Romaine is the workhorse lettuce: crisp, upright heads that stand up in salads and sandwiches, and one of the easiest leafy crops to start indoors. The trick with romaine is temperature, not technique — it germinates best in cool soil and bolts the moment summer heat arrives, so indoor starting is mostly about getting a head start on spring and a second crop in late summer.

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

Romaine Lettuce at a glance

Growing factor What this crop needs
Days to germination 7–10
Sowing method both
Days to maturity 60–70
Soil pH 6.0–7.0
Spacing 8–12 inches
Sun full sun
Cold tolerance hardy

When to start Romaine Lettuce seeds indoors

Start Romaine 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. Romaine is a cool-season crop, so the calendar runs backwards from heat rather than from frost: in Zones 3–5 start indoors 4–6 weeks before your last frost for spring heads, and sow a second batch in late summer for autumn. In Zones 8–9, autumn and winter are the main lettuce seasons. In frost-free Zones 10–13, grow romaine through the cooler months and give it afternoon shade. Lettuce seed germinates poorly above 24°C (75°F), so keep trays cool.

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 8–12-inch spacing. Check old packets with the seed-viability checker.

Step by step: starting Romaine Lettuce seeds indoors

Romaine 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. Romaine 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 8–12 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 Romaine Lettuce once soil is workable. Check old packets with the seed-viability checker.

Light for seedlings

Light separates stocky seedlings from leggy ones. Give Romaine 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 Romaine 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. Romaine 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 8–12 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 Romaine 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
Seeds not germinating Buried too deep or too warm Surface-sow and keep trays below 24°C (75°F).
Leggy seedlings Too little light Move to the brightest spot or under lights, 2–4 inches above.
Seedlings bolting indoors Too warm Move trays to a cooler spot; lettuce wants cool growing conditions.
Damping off Soggy compost, poor airflow Water from below and thin seedlings early for air movement.

Seeds not germinating

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

Leggy seedlings

Too little light is the usual culprit. Move to the brightest spot or under lights, 2–4 inches above. If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Seedlings bolting indoors

Too warm is the usual culprit. Move trays to a cooler spot; lettuce wants cool growing conditions. If badly affected, re-sow — a fresh sowing usually overtakes a struggling seedling within two weeks.

Frequently asked questions

Do romaine lettuce seeds need light to germinate?

Yes — barely cover them, or press them onto the surface and mist. Seed buried deeply often fails to sprout at all.

How long until romaine is ready to pick?

Around 60–70 days from sowing to a full head, but you can pick outer leaves much sooner, and baby leaves in about a month.

Why did my romaine bolt?

Heat. Once daytime temperatures stay high, romaine switches to flowering and turns bitter. Grow it in spring and autumn, and give it afternoon shade in warm zones.

Can I grow romaine indoors all the way to harvest?

Yes, in a bright spot or under lights — it is one of the better crops for indoor growing. But it still needs 60–70 days, so most gardeners start indoors and move plants out.

How far apart should romaine seedlings go?

Thin or transplant to 8–12 inches apart. Crowded plants stay small and airless, which invites rot in the heart.

Related guides