How to Grow Romaine Lettuce Indoors

Mix rich, slightly acidic potting soil with perlite -- 7 parts of soil to 3 parts of perlite -- so your planting mix holds water but also drains well and allows air to enter the soil.

Separate lids from the bottoms of each clamshell container. Cut five to eight, 1-inch-long gashes in the bottom of each lettuce clamshell. Place the lid of every clamshell beneath it as a drain tray.

Fill each container with 3 or 4 inches of soil mix and place containers in the sink. Thoroughly water the soil and let all moisture drain away.

Loosely scatter romaine lettuce over the surface of each container, so you spread about 25 seeds per lettuce box. Barely cover the seeds with potting soil and mist the surface to moisten the seeds.

Place the clear cover on each clamshell, and locate “lettuce boxes” in a bright south- or west-facing window, if available. Use grow lights 6 to 8 inches above containers also, so lettuce seeds and seedlings receive 12 hours of bright direct light per day.

Remove the container lids and place them beneath each box – as drain trays – after five or six days, when the romaine seeds have sprouted. Water lightly every day, to keep potting soil moist.

Fertilize with half-strength liquid fish or complete water-soluble fertilizer once a week as seedlings develop into vertical “heads” of lettuce. Thin seedlings to stands 6 inches apart as they grow and start to crowd each other – using thinned plants in salads.

Harvest romaine lettuce when heads are mature but still tender, in about 60 to 70 days. Plants may grow to 10 inches. Cut off the plant with a sharp knife about 1 inch above the potting soil. Roots may grow new leaves, for a second harvest.