Lettuce is a leafy vegetable that belongs to the Asteraceae family, together with celery, parsley and endive.
Two types, the heading and non-heading, exist. The heading type forms a compact head like cabbage while the non-heading has loose leaves. The edible part are the green leaves that are eaten in salads.
Lettuce is mainly used in hotels and restaurants for preparing salads. However, there is little acceptance and consumption at household level.
The vegetable grows well in Kiambu and Nairobi.
Climatic and cultural requirements
Lettuce thrives at relatively cool temperatures and requires ample sunlight, uniformly cool nights, and plenty of moisture in the soil for well developed, solid heads.
Optimum temperatures for growth are 18.8– 22.70C during the day and 7.2–110C at night. High temperatures generally above 290C are conducive to early flowering resulting in low quality and bitter flavour.
At temperatures above 23.80C, firm heads are difficult to obtain, and stems tend to elongate. Plants will withstand freezing, but below 7.20C, growth is extremely slow.
As plants approach maturity, they are more easily injured by freezing than when they are young and injured tissues become more susceptible to soft rot.
Lettuce can be grown on a wide range of soil types, depending on irrigation, drainage, and local climatic conditions. The soil pH should be 6.5-7.2.
Nutrient removal from soils by lettuce is relatively modest, but fertility requirements are generally high because of the plant’s limited root system and necessity for rapid, continuous growth, which is essential for crispness and high quality.
Nitrogen is the most important fertiliser nutrient. A crop may remove 36–130kg N/ha, but fertiliser recommendations may be higher to account for losses due to leaching and denitrification.
Apply as a top dress in two splits during the growing season. A lettuce crop will also remove about 12–17kg of phosphorous and 130–220kg of potassium per ha. Lettuce is moderately tolerant to salts but excess salts can be a problem.
PLANTING AND ESTABLISHMENT
They are established from seeds that are mostly planted directly in the final field. Transplanting is also done but is not the most common method.
It is only suitable in areas where the climate is of short duration for successful direct seeding. Lettuce grown in greenhouses or other protective structures is usually transplanted.
Seed germination occurs best at 4–260C with optimal germination occurring at 23.80C.
Space 30x30cm, 30x25cm or 25x25cm depending on production area and variety.
Done to keep heads free from soil splashes and moisture conservation.
Should be regular until crop foliage covers ground. Thereafter, it can be reduced to pulling out individual weeds when they appear.
Keep ground moist to ensure even and rapid growth, otherwise lettuce may become bitter or flower prematurely.
Use nitrogenous fertilisers for lettuce grown in soils, with high organic matter about 80kg N/ha, while those grown in light soils require 135kg N/ha.
Phosphorus and potassium application should be done at planting time and guided by soil test results.
It takes 1.5–3 months from planting to harvesting, which continues for about four weeks. The lettuce is ready when the heads are firm or leaves are fully developed.
Harvesting should preferably be done early in the morning. Cut off heads with a sharp knife and remove dirty or damaged leaves.
Lettuce grading is based on leaf appearance. The leaves should be fresh and free of discolouration. Leaf colour may differ but no sign of leaf tip-burn should be present, particularly in the inner leaves.
Seed stalk should be absent. Heads are packed in crates not more than two layers thick. Lettuce should always be kept under shade after harvest to avoid wilting.
Transportation should be over a short distance or very fast or refrigerated but done with great care. Yields of 10 – 14 tonne/ha may be obtained.
i) Basal rot (slimy soft rot) caused by Erwinia sp. It is characterised by small water soaked area on the plant. Controlled by rouging, avoid excess watering, ensure proper drainage, avoid lettuce related crops in the neighbourhood and use certified seeds.
ii) Downy mildew caused by fungus Bremia lactucae. It is characterised by light green to yellow areas on upper surface of wrapper leaves. Infected tissues later turn brown. Controlled by ensuring good drainage, removing crop residues promptly, spraying with Ridomil, Antracol every 7 – 14 days.
iii) Cercospora leaf spot caused by fungus Cercospora longissima. It is characterised by minute leaf spots which gradually enlarge to circular then irregular spots that turn various shades of tan to brown. Control: Crop rotation, rouging, good drainage, use certified seeds and fungicides.
iv) Lettuce mosaic virus: Young affected plants are stunted and have mottled leaves, which may be distorted.
Control:Weeding to destroy alternative hosts like sow thistle, groundsel, control aphids, use resistant cultivars and certified seeds.
Other diseases are white mould, leaf spot, damping off, Aster yellows and Big vein disease.
i) Aphids are the most serious pests and can cause physical damage, as well as serving as vectors for lettuce mosaic virus. The caterpillars include loopers, variegated cutworms, and various armyworm species.
ii) Worms hatch from eggs laid by grayish-brown moths. The cabbage looper (Trichoplusia ni) begins feeding on the underside of the foliage and eventually develops into a green caterpillar.
iii) The beet armyworm (Spodoptera exigua) and the yellow-striped armyworm (S. ornithogalli) can cause severe seedling damage at the ground by feeding on emerging larvae.
Later generations or older larvae of caterpillars can enter the head, making control difficult. Control of all these insect pests is by use of insecticides that should be applied early in the season.
Biological control with Bacillus thuringiensis is frequently used to control certain caterpillars.
Department of Crops, Horticulture and Soils, Egerton University.
- Lettuce lowers cholesterol levels hence help in weight and blood sugar management.
- Induces sleep, thus helps fight insomnia.
- Prevents growth of cancerous cells.
- Has antioxidant properties.
- has antimicrobial properties.
- Controls anxiety
Nutritional Value of Lettuce
- Contains fat, proteins, moisture, carbohydrates, sugar and dietary fibre.
- Contains minerals like calcium, zinc, magnesium, phosphorous, potassium, sodium and iron.
- Contains vitamin A, B, C, E and K