Most varieties of lettuce require cool weather or slight shading for best growth. Grow lettuce in the cool part of the year, when temperatures range in the 50s and 60sF. You can plant lettuce as soon as the ground can be worked in spring. Grow leafy varieties where the weather is warmer.
Lettuce grows well in average, but loose and well-drained soil. Don't crowd lettuce; let it leaf out and grow steadily and quickly for best flavor. Lettuce that is crowded or grown in poor soil will be tough and bitter tasting.
For lettuce growing tips see Lettuce Growing Success Tips at the bottom of this post and How to Grow Lettuce.
Here are common lettuce growing problems with cures and controls:
• Seed planted in mid summer or warm weather fails to germinate. Temperatures are too high. Lettuce seed has a germination rate of 99 percent at 77°F; the germination rate drops to 87 percent 86°F. Use an organic mulch to reduce soil temperature. Plant varieties that tolerate warm soil temperatures: Black Seeded Simpson, Progress, Great Lakes, Imperial 615.
• Seedlings wilt and collapse with dark water-soaked stems as soon as they appear. Damping off is a fungus that lives in the soil, particularly where humidity is high. Do not plant in cold, moist soil. Make sure soil is well drained.
• Seedlings uprooted; leaves torn. Birds pull up seedlings to feed on seed. Cover seedlings with bird block or floating row covers until established.
• Young stems chewed. Young earwigs feed on plant shoots and eat holes in foliage. Most often the damage is tolerable and the infestation is light. Heavy infestation use traps of rolled wet paper or old flowerpots stuffed with paper to catch earwigs at night. Dump them in soapy water. Keep garden free of plant debris. Spray with hot pepper and garlic repellent.
• Leaves are distorted or curled under with small shiny specks. Aphids are tiny, oval, and yellowish to greenish pear-shaped insects that colonize on the undersides of leaves. They leave behind sticky excrement called honeydew which can turn into a black sooty mold. Blast aphids away with water from hose. Use insecticidal soap. Mulch with aluminum foil which will disorient aphids.
• Leaf margins appear scorched and wilted. Leafhoppers are green, brown, or yellow bugs to ⅓-inch long with wedge-shaped wings. They suck the juices from plants. Use insecticidal soap. Cover plants with floating row covers to exclude leafhoppers. Use Sevin, pyrethrum, rotenone.
• Trails of silver slime on leaves; leave eaten. Snails and slugs feed on leaves. Reduce hiding places by keeping garden free of debris. Handpick from under board set in garden as shelter-trap. Use a shallow dish of beer with lip at ground level to attract and drown snails and slugs.
• Small ragged holes eaten in leaves. Cabbage looper is a light green caterpillar with yellow stripes running down the back; loops as it walks. Keep garden clean of weeds and debris where adult brownish night-flying moth can lay eggs. Cover plants with spun polyester to exclude moths. Pick loppers off by hand. Use Bacillus thuringiensis. Dust with Sevin or rotenone.
• Holes in leaves; leaves skeletonized; seedlings eaten. Armyworms or corn earworms:
(1) Armyworms are dark green caterpillars the larvae of a mottled gray moth with a wingspan of 1½ inches. Armyworms mass and eat leaves, stems, and roots of many crops. Handpick caterpillars and destroy. Cultivate after harvest to expose the pupae. Use commercial traps with floral lures.
(2) Corn earworm is a white, green, or red caterpillar with spines to about 1½ inches long. Tomato hornworm is a green caterpillar 3 to 5 inches long with white stripes. These pests will eat holes in leaves and fruit. Handpick and destroy. Spray with Bacillus thuringiensis. Spray with pyrethrum or rotenone.
• Leaf veins are swollen and light yellow; leaves are puckered, ruffled, and brittle. Big vein is a viral disease associated with fine-textured, poorly drained soil. Remove and destroy infected plants. Do not overwater. Keep soil on the dry side. Plant tolerant varieties. Plant when the air temperature is 60°F or greater to lessen the severity of symptoms.
• Leaves faintly mottled; plants are yellow and stunted. Mosaic virus is transmitted by aphids and leafhoppers. There is no control after symptoms occur. Remove infected plants. Control insects. Keep garden free of weeds where insects harbor. Use aluminum mulch to disorient aphids. Plant resistant varieties: Parris Island. Valmaine Cos.
• Plants yellow; dark brown steaks inside stems and larger veins; plant wilts. Fusarium wilt is a soil fungus which infects plant vascular, usually where the soil is warm. If you cut the plant at the base, the stem will be dark reddish brown instead of ivory color. Grow resistant varieties. Rotate crops. Remove and destroy infected plants. Solarize the soil in late spring or summer. Fungicides are not effective.
• Pale yellowish spots develop on upper leaf surfaces; gray-purple powder or mold on leaf undersides. Downy mildew is caused by a fungus. Improve air circulation. Rotate crops. Keep garden free of plant debris. Plant resistant varieties: Arctic King, Big Boston, Salad Bowl, Imperial.
• Sunken, water-soaked spots appear on lower leaves which turn brown and slimy. Rhizoctonia bottom rot is caused by a soilborne fungus. Remove infected plant debris that harbors fungus. Rotate crops. Plant in well-drained area. Solarize the soil in late spring or summer.
• Leaves rot becoming water soaked and turning brownish-black. Bacterial leaf or fungal leaf spot cannot be cured. Plant treated seed. Prune away infected leaves. Keep garden and tools clean. Avoid overhead watering. Plant resistant varieties.
• Stem, lower leaves rot; dense grayish green mold on affected areas. Botrytis gray mold is a fungal disease. Remove and destroy infected plants. Keep weeds out of garden where fungal spores may harbor. Plant in well-drained area.
• Lettuce bolts: flowers and goes to seed before it is ready to eat. Long hot days and warm nights will trigger bolting. Sow lettuce so that it grows and matures in cool weather. Sow lettuce in spring 2 weeks before the last frost or plant in late summer for a fall crop. Plant heat resistant varieties: Great Lakes, Salad Bowl, Slowbolt.
• Leaf tips turn brown and look burned. Sunburn happens when leaves receive full sun in summer. Give plants partial shade in afternoon. Use shade cloth or plant where bed is shaded in afternoon. Plant lettuce in shadows of taller crops.
• Edges of inner head lettuce leaves are brown and rotten; usually not visible form outside of head. Tipburn is a physiological disorder caused by soil calcium deficiency. High temperatures and too much nitrogen can aggravate tipburn injury. Test soil for calcium levels; adjust as necessary. Avoid water stress. Plant lettuce so that it comes to harvest in cool weather.
• Leaves at center of heads are stunted, twisted, narrow, yellowed. Aster yellows is a mycoplasma disease spread by leafhoppers. Remove infected plants. Control leafhopper. Keep the garden free of weeds which can harbor disease. Destroy infected plants. Use Sevin.
• Romaine lettuce does not form heart. Seed was planted too deep; place seed on seedbed and cover with ½ inch of soil.
• Head lettuce does not form head. Plants are crowded. Thin head lettuce to stand from 12 to 14 inches apart.
• Leaves of looseleaf varieties are small and bitter tasting. Plants are crowded. Thin plants to stand from 6 to 10 inches apart.
• Leaf undersides are silver; tiny brown spots on leaves. Smog or air pollution; grow lettuce during cool, clear months.
• Silvery leaves after cold weather. Frost injury; freezing and temperature drops will injure leaves.
Lettuce Growing Success Tips:
Planting. Grow lettuce in full sun where the growing season is cool. In very warm to hot growing regions, grow lettuce in partial shade–between taller crops is good. Lettuce requires a minimum of 4 hours of sun each day. Lettuce will grow in average soil, but soil amended with aged compost that is well-drained is optimal. Sow seed directly or set out transplants.
Planting time. Lettuce can be planted in the garden as early as 4 weeks before the last expected frost in spring. For a continuous supply, plant lettuce every couple of weeks until about 4 weeks before the average daytime temperature exceeds 80°F (after that plant lettuce in the shade or sow heat-resistant varieties). Begin sowing lettuce again towards the end of the growing season, about 8 weeks before the first expected frost in fall.
Care. Lettuce is shallow-rooted and requires consistent, even moisture. Do not let the soil dry out, but avoid keep the surface soil constantly wet. In hot weather or dry conditions, lettuce may require watering every day. Keep growing beds weed free; cultivate shallowly to avoid disturbing lettuce roots.
Harvest. Pick lettuce on a cut-and-come-again basis; pick the outside leaves as soon as they are big enough to eat. For heading lettuce–crisphead and Romaine varieties–cut heads as soon as they are solid and firm.
More growing tips at How to Grow Lettuce.