Lettuce

Lactuca sativa

Quick Facts

Plant family: Asteraceae (Aster or Sunflower)
Season: Cool
Life Cycle: Annual
Seed to first harvest: 30-60 days

History

Asteraceae-plant-family — Figure 1. Lettuce belongs to the Asteraceae plant family.

Lettuce is a member of the Asteraceae family, also known as the aster or sunflower family, which includes crops such as globe artichoke and endive, but also garden ornamentals like sunflower, dahlia, marigold, and zinnia (Figure 1).

Cultivated lettuce likely is derived from wild lettuce from the Mediterranean area of the Middle East and was first recorded in ancient Egypt around 2500 B.C. in tomb paintings but may have existed as early as 4500 B.C. This original lettuce had two main types: one with much thicker stems (like celery) and another that produced seeds used for cooking oil. The stem lettuces (similar to present-day romaine lettuce) were documented in China between the 5^th and 7^th centuries. Non-heading lettuces (similar to leaf lettuce) were common in the Mediterranean area in red and green leaf forms. Around the 15^th century, new butterhead and crisphead lettuce varieties moved into Western Europe and were likely brought to North America by Christopher Columbus in 1494. By the 1600s romaine lettuce was very common in Italy, and by the 1700s it was cultivated in the Americas. See Figure 2.

Map-showing-the-origin-and-migration-of-lettuce-to-the-U.S. — Figure 2. Map showing the origin and migration of lettuce to the U.S.

In the U.S. until about 1970, the crisphead (or iceberg) type was the most popular lettuce and dominated the market. After 1970, butterhead, romaine, and leaf lettuce types began to gain in popularity. Along with these market changes came the salad bar, where mixed leaf lettuce became the more practical type, along with the popular Caesar salad, which uses romaine lettuce. During this same time period, processed and value-added lettuce became the market standard, such as bagged, shredded salad mixes containing toppings and dressing.

Varieties

Lettuce is a fast and easy-to-grow cool season crop generally grouped into four main types: (1) butterhead, (2) crisphead, (3) leaf (red or green), and (4) romaine/cos (see Figure 3). These types vary in appearance, taste, and resistance to disease and weather extremes.

Butterhead lettuce, also known as Boston or Bibb lettuce, forms a loose head with sweet, buttery, and tender leaves. Butterhead lettuce is considered more delicate than romaine and much tastier than crisphead.

Crisphead lettuce, also referred to as iceberg lettuce, forms a large, dense head (similar in shape to cabbage) with crisp, crunchy leaves and ribs. Crisphead lettuce is not heat tolerant and therefore is most prone to bolting. Therefore, due to this intolerance to heat and lengthy harvest time (60-85 days), crisphead is not recommended to plant in Louisiana. There are a few moderately heat-tolerant varieties (Crispino, Great Lakes, and Ithaca), but planting should be restricted to the coolest recommended months of planting (e.g., January and October) for the highest chance of success.

Leaf lettuce is a loose-leaf lettuce that grows in a rosette and varies in leaf shape, texture, and color (green, red, and even freckled hues). Leaf lettuce is commonly grown as baby lettuce, which can be harvested from seed in as little as 28 days. A new innovation in leaf lettuce is Salanova, which is a mix of several varieties selected for productivity, uniformity, extended storage ability, and excellent leaf flavor and texture. See this video for an overview of growing and preparing Salanova leaf lettuce mix.

Romaine lettuce, also known as cos lettuce, has a tall, dense head with crispy leaves and a tender heart. Romaine lettuce is less prone to bolting (more heat tolerant) than crisphead or butterhead.

Main-types-of-lettuce — Figure 3. The main types of lettuce (left to right, top to bottom): butterhead, crisphead, leaf, and romaine.

Lettuce has either open-pollinated (including heirloom) or hybrid varieties. Some lettuces are heirloom varieties, like Oak Leaf, Black Seeded Simpson, and Rouge D’Hiver, meaning the seeds have been saved for at least 50 years, can be saved each season and replanted, and are open-pollinated. Lettuce produces perfect (both male and female flower parts), self-pollinating flowers, but a small amount of natural cross-pollination can occur when two varieties are grown side by side. If saving seed, different varieties should be separated by a distance of 10-20 feet to avoid cross-pollination. Generally, it is not recommended to save seed for future planting with hybrid varieties as they are usually not expressed properly in the next generation.

When selecting lettuce varieties for Louisiana’s warm climate, look for resistance to bolting (e.g., heat tolerance/resistance), which causes bitter lettuce, and opt for varieties with shorter days to harvest.

See the recommended lettuce varieties for Louisiana in Table 1.

When and How to Plant

Lettuce is an annual (with a life cycle of one season/year), cool season crop and can be grown in Louisiana during the cooler spring, fall, and winter months. Lettuce is best seeded in late winter or early fall as plants thrive in temperatures during this time. If the soil temperature is at least 40 degrees F, lettuce may be direct seeded outside; however, starting seeds indoors may speed plant growth.

Starting seeds inside: Begin seeding indoors approximately 4 weeks before the first planting date (refer to the Lettuce Planting Guide, Table 2). Using seed germination trays (with at least 1.5-inch diameter cells), plant 1 seed per cell (unless germination rate is low or conditions are less than ideal; then plant 2-3 seeds per cell) at a very shallow depth (1/8 inch), and lightly cover with a thin layer of soilless potting mix. Make sure to keep the seed trays in a well-lit area around 75 degrees F. Keep the soil moist, which usually requires daily light watering. A seedling heat mat and plastic dome lid are helpful in maintaining ideal germination conditions. If multiple seeds were planted per cell, thin seedlings to one plant per cell once a few true leaves develop. Lettuce is generally very transplant hardy, and seedlings can be transplanted outside approximately 2 weeks later or when 4-6 true leaves develop, if soil temperature is above 40 degrees F. A few days before planting transplants outside, it is recommended to follow a hardening-off process to transition seedlings to outdoor conditions.

Direct seeding or transplanting outside: The optimum temperature to direct seed outside or transplant outside is a soil temperature of 40-85 degrees F (optimum 75 degrees F). The use of a soil temperature map can help guide planting decisions. Either follow the recommended plant spacing when transplanting (refer to the Lettuce Planting Guide, Table 2) or scatter seeds (broadcast) in a 2-inch-wide furrow or line that is very shallow (1/8-1/4 inch deep) when direct seeding. Lightly cover with soil and water in. Some of the seeds will actually be uncovered and visible. Thin to the recommended plant spacing after true leaves develop, if needed. Adequate irrigation is needed to ensure coolness, moisture, and germination success. Since lettuce grows so quickly, multiple successions can be planted within the recommended planting dates. For a steady supply of lettuce, seed every 2-3 weeks.

The optimum temperature for lettuce growth and development is 60-65 degrees F with a minimum of 45 degrees F and maximum of 75 degrees F. Properly hardened lettuce transplants can tolerate temperatures as low as 20-25 degrees F.

Table 2. LETTUCE PLANTING GUIDE

Category	Planting Outside Dates	Spacing (inches)		Days to Harvest*
		Plants	Rows
Head (Butter, Crisp)	North LA: Jan-Feb; Sept-Oct South LA: Sept-Feb	10-15”	12-18”	60-80 days (45-60 days)
Leaf	North LA: Jan-Feb; Sept-Oct South LA: Sept-Feb	4-8"	12-18”	60-80 days (45-60 days)
Romaine	North LA: Jan-Feb; Sept-Oct South LA: Sept-Feb	10-14"	12-18”	60-80 days (45-60 days)

*Days from seed to harvest; days in parentheses are transplant to harvest
Note: Table adapted from LSU AgCenter, UF Extension Planting Guides and Southeastern U.S. Vegetable Production Handbook

Where to Plant

Lettuce requires well-drained soil and prefers a sandy loam soil with a pH between 6.0 and 6.8. Lettuce fairs well in a wide range of soil types — sandy to clay — but will not tolerate acidic soil. Lettuces prefer full sun (6 hours/day) but some varieties may tolerate partial shade. It is recommended to plant lettuce in box beds or in traditional raised garden rows that are about 12 inches tall to ensure good drainage. In all types of gardens, it is recommended to add a layer of compost, peat moss, rotted hay, or other organic matter and mix into the soil to optimize plant health. This is especially important for lettuce, which thrives on soils with a high level of organic matter.

Reflective plastic mulch — or a plastic fabric/film — is recommended to deter aphids that transmit viruses, to increase soil temperature, and to control weeds. Drip irrigation is also recommended when using plastic mulch to maintain ideal soil moisture and to encourage productive plants.

Lettuce tends to bolt (go to seed), causing very bitter-tasting lettuce, when temperatures are above 80-85 degrees F for several days and/or the days are longer. Consider applying mulch, using shade cloth, and/or intercropping lettuce with taller vegetables to provide shade to keep the soil cool and prevent bolting.

Each season, rotate plant families — avoid planting crops from the same plant family in the same area of the garden — to reduce disease and pests.

Plant Care

Watering

Lettuce needs regular, light watering due to a shallow root system — usually several times a week or daily in warmer temperatures. If the leaves look wilted, increase watering frequency and duration. Avoid inefficient watering during the heat of the day as water will evaporate before it reaches the roots. Take care not to overwater as this may cause tipburn and other diseases.

Fertilization

Do not overfertilize lettuce. Fast growth caused by nitrogen can lead to tipburn, the most common defect of lettuce. Tipburn is caused by calcium being unable to move through the leaf quickly enough towards the end of the crop, when heads are large and nearly ready to harvest. Although it is a calcium deficiency, it is more related to how quickly calcium is able to move through leaves that are expanding too fast, rather than to calcium levels in the soil. To avoid overly fast growth, avoid adding more than the recommended amount of nitrogen or use a slower-release organic form. Inadequate potassium may interfere with proper head formation. Soil testing may be helpful when growing head lettuce.

Organic fertilizers such as compost, fish emulsion, composted poultry litter or manure, worm castings, and blood or bone meal originate from living organisms. They are far more environmentally sustainable and safe than traditional synthetic fertilizers. They naturally release nutrients more slowly and over a longer period of time. When applying organic fertilizer, it is important to use in unison with compost, cover crops, and crop rotation, which all work together to build soil health. Learn how to convert inorganic fertilizer recommendations to organic fertilizers here.

Alternatively, a synthetic fertilizer may be used at a rate of about 1.5 pounds (3 cups) of 13-13-13 for every 25 feet of row or 75 square feet. Broadcast, or sprinkle evenly, over the soil and then mix in about 3-6 inches deep using a rake. Supplemental sidedressing, or reapplication of synthetic fertilizer, is recommended 3-4 weeks after transplanting. Sidedressing is the addition of fertilizer to the soil around already established plants when the plant begins to fruit or vine, primarily to provide nitrogen. When using synthetic fertilizer, sprinkle lightly around each plant or along the side of the row, keeping it about 6 inches away from the plant stem; water into the soil. Additional sidedressing may be applied every 3-4 weeks. Because of their slow, steady release of nitrogen, crops fertilized with organic fertilizer do not usually need to be sidedressed, but fish emulsion, a quick-release form of organic nitrogen, may be used if needed.

Weeds

Plastic mulch will control most of the weeds; hand-pull weeds close to the plant, especially in the planting holes. It is especially important to control weeds since the shallow roots compete directly with weeds; take care not to damage the plant when weeding.

Insect Pests and Diseases

Common insect pests for lettuce are aphids and cabbage worms. Regular monitoring can help to identify symptoms of these insect pests and allow for early treatment and further prevention. Lettuce is fairly susceptible to diseases such as damping off, downy mildew, lettuce drop, bottom rot, lettuce mosaic virus, and tipburn. Many varieties of lettuce are resistant to specific diseases and these should be selected and planted. Refer to Table 3 to aid in diagnosis and management of some common lettuce insect pests and diseases.

Table 3. Organic and Natural Management for Common Lettuce Insect Pests and Diseases

Symptoms	Diagnosis	Organic and Natural Pest Management
• Curled and yellowed leaves • Stunted crops • Sticky honeydew on leaves	Aphids	Timely planting and harvest Reduce water stress Weed control Use water jet to dislodge Reflective mulches; insect barrier fabric Beneficial insects: lady bugs, lacewings, predatory stink bugs, syrphid flies Insecticidal soap, neem oil, pyrethrin, Azera
• Seedling rots and suddenly dies (before or after germination) • Cool and wet weather conditions	Damping off	Plant fungicide-treated seed Increase plant spacing
• Damp, cool conditions • Small, yellowing angular patches on leaves • Damping off	Downy mildew	Crop rotation (2+ years) Plant resistant varieties Reduce leaf moisture by improving air circulation; morning irrigation Remove crop debris and weeds Use organic/natural fungicides
• Slimy or rotted leaves at the base of plant	Lettuce drop and bottom rot	Mulch Manage soil fertility Drip irrigation; morning watering
• Stunted or deformed plants • Mosaic or mottling pattern • Incomplete heads	Lettuce mosaic virus (LMV)	Plant resistant varieties Monitor and treat for aphids Remove infected plants and weeds Mineral oil or rapid-acting insecticide spray
• Edges of leaves turn brown or speckle	Tipburn	Plant resistant varieties Avoid over-fertilizing Maintain uniform soil moisture

Note: Table adapted from LSU AgCenter; University of Illinois Extension; Alabama A&M and Auburn Universities Extension; and UMass Extension Vegetable Program. The Louisiana Pesticide Law regulates the use of pesticides in schools to protect children and staff from harmful exposure to chemicals and is enforced by LDAF. The recommended alternative to routine pesticide use is Integrated Pest Management (IPM), which combines pest control, disease management techniques and organic/natural alternatives, many of which are found in this table.

Harvest and Storage

Harvest lettuce when heads reach desired size (baby or full size), though it’s best to harvest just before full maturity. There are three main methods to harvest lettuce: (1) single cut, (2) cut and come again, or (3) outer leaf harvest.

For butterhead, crisphead, and romaine lettuces, the single cut method (i.e., harvesting a head of lettuce only once) is most used since the entire head is harvested (rather than loose leaves). For this method, use a knife or shears to cut the root right below or at the soil level to keep all the leaves attached to the root. Discard any bad outer leaves from the head.

For leaf lettuces, the cut and come again method is a great technique to potentially allow for more than one harvest of a single lettuce plant. For this method, cut the leaves a few inches above the crown (where the leaves begin to grow off the stem). When done correctly and under good growing conditions, the new leaves should regrow from the crown in about two weeks. The outer leaf harvest method can also be used for romaine and leaf lettuces, which is simply removing the outer leaves when harvesting and leaving the center leaves and growing point to continue to grow.

Here are some helpful videos showing the three harvesting methods:

Single cut and outer leaf harvest methods
Cut and come again harvest method

Lettuce is extremely perishable, especially in warm temperatures. It is recommended to harvest in the early morning when temperatures are lowest, and immediately store in the refrigerator (without washing) until use. After harvest, the crop needs to be cooled down to remove field heat and placed in a refrigerator or cooler where there is high humidity. Removing field heat will help prevent moisture loss and wilting and will preserve quality and shelf life.

Lettuce stores best in a loose plastic bag to retain moisture. At an ideal storage condition of 32 degrees F with high (98-100%) humidity, lettuce will store for 2-3 weeks postharvest. Avoid storing lettuce with fruits that release ethylene gas (a natural ripening agent) as this decreases shelf life; examples include apples, bananas, and pears.

When ready to use the lettuce, wash with cold water and dry in a salad spinner or with a towel. If the lettuce appears wilted, place the head or leaves in a cold ice water bath for about 15 minutes to revive. Avoid cutting/tearing the lettuce until just before serving. Lettuce cannot be preserved, so use when fresh!