Family: Asteraceae
Genus and Species: Lactuca sativa
Climate
Lettuce grows in many climates given the right soil and season conditions. Lettuces generally like to grow in partial to full shade. Lettuce is best when it is grown quickly and pampered a bit with good soil and adequate moisture so it doesn't bolt (send up a flowering stalk) or become bitter and tough.
In warmer climates grow lettuce varieties that are heat tolerant. During the summer and grow cool season lettuces in the fall, winter, and early spring.
In cool climates, grow warm season cultivars in the summer and cool season lettuces in the early spring and fall- you can even sow seeds in late fall for an early spring crop.
Soil
Lettuce likes a partly sunny to shady spot with soil rich in humus that retains moisture. Lettuce grows best in temperatures ranging from 55°-65°. The optimum pH is 6.5 to 6.8.
Spacing
Spacing for lettuce depends somewhat on the kind of lettuce planted. If you are planning to harvest the entire head of lettuce then give the plants more room. For example, lettuce grown to produce heads should be given 8"-12" apart in all directions. Lettuce that will be harvested leaf by leaf should be planted much closer, with ½" between the starts. Spacing can even be random and tight if you intend to harvest the lettuce very young.
Lettuce is pretty forgiving and great crop to use for interplanting - planting in and among other vegetables. Give the main crop room to grow and allow enough space for good air circulation.
Direct Seeding
Lettuce is a good crop for direct seeding. Make sure the bed is prepared well and the soil is moist. Lettuce germinates best in cool soil (40°-60°) and becomes temporarily dormant if it is too hot.
Sow lettuce seeds 1/2" deep in rows 1-1/2" apart. Lettuce plants have a shallow, compact root system. Make sure there are enough nutrients available by mixing in compost before sowing the seeds.
Broadcast lettuce seeds over the bed and rake lightly so they are covered with a very thin layer of topsoil for harvest as young lettuce.
Seeding For Transplants
Lettuce can be started indoors for early planting in the spring or for succession planting. Start lettuce seeds in trays with individual cells. Start them under lights if available as lettuce seeds need light to germinate. Be careful not to cover the seeds with soil, gently pressing the seeds into the moist starting mix is enough to ensure good germination. For best results, start seeds one month before planting out.
Try to start successive batches of lettuce instead of starting a whole tray or seed package. You will be able to use more lettuce if you have a continuous supply rather than a huge crop ready all at once.
Germination
These seeds germinate best in soils around 40°F-60°F.
Germination will take 7-14 days.
Transplanting Into the Garden
Transplant lettuce to the garden when there are at least 4 true leaves on the starts. Make sure the soil is moist before planting.
Watering
Lettuce does not need a lot of water but it does need to be continuously moist. It is important to make sure your lettuce bed does not dry out as this will cause the lettuce to bolt and become bitter. Growing lettuce in a semi-shaded to shaded location and using a straw mulch around the plants helps retain moisture. Water lettuce with a watering wand and concentrate the water at the base of the plant, not on the leaves. Watering the leaves encourages diseases and may damage some varieties of the more delicate lettuces.
Harvesting
Lettuce grows quickly and is ready before flowers appear. The trick to harvesting lettuce is picking it before it is bitter. If flowering stalks appears, the plant is past its prime. Lettuce can be harvested as soon as true leaves appear but let the lettuce plants develop enough so that harvesting 3-4 leaves from the outside of the rosette of each plant will not harm the plant's growth. If you are harvesting the whole lettuce head, wait until it is bigger than the size of your fist and harvest the lettuce before it becomes bitter. If in doubt, try a leaf!
Harvest either the largest, outside leaves or the whole plant. If harvesting tender young lettuce that is tightly spaced use a pair of scissors to cut the lettuce above the soil line.
Post-Harvest Handling
Clean the lettuce of dirt and cool using hydro cooling. Hydro cooling is the process of spraying or immersing vegetables in chilled water.
Storage
Clean, dry lettuce lasts 3-5 days in optimum conditions though lettuce is best eaten fresh. Lettuce can be stored for 2-3 weeks at 32° and 98%-100% relative humidity.
Lettuce is very sensitive to ethylene gas so do not store lettuce with vegetables and fruits that give off ethylene gas such as apples and pears.
Diseases
Big Vein, Damping-Off, Downy Mildew, Mosaic Virus, Nematodes, Sclerotinia Drop, Soft Rot, and Tip Burn
Pests
Bulb Mites, Cutworms, Darkling Beetles, Field Cricket, Garden Symphylans, Leafminers, Springtails, Armyworm, Beet Armyworm, Corn Earworm and Tobacco Budworm, Loopers, Saltmarsh Caterpillar, Foxglove Aphid, Green Peach and Potato Aphids, Lettuce Aphid, Lettuce Root Aphid, Silverleaf Whitefly, and Slugs
Comments
The key to growing lettuce is starting successive batches a week or two apart to ensure that you have a continuous supply throughout the growing season.
An insecticidal soap or handpicking usually alleviates the majority of lettuce pests. Use a beer trap for the slugs. A beer trap is a shallow dish placed level with the soil and filled with beer.
References
Bradley, F. M. and Ellis, B. W.(Ed.). (1992), Rodale's All-New Encyclopedia of Organic Gardening: The Indispensable Resource for Every Gardener, Emmaus, Pennsylvania: Rodale Press.
Johnny's Selected Seeds (2002), "Lettuce", Johnny's Selected Seeds.
Oregon State University, Lettuce , Commercial Vegetable Production Guides, Last modified 1999-01-02, http://www.oregonstate.edu/Dept/NWREC/lettuce.html, Accessed 2003-5-11
Smith, E.C. (2000), The vegetable gardener's bible: discover Ed's high yield W-O-R-D system for all North American gardening regions., Storey Books: Pownal, VT.