How to Grow Lettuce in a Window Box Year Round

You can grow lettuce in a window box year round if you sow every two weeks and keep the soil cool. Cut outer leaves, avoid hot noon sun, and one box can feed salads all season.

Quick answer

You can grow lettuce in window box culture if the trough is at least 6 to 8 inches deep and 24 to 36 inches long, filled with potting mix, not garden soil. Sow a pinch of looseleaf seed every 14 days. Cut outer leaves and leave the crown so the plant keeps growing. Lettuce bolts when days sit above 75°F to 80°F, so give afternoon shade in summer and use a south or west window in winter. Black Seeded Simpson, oakleaf, and buttercrunch are the types that earn their keep in a box.

Build the box to the crop, not the other way around

If you grow lettuce in window box troughs on a sill or rail, depth is the first measurement. A box 6 to 8 inches deep holds enough mix for lettuce roots, which run 4 to 6 inches. A 4-inch-deep decorative box dries by noon and turns bitter. Length of 24 to 36 inches gives you a real salad row.

Drainage holes every 4 to 6 inches along the bottom are required. Set a saucer under it so dirty water does not stain the building. A wet 36-inch box 8 inches deep can weigh 25 to 40 pounds. If you rent, use a floor box on a caddy instead of drilling into masonry.

Fill with potting mix. Budget about 10 to 16 quarts for a 24- to 36-inch box at 6 to 8 inches of depth, leaving 0.5 inch of headspace. Garden soil crusts and puddles on a sill.

Sun: 4 to 6 hours is plenty. Morning sun and afternoon shade is the summer pattern that delays bolting. In winter, a south or west window keeps growth moving. A light frost at 32°F nips baby leaves; a hard freeze at 25°F can kill a shallow box overnight. Move it inside for those nights.

This is not your problem if

• You already have a shady in-ground bed and can sow lettuce in rows 12 inches apart. You do not need a box.
• Your only window is a dark north well with no sky, and you will not add a simple LED bar.
• You want heading iceberg the size of a grocery ball from a 6-inch-deep box. Grow looseleaf instead.
• Your HOA or lease bans all railing boxes and you cannot place a 24-inch trough on the floor of the balcony.

Sow, cut, and sow again

Looseleaf types are the right tool: Black Seeded Simpson, oakleaf, and buttercrunch. Skip large crisphead types. They want 10 to 12 inches of depth and a long cool season you may not have on a sill.

Sow every 14 days. Sprinkle seed 0.25 inch deep, 1 inch apart, in two staggered rows if the box is 8 inches wide. Thin to 4 inches apart for leaf types, 6 inches for buttercrunch. Eat the thinnings.

Cut-and-come-again means you harvest outer leaves with scissors 1 inch above the mix, and you leave the growing point. A plant started in week 1 can give leaves in 30 days and keep giving for 3 to 4 weeks if temperatures stay under 75°F. Then it turns bitter or bolts, and your 14-day succession sowing is already up.

Yield from one 36-inch box: at peak you can cut a salad bowl every 3 to 4 days. That is one to two people, not a household of five. Two boxes, staggered by 14 days, smooth the gaps.

Bolting starts when daytime temps hold above 75°F to 80°F. Give afternoon shade with shade cloth, a taller pot on the west side, or by sliding the box off a hot rail. Water in the morning so leaves are dry by night.

Apartment constraints: do not let mix drop on a shared walkway. Empty the saucer after watering. In winter, a south window can replace the balcony for 8 to 12 weeks. Keep the box 2 inches off cold glass at night.

Water, heat, and a 14-day habit

Check the top inch of mix every morning. In 70°F weather a 6-inch-deep box may need water every 1 to 2 days. In 85°F wind, water every morning and add a splash at 5 p.m. Water the mix, not the leaves.

Feed lightly. A half-strength liquid fertilizer every 14 days is enough. If leaves pale after 3 weeks of cuts, feed once, then resume.

Pests are usually aphids and slugs. Rinse aphids in the morning. Lift the box once a week and pick off slugs.

HOA notes: a matching 24- to 36-inch box with a saucer looks like a flower trough if you tuck nasturtiums at the ends. Keep height under 12 inches.

Container vs in-ground

In-ground lettuce has a bigger cold buffer. A 6-inch box freezes solid at 25°F; garden soil 8 inches down may stay in the 30s. Move the box on hard-freeze nights.

In summer you can slide the box 3 feet into afternoon shade. In-ground rows cook unless you planted them east of a taller crop. Mobility is the point.

You will not grow 1-pound icebergs. You will grow 4- to 6-inch leaves on a 14-day cycle. Yield per square foot can beat a garden row because you never leave a gap empty.

A wet 36-inch box at 40 pounds is still less than one 5-gallon tomato bucket. Put it on the wall side of the balcony, not on a weak railing cap.

Myths that waste time

Myth: lettuce needs full all-day sun. More than 6 hours of hot sun in July is how you get bitter, bolted plants. Four to six hours with afternoon shade is the target.

Myth: you cut the whole head once and you are done. In a box, cut outer leaves and sow every 14 days.

Myth: garden soil is richer so the box will be stronger. Garden soil in a 6-inch trough becomes mud, then brick. Use potting mix.

Myth: you can keep one sowing from April to October. Above 75°F to 80°F it bolts. Succession sowing is the method.

Myth: any window is a winter greenhouse. A north interior window without added light gives pale seedlings. Use a south or west sill, or wait for March.

Do this today / this week

1. Measure your box or buy one: 6 to 8 inches deep, 24 to 36 inches long, with drainage holes.
2. Fill with 10 to 16 quarts of potting mix and set a saucer underneath.
3. Sow Black Seeded Simpson, oakleaf, or buttercrunch 0.25 inch deep, 1 inch apart.
4. Write the next sowing date on tape: 14 days from today.
5. Place the box where it gets morning sun and afternoon shade in the warm season.
6. If you rent, skip railing screws. Use a floor trough on a caddy rated for 50 pounds.

Frequently asked questions

Can I grow lettuce in window box trays that are only 4 inches deep?

You can start seedlings there, but plants dry out and stay bitter. Step up to 6 to 8 inches of depth. Roots want 4 to 6 inches of mix plus a buffer so a workday does not cook them.

How many plants fit in a 36-inch box?

At 4-inch spacing you can hold about 16 to 18 looseleaf plants in two staggered rows. Buttercrunch at 6-inch spacing holds about 10 to 12. Crowding past that invites rot.

What temperature makes lettuce bolt?

Daytime highs that stay above 75°F to 80°F. Night temps above 70°F speed it up. Shade the box from noon to 4 p.m. and keep the mix moist, not soggy. Sow oakleaf in summer; save buttercrunch for spring and fall.

Can I grow lettuce indoors all winter without a grow light?

On a south or west window, yes, slowly. Expect 40 to 50 days to a small harvest instead of 30. A north room without sky will not do it. Pull the box 2 inches back from cold glass at night.

How often should I replace the mix?

After two or three sowing cycles, blend in 2 to 3 quarts of fresh potting mix and pull old roots. Replace the whole fill once a year, usually in September before the fall sowing push.

Is buttercrunch still a cut-and-come-again crop?

Yes, if you take outer leaves and leave the crown. If you wait for a tight 6-inch head and cut the whole plant, that sowing is over. In a window box, the outer-leaf method gives you more salads per 14-day cycle.

Key takeaways

• Box size: 6 to 8 inches deep, 24 to 36 inches long; about 10 to 16 quarts of potting mix.
• Sow every 14 days; cut outer leaves and leave the crown.
• Bolt risk starts above 75°F to 80°F — give afternoon shade in summer.
• Use looseleaf types: Black Seeded Simpson, oakleaf, and buttercrunch.
• A wet 36-inch box can weigh 25 to 40 pounds; use a saucer and a rated bracket or a floor caddy.

Related reading

• Best Herbs for a North Facing Balcony (Low Light)
• Best Vegetables to Grow in 5 Gallon Buckets
• How to Water Container Vegetables While on Vacation