Garden lettuce (Lactuca sativa) is a hardy annual plant commonly cultivated as a leafy vegetable. The earliest evidence of its cultivation goes all the way back to Ancient Egypt. Its leaves are often used for making salads, and they are an excellent source of vitamins K and A.
I. Appearance and Characteristics
Lettuce (Lactuca sativa) is an annual plant of the family Asteraceae.
Lettuce was originally farmed by the ancient Egyptians, who transformed it from a plant whose seeds were used to obtain oil into an important food crop raised for its succulent leaves and oil-rich seeds. Lettuce spread to the Greeks and Romans; the latter gave it the name lactuca, from which the English lettuce is derived. By 50 AD, many types were described, and lettuce appeared often in medieval writings, including several herbals. The 16th through 18th centuries saw the development of many varieties in Europe, and by the mid-18th century, cultivars were described that can still be found in modern gardens.
Lettuce’s native range spreads from the Mediterranean to Siberia, although it has been transported to almost all areas of the world. Plants generally have a height and spread of 15 to 30 cm (6 to 12 in). The leaves are colorful, mainly in the green and red color spectrums, with some variegated varieties. There are also a few varieties with yellow, gold or blue-teal leaves.
Lettuces have a wide range of shapes and textures, from the dense heads of the iceberg type to the notched, scalloped, frilly or ruffly leaves of leaf varieties. Lettuce plants have a root system that includes a main taproot and smaller secondary roots. Some varieties, especially those found in the United States and Western Europe, have long, narrow taproots and a small set of secondary roots. Longer taproots and more extensive secondary systems are found in varieties from Asia.
Depending on the variety and time of year, lettuce generally lives 65–130 days from planting to harvesting. Because lettuce that flowers (through the process known as “bolting”) becomes bitter and unsaleable, plants grown for consumption are rarely allowed to grow to maturity. Lettuce flowers more quickly in hot temperatures, while freezing temperatures cause slower growth and sometimes damage to outer leaves.
Once plants move past the edible stage, they develop flower stalks up to 1 m (3 ft 3 in) high with small yellow blossoms. Like other members of the tribe Cichorieae, lettuce inflorescences (also known as flower heads or capitula) are composed of multiple florets, each with a modified calyx called a pappus (which becomes the feathery “parachute” of the fruit), a corolla of five petals fused into a ligule or strap, and the reproductive parts. These include fused anthers that form a tube which surrounds a style and bipartite stigma. As the anthers shed pollen, the style elongates to allow the stigmas, now coated with pollen, to emerge from the tube. The ovaries form compressed, obovate (teardrop-shaped) dry fruits that do not open at maturity, measuring 3 to 4 mm long. The fruits have 5–7 ribs on each side and are tipped by two rows of small white hairs. The pappus remains at the top of each fruit as a dispersal structure. Each fruit contains one seed, which can be white, yellow, gray or brown depending on the variety of lettuce.
The domestication of lettuce over the centuries has resulted in several changes through selective breeding: delayed bolting, larger seeds, larger leaves and heads, better taste and texture, a lower latex content, and different leaf shapes and colors. Work in these areas continues through the present day. Scientific research into the genetic modification of lettuce is ongoing, with over 85 field trials taking place between 1992 and 2005 in the European Union and the United States to test modifications allowing greater herbicide tolerance, greater resistance to insects and fungi and slower bolting patterns. However, genetically modified lettuce is not currently used in commercial agriculture.
Lettuce grows best in full sun in loose, nitrogen-rich soils with a pH of between 6.0 and 6.8. Heat generally prompts lettuce to bolt, with most varieties growing poorly above 24 °C (75 °F); cool temperatures prompt better performance, with 16 to 18 °C (61 to 64 °F) being preferred and as low as 7 °C (45 °F) being tolerated. Plants in hot areas that are provided partial shade during the hottest part of the day will bolt more slowly. Temperatures above 27 °C (81 °F) will generally result in poor or non-existent germination of lettuce seeds.
II. How to Grow and Care
Sunlight
Lettuce grows best in areas with full sun exposure, typically at least four to six hours daily, but in areas with the harsh afternoon heat, lettuce benefits from partial shade. Cold-adapted lettuce varieties can survive in places with lower soil temperatures, but most prefer moderate temperatures to prevent produce from growing leggy or weak.
Temperature and Humidity
While some varieties are cold-tolerant, lettuce will not tolerate freezing temperatures. Avoid summer bolts or bitter leaves by stopping growth at least a month before the intense summer heat. The ideal growing temperature is around 60°F to 65°F. Temperatures that exceed 65°F will start impacting development, but at long as lettuce gets a break during the evening with cooler nights, it should be somewhat tolerant. Keep humidity in check by providing adequate airflow and moist soil.
Watering
Prevent disease or weak growth by keeping the soil moist but not overwatered. Soil that’s too dry causes lettuce to bolt or scorch. Use your hands to feel the top inches of soil to maintain an evenly moist environment. A continuous moisture supply will yield the best results.
Soil
Lettuce grows best in loamy, loose soil that’s well-draining and rich in organic matter. Aim for a soil pH that’s slightly acidic to neutral, similar to other vegetable growing conditions. Soil temperatures between 45°F and 65°F are best. Amend soil with compost or manure to achieve healthy plants.
Fertilizing
Lettuce thrives in soil amended with organic compost or manure. Amended soil helps preserve moisture, moderates temperatures, and prevents weeds from growing. Add fertilizer high in nitrogen before planting, including alfalfa or blood meal products. During the growing season, add a balanced, water-soluble fertilizer like a 5-5-5 or a 10-10-10 every two weeks to promote healthy leaves.
Pruning
Lettuce does not require additional pruning, as harvesting the leaves will help trim the plants throughout the growing season. Keep up with maintenance by removing dead or broken foliage. Provide adequate airflow to prevent diseases or pests from spreading.
Propagation
Lettuce easily propagates from scraps. While it won’t yield massive results, this method is a quick and interesting method for propagating lettuce in water:
- Cut the leaves using a lettuce head using a sharp, clean knife about half an inch from the bottom.
- Place the leaves (about half to one inch tall) in a shallow bowl of clean water.
- Position the bowl near a sunny window, requiring at least four to six hours of daily sunlight.
- Change the water every two to three days by carefully draining and replacing it in the shallow bowl. Be mindful not to disturb the roots once they emerge.
- In about seven to 10 days, lettuce scraps will fully form. Lettuce does not grow beyond this point and will likely decline in health if left in the water.
How to Grow from Seed
The most economical way to grow the best lettuce selections is from seeds. Purchased transplants are faster, but your choices may be fewer. Lettuce seeds germinate best in cool soil. If you want to be sure to get the earliest harvest possible, start two to four weeks before the last expected frost. That’s February in the Middle South and March in the Upper South. Lower and Coastal South gardeners can grow lettuce all winter, beginning with the first planting in October. Seeds are inexpensive, so sow them in different beds every two to three weeks. You will have an early crop in the years you don’t get a hard freeze.
Lettuce seeds require light to germinate. Sprinkle seeds on top of the soil, and lightly cover or scratch them into the bed just below the soil’s surface. Lettuce must be kept moist throughout its growing season. As seedlings mature, thin them when they are two or three inches tall by gently pulling out the largest plants. These will make your first salad, even though they are small. Leave six to eight inches between the remaining plants for sufficient room to mature. If pulling the seedlings is difficult, pinch them off instead. Remember to start with the more cold-tolerant lettuce selections in the cool months and then sow the heat-tolerant ones later to help carry you through the warming days of spring.
Expect to harvest lettuce 45 days after planting and semi-heading selections in about 50 days. Pick the outer leaves first, as the plant will continue growing and producing, or cut the entire plant two inches above the base. The new plants may not have the lovely form of the original, but this will help produce more leaves. To store lettuce, soak it immediately in an ice water bath for five minutes. Then drain, and keep in zip-top plastic bags in the refrigerator.
Mulching
Lettuces will also benefit from a good layer of mulch. Install a 2- to 3-inch layer of organic mulch, such as wood chips, straw, leaves, or grass clippings around the lettuce, being sure to leave a little space around the plant to prevent rot. This layer of mulch will help retain moisture, keep the soil cool, reduce the amount of weeding you have to do, and keep the lettuce clean by preventing soil from splashing up on the leaves when you water.
Potting and Repotting
A quality all-purpose potting mix is typically fine for lettuce. But a mix marked specifically for lettuce is better, as it usually has components to retain adequate soil moisture. Aim to plant lettuce initially in a container that will accommodate its mature size to avoid having to disturb its roots with repotting.
Overwintering
As an annual plant, it is optional to overwinter lettuce. Lettuce will not survive in freezing temperatures, so harvest the produce in the fall before the first frost. Gradually introduce cold-tolerant varieties to lower temperatures to extend the growing season. Additionally, planting lettuce in containers is another way to extend the growing season, as you can move the plant indoors during extreme cold. If growing outdoors, cover the ground in organic matter such as wood chips, straw, or pine needles to help retain moisture and protect from frostbite.
Pests and Diseases
Common Diseases
Lettuce typically doesn’t have any serious disease problems, except for potentially powdery mildew and downy mildew. Growing lettuce in the environment it likes can help to prevent these issues.
Common Pests
Common pests that might affect lettuce include the following:
Aphids
Aphids can also be a problem. If they are, try knocking them off with a blast of water from the hose or try a homemade spray to get rid of them.
Cutworms
Cutworms can also be a problem, and the best way to protect against them is to install a collar made of thick paper or cardboard around the base of any newly planted lettuce seedlings.
Slugs
Slugs and snails are its biggest enemy, and they can be thwarted by setting out a saucer of beer to trap them in, or by sprinkling diatomaceous earth or crushed eggshells around your plants. These sharp substances cut the slugs’ underbellies when they slide across them and kills them.
Rabbits
If your pests are of the long-eared, four-footed variety that like to nibble on lettuce leaves, the best defense against rabbits and other critters is to install a metal fence around the garden, or around the bed in which you are growing your lettuce. You can also try sprinkling cayenne pepper on the plants to deter the rabbits.
III. Uses and Benefits
As described around 50 AD, lettuce leaves were often cooked and served by the Romans with an oil-and-vinegar dressing; however, smaller leaves were sometimes eaten raw. During the 81–96 AD reign of Domitian, the tradition of serving a lettuce salad before a meal began. Post-Roman Europe continued the tradition of poaching lettuce, mainly with large romaine types, as well as the method of pouring a hot oil and vinegar mixture over the leaves.
Today, the majority of lettuce is grown for its leaves, although one type is grown for its stem and one for its seeds, which are made into an oil. Most lettuce is used in salads, either alone or with other greens, vegetables, meats and cheeses. Romaine lettuce is often used for Caesar salads. Lettuce leaves can also be found in soups, sandwiches and wraps, while the stems are eaten both raw and cooked.
The consumption of lettuce in China developed differently from in Western countries, due to health risks and cultural aversion to eating raw leaves; Chinese “salads” are composed of cooked vegetables and are served hot or cold. Lettuce is also used in a larger variety of dishes than in Western countries, contributing to a range of dishes including bean curd and meat dishes, soups and stir-frys plain or with other vegetables. Stem lettuce, widely consumed in China, is eaten either raw or cooked, the latter primarily in soups and stir-frys. Lettuce is also used as a primary ingredient in the preparation of lettuce soup.
IV. Harvesting and Storage
The best time of day to harvest is in the morning when the lettuce is still plump and the sun hasn’t caused it to wilt at all. You can harvest cut-and-come-again lettuce types as soon as the outer leaves reach about 6 inches long. Trim off these outer leaves, leaving the inner leaves to continue to mature. If you are growing head lettuce, be sure to harvest before the head starts to elongate. That means it’s ready to bolt, and the flavor will suffer. It can be stored in the refrigerator in a plastic bag for up to 10 days.
Find Where to Buy the Best Lettuce (Lactuca sativa)
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