of space and grow in the cool season.
Seeds not germinating Buried too deep or too warm Surface-sow and keep trays cool, below 24°C (75°F).
Bitter leaves Heat stress or bolting Harvest promptly and grow in spring or autumn.
Rot in the heart Overhead watering, no airflow Water at soil level and space plants properly.

No head forming

Heat or crowding is the usual culprit. Give 12–18 inches of space and grow in the cool season. If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Seeds not germinating

Buried too deep or too warm is the usual culprit. Surface-sow and keep trays cool, below 24°C (75°F). If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Bitter leaves

Heat stress or bolting is the usual culprit. Harvest promptly and grow in spring or autumn. If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Frequently asked questions

Why won't my iceberg form a head?

Usually heat, or plants set too close together. Iceberg needs 70–85 days of cool weather and 12–18 inches of space to head up properly.

Do iceberg seeds need light to germinate?

Yes — sow on the surface and barely cover. Deep-sown lettuce seed often never emerges.

How long does iceberg take from seed?

Around 70–85 days to a full head. Start indoors to buy the extra weeks a cool season demands.

Can I grow iceberg in summer?

Not usually — it bolts and turns bitter in heat. In hot climates, treat it as an autumn and winter crop.

Should I start iceberg indoors or direct sow?

Both work, but indoor-started plants head more reliably because you control germination temperature and transplant at exactly the right size.

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