Growing lettuce

When to plant

Lettuce is a cool-weather crop. It grows best at about 60–70°F ^[3], and other sources give a similar range of about 50–75°F ^[5][9]. In heat it slows down, turns bitter, and sends up a flower stalk. That’s why most gardens get two lettuce seasons, spring and fall, with timing set by your typical frost dates. Your hardiness zone doesn’t set planting dates for lettuce.

Spring. Sow seed outdoors as soon as the soil is dry enough to work, often 2–4 weeks before your typical last spring frost ^[2][6]. Seed can sprout in soil as cool as about 35–40°F, but it comes up faster in warmer soil ^[6][7]. Set out transplants near the typical last spring frost ^[2]. If you grew them indoors, harden them off first by moving them outside a little longer each day over about 5–14 days ^[14]. Try to finish spring sowings at least a month before hot weather usually starts ^[3]. Sowing a short row every 10–14 days keeps a steady supply coming ^[3].

Fall. Count back from your typical first fall frost. Minnesota Extension suggests sowing about three months before it ^[1]. Utah State times fall crops to mature a week or two before it ^[2]. Late-summer soil is the main hurdle. Lettuce sprouts poorly when soil is above about 80°F ^[2] and not at all above about 95°F ^[5]. Starting seeds indoors in midsummer and setting out transplants later is one way around it ^[1].

Through the season

Sowing and sprouting. Plant seed about 1/4 to 1/2 inch deep ^[1][2]. Seedlings usually show in about 4–10 days ^[6], and more slowly in cold soil.

Seedlings. Thin when seedlings are about 1–2 inches tall ^[5] or have 3–4 true leaves ^[2]. How far apart depends on the type and how big you want the plants ^[1][3]:

• Leaf lettuce: about 4 inches, or about 5 inches if you’ll pick it as baby leaves. Give plants you want full size 10–12 inches.
• Romaine and butterhead: about 6–8 inches.
• Crisphead (iceberg): about 10–12 inches.

Sources don’t fully agree. Utah State suggests 8–12 inches for all types ^[2], while Maryland gives 4–10 inches for leaf, romaine, and butterhead ^[4]. Closer spacing works if you’ll harvest young. The thinnings make good baby greens ^[10].

Growing. Lettuce wants steady moisture. About 1 inch of water a week is enough in Minnesota ^[1]; Utah State suggests 1–2 inches ^[2]. Moisture matters most while young plants get established ^[7]. Uneven watering, especially a dry spell followed by fast regrowth, can brown the leaf edges (tipburn) ^[1][10]. Too much water just before heads mature can make them soft ^[7]. Pots dry out much faster than beds, so check them often in warm weather. Water when the top inch is dry, until water runs out the bottom ^[16].

Lettuce likes full sun in cool weather but tolerates part shade, about 4–6 hours of direct sun ^[4]. Once it’s hot, afternoon shade helps ^[9].

Bolting. When the center of a plant starts to stretch upward into a thick stalk, it’s bolting. Harvest it right away (see Harvest).

Weather that matters

Cold. Lettuce handles light frost well. Sources put the point where leaves may be damaged at about 28–32°F ^[11], the upper 20s°F ^[12], or about 26–31°F for a short time, often with some leaf damage ^[13]. Young plants take 32°F without serious harm ^[2]. Transplants fresh from indoors are more tender. While you’re hardening them off, bring them in if a freeze is expected ^[14]. A light row cover adds about 4°F of protection ^[5]. Cold-hardened fall lettuce in western Oregon can survive about 20°F ^[15], while Florida guidance is more cautious and suggests covering below about 40°F ^[9].

Heat. Heat is usually the bigger problem. Sources disagree on the exact point. Several days above about 75°F can make lettuce flower ^[1]. Above about 80°F, plants start seed stalks and turn bitter ^[7]. Around 85°F, bolting and bitterness are likely ^[8]. Longer days add to the push to bolt ^[4]. Before a hot spell, pick the larger leaves first. In a Delaware trial, 30% shade cloth made lettuce less bitter. It didn’t reliably prevent bolting, and in some heat-sensitive varieties it made bolting more likely ^[8]. For late-spring sowings, choose varieties sold as heat tolerant or slow to bolt ^[10].

Dry spells. Lettuce is very sensitive to dry soil ^[10]. After a dry stretch, push a finger into the soil and water if the top inch is dry ^[16]. Check containers first.

Heavy rain. Soggy soil favors disease ^[9]. Rot of the lower leaves is worse when leaves stay wet, and hard rain can bruise tender leaves ^[10].

Harvest

Lettuce can be picked as soon as leaves are big enough to use ^[10]. Most types take about 40–80 days from seed ^[4]. Leaf types are often ready in 50–60 days, while head and romaine types take about 65–85 ^[10].

• Leaf lettuce: Start picking outer leaves once plants are about 5–6 inches tall ^[4][5]. Plants cut back at 6–10 inches tall often regrow in 2–3 weeks ^[4].
• Butterhead (Bibb): Harvest when the inner leaves cup together into a loose head ^[5].
• Romaine: Harvest at about 6–8 inches tall ^[4][5].
• Any type that starts to bolt: Harvest the whole plant before bitterness sets in ^[10].

Chill lettuce soon after picking. It keeps about 1–2 weeks in the refrigerator ^[2]. Storage near 32°F with high humidity is ideal ^[3][4].

Common problems

Bolting and bitter leaves

What it looks like: The plant stretches upward, a thick stalk forms in the center, and the leaves taste bitter.

Likely causes: Hot weather ^[1][7][8] and long days ^[4].

What to do: Harvest right away ^[10]. Next time, sow earlier in spring or wait for fall, choose slow-bolting varieties, and give afternoon shade in warm spells ^[9][10]. Crisphead and butterhead types tend to bolt later than leaf and romaine types ^[20].

When to ask an expert: If lettuce bolts while the weather is still cool, ask your county extension office which varieties do well in your area.

Tipburn (brown leaf edges)

What it looks like: The tips and edges of leaves turn brown and dry ^[1][10].

Likely causes: Heat combined with dry soil, regrowth after a dry spell, or very fast growth from heavy feeding. In strongly acidic soil, low calcium can play a part ^[1][10].

What to do: Keep soil evenly moist, go easy on fertilizer, and try varieties that resist tipburn ^[10]. Trim off the brown edges before eating.

When to ask an expert: If tipburn keeps coming back even with steady watering, a soil test through your extension office can check pH and calcium ^[10].

Seeds didn’t come up

What it looks like: Few or no seedlings two weeks after sowing.

Likely causes: Soil that is too warm. Lettuce sprouts poorly above about 80°F ^[2] and not at all above about 95°F ^[5]. Seed sown too deep or a seedbed that dried out can also cause gaps.

What to do: Sow about 1/4 to 1/2 inch deep and keep the seedbed moist ^[1][2][10]. In late summer, wait for cooler soil or start seeds indoors and transplant ^[1].

When to ask an expert: If fresh seed keeps failing in cool, moist soil, your extension office can help you find the cause.

Slugs and snails

What it looks like: Ragged holes chewed in the leaves.

Likely causes: Slugs and snails, a common lettuce pest ^[5][10].

What to do: Remove boards, debris, and other hiding spots, pick them off by hand, and use baits labeled for vegetable gardens if needed ^[10].

When to ask an expert: If damage keeps spreading after a week or two of cleanup and hand-picking.

Rotting lower leaves

What it looks like: The lower leaves turn slimy and rot. The rot moves into the stem and can take the whole plant ^[10].

Likely causes: Soil fungi that cause diseases called drop and bottom rot, helped along by wet leaves and soggy soil ^[5][9][10].

What to do: Water in the morning and at soil level, keep leaves as dry as you can, and avoid overfeeding, which makes soft growth ^[10]. Pull badly rotted plants.

When to ask an expert: If many plants collapse, your extension office can identify the disease from a sample.

Aphids

What it looks like: Clusters of tiny, soft-bodied insects on the leaves.

Likely causes: Aphids, which are among the insects that feed on lettuce ^[5][9][10].

What to do: In home gardens they rarely need treatment. If they build up, insecticidal soap works on soft-bodied insects. Follow the label ^[10].

When to ask an expert: If plants are stunted or leaves curl and yellow despite treatment.

Related plants

Endive, escarole, and radicchio are grown much like lettuce and like the same cool weather ^[1]. Spinach is another cool-season green for spring and fall ^[6], and arugula fits the same beds.