Introduction
Lettuce (Lactuca sativa) is an annual plant of the family Asteraceae, together with celery and parsley that is most often grown as a leaf vegetable, but sometimes for its stem and seeds. It is mostly used for salads and in other kinds of food, such as soups, sandwiches and wraps and can also be grilled.
Although lettuce is least known in most Kenyan households, which makes it less consumed, it is slowly gaining popularity and acceptance.
Nutritional & Health Benefits of Lettuce
Some of the benefits of lettuce include the following;
- Certain proteins in lettuce like lipoxygenase and carrageenan help in controlling inflammation
- Lettuce’s vitamin c and beta-carotene work together to prevent the oxidation of cholesterol
- It helps with insomnia (has Lactucarium which has relaxing and sleep inducing properties)
- It is a rich source of vitamin k which has a potential role in the bone metabolism.
- Fresh lettuce leaves contain good amounts of folates and vitamin c.
- It contains zeaxanthin, an important dietary carotenoid.
- It is rich in healthy amounts of minerals like iron, zinc, calcium, potassium and magnesium which are very essential for body metabolism.
- It is a rich source of the B-complex group of vitamins like thiamin, vitamin b-6 and riboflavins.
- It is rich in fiber and cellulose
- The latex of lettuce has antimicrobial properties that completely deform yeasts using terpenes and cardenolides and enzymes like glucanases.
- Lettuce has anti-cancer properties.
Seed selection
There are two major varieties of lettuce, that is, heading and non-heading varieties. The heading variety forms a compacted head like a cabbage while the non-heading one has loose leaves.
Some of the commonly grown lettuce varieties include;
- Butterhead; with loose leaves and buttery texture. Ready for first harvest in 45-55 days after sowing.
- Crisp-head; forms tight, dense heads that resemble cabbage. It is the mildest form and valued for its crunchy texture than flavour. Ready for first harvest in 70-100 days after sowing.
- Loose-leaf; has tender, delicate and flavourful leaves with a loose bunch and is mainly used for salads. Ready for first harvest in 45-55 days after sowing
- Romaine lettuce; grows to a long head of sturdy leaves with a stout rib almost reaching to the tip of the leaf blade. It is used mainly for salads and sandwiches and is ready for first harvesting in 75-85 days after sowing.
- Chinese lettuce/celtuce; forms long, tapering, non-head forming leaves and has strong-flavoured tuff of terminal leaves.
- Summer crisp; forms moderately dense heads with a crunchy texture. The variety is intermediate between crisp-head and loose-leaf varieties.
Ecological requirements
Lettuce is a cool season crop that grows best with temperatures of 12-20OC. Temperatures above 27oC affect head development and plant edible quality and also promote premature seed stalk development. Although the crop can thrive on a wide range of soils, optimal production is achieved on well-drained fertile loam soils that are rich in organic matter, have good water-holding capacity and a pH between 5.5-7.0.
Propagation & Planting
Lettuce is propagated from seeds, which can be sown directly into the main field or raised in a nursery bed and transplanted thereafter.
Raising seedlings
Procedure
- Prepare the nursery bed, about 1 metre wide against the required length.
- Drench the soil with a mixture of Pyramid® 700WP 100g + Loyalty® 700WDG 10g + Optimizer® 20ml in 20 litres of water. This mixture controls soil pest and diseases, provides nutrients, breaks seed dormancy and promotes uniform germination.
- Sow seeds at a depth of about 2cm and cover them lightly with soil.
- Cover nursery bed with a thin layer of mulch.
- Water the nursery bed.
It takes 3-4 days for germination of seeds.
Transplanting
Lettuce seedlings are ready to transplant in 4-5 weeks with 4 to 6 leaves and a well-developed root system. To harden the young plants, the rate of irrigation should be reduced, a week before transplanting, and the shading withdrawn. Before lifting the seedlings, the nursery bed should be irrigated.
Procedure
- Make raised or sunken beds on the prepared land.
- Mix soil with manure and DAP. For efficient and improved nutrient uptake and stimulation of root development, among other benefits, mix 1kg Humipower® with 50kg DAP and /or 1 ton of manure.
- Irrigate the beds to allow for easy planting.
- Plant the seedlings in the beds at a spacing of 45*30cm
Tips!
- To overcome transplanting shock, spray seedlings with Optimizer® 10ml/20L.
- Transplanting is best done early in the morning or in the evenings.
- Transplant only the healthy and strong seedlings.
Cultural practices
Thinning; When seedlings sprout to about 2-3 inches, they should be thinned to 3-4 inches apart. This helps in reducing competition as well as ensuring sufficient air circulation within the crops.
Weeding; Lettuce does not compete well with weeds and therefore weeding should be done regularly until the crop foliage covers ground. Thereafter, it can be reduced to pulling out individual weeds when they appear.
Mulching; The crop should be mulched to retain soil moisture and keep the produce clean. When decomposed, the mulch releases nutrients into the soil which are absorbed by the crop.
Watering/Irrigation; In order to ensure even and rapid growth, the soil should be kept moderately moist throughout the season. Otherwise, lettuce may become bitter and flower prematurely.
Crop rotation; Strict crop rotation should be practised using non-family crops like cereals, legumes, brassicas, among others. This helps prevent build-up of pests and diseases among other benefits.
Fertilizer application; Timely application of both basal and foliar fertilizers is highly recommended in order to achieve optimum yields. Well decomposed farm yard manure should also be added especially for soils with little or no organic matter as it boosts productivity and does not destroy soil structure or pollute the environment. It also has residual effect in the soil which benefits the next season’s crop.
For instance;
During transplanting, apply 50kg of DAP or TSP in one acre. These are rich in phosphorous which is highly needed at early developmental stages of the crop. After 2 weeks, spray the young plants with Lavender Super Starter® 20ml/20L or Goldchance Super Start® 50g/20L to promote rapid root development, ensure balanced crop growth and increase the crops’ resistivity to infections and weather stress.
Top dress using 50kg of CAN or NPK in one acre two weeks after transplanting and after 4 weeks, at rate of 50kgper acre. Consequently, spray the crop with Goldchance Super Growth® 50g/20L or Lavender Super Growth & Vegetative® 20ml/20L to promote a faster vegetative growth, increase crop resistance to infections and weather stress and ensure a balanced growth of the crop.
Other foliar fertilizers which can be used include Optimizer® 10ml/20L, Goldchance Multisuper K® 50g/20L, Zinc Gold® 10ml/20L and Biodistinction Xtra®.
Maturity, Harvesting & Post-Harvest Handling
Depending on the variety and the ecological conditions, lettuce takes 45–130 days from planting to harvesting. Besides, the stage of harvest maturity depends on the variety of lettuce and the purpose for which it is grown. For instance, since lettuce that flowers through bolting becomes bitter and unsaleable, plants grown for consumption are rarely allowed to grow to maturity.
The heading
types are harvested when the heads are fully grown and firm while the loose
leaf types are picked when the leaves have reached the required size.
Harvesting is done by cutting off the plant just above the soil surface to keep
most of the outer leaves around the head and should be done very early in the
morning because lettuce wilts rapidly. Generally, harvesting lettuce in cooler
weather makes for much sweeter tasting, crisper leaves.
If lettuce is left in the ground for too long, it begins to form a seed stalk, a process known as bolting, and the lettuce leaves will have turned bitter.
After harvesting lettuce, the leaves or heads should be sorted and graded, then packed in crates or cartons. Lettuce can also be stored for up to three weeks at temperatures of 4°c at 95% relative humidity.
Major Pests & Diseases
Insect pests
Aphids; These are the most serious pests that cause physical damage and serve as vectors for lettuce mosaic virus. Heavy populations of aphids can result in stunted growth of young plants and their presence also contaminates the heads, as they secrete honeydew which encourages the development of sooty mold, thereby, affecting the appearance of the produce.
Spray Kingcode Elite® 50EC 10ml/20L or Lexus® 247SC 8ml/20L or Pentagon® 50EC 10ml/20L or Loyalty® 700WDG 5g/20L or Emerald® 200SL 10ml/20L
To cleanse the sooty mold, spray Jambo Clean® 100ml/20L
Cutworms; These are problematic mostly during the seedling stage and are found 2-5cm below the soil surface. Cutworms are active during the night and they cut stems just above or below the soil surface.
Drench soil with Profile® 440EC 60ml/20L or Pentagon® 50EC 20ml/20L
Diamond Back Moth (DBM); The larva is a green caterpillar which feeds on the leaf tissue except the veins. Infestation causes significant losses if the pest is not controlled.
Spray Escort® 19EC 10ml/20L or Baciguard® 16WDG 15g/20L or Lexus® 247SC 8ml/20L
American bollworm; The larvae penetrate at the bottom of the leaves and eat their way into the heads.
Spray Kingcode
Elite® 50EC 10ml/20L or Pentagon® 50EC 10ml/20L or Baciguard® 16WDG 15g/20L
Diseases
Damping off; This is a soil borne disease which attacks seeds and seedlings. Diseased seeds do not emerge while seedlings rot and eventually die. The roots of these infected seedlings exhibit a white cottony growth.
Drench the planting holes/soil with Pyramid® 700WP 100g/20L
Spray the plants with Absolute® 375SC 10ml/20L
Downey mildew; It is characterized by light green to yellow areas on the upper surface of the leaf. As the disease progresses, the infected tissues turn brown and a white downey mold also forms on the lower leaf surface.
Spray Gearlock Turbo® 250WP 25g/20L or Fortress Gold® 720WP 40g/20L or Tower® 720WP 50g/20L or Katerina® 720SC 40ml/20L
Leaf spot; Infected plants have small, yellowish spots on the outer leaves. These spots grow and become large, irregular and brown. The centre of the spot is pale creamy brown with many black dots.
Spray Chariot® 500SC 20ml/20L or Ransom® 600WP 15g/20L or Domain® 250EC 10ml/20L or Absolute® 375SC 10ml/20L
Powdery mildew; This disease occurs frequently and it reduces the quality of the crop. Infection symptoms are characterized by spots or patches of white to greyish, talcum powder like growth. Tiny pinhead sized, spherical structures that are first white, later yellow brown and finally black, may be present singly or in groups.
Spray Ransom® 600WP 15g/20L or Chariot® 500SC 20ml/20L or Domain® 250EC 10ml/20L or Absolute® 375SC 10ml/20L
Sclerotinia rot; Infected plants initially wilt on
hot days. A severe wet rot is seen on the stem near the soil. This rot spreads
to the roots and into the head, becoming wet and slimy. A white, cottony mold
develops on the rotten tissue and hard, irregular, black sclerotia can also be
seen in the white mold on rotten tissues. The outer leaves whither and drop
flat, then the inner ones, if the base of the leaf has rotted.
Drench soil with Gearlock
Turbo® 250WP 50g/20L or Pyramid® 700WP 100g/20L
Bacterial rot; Disease symptoms include leaf spots which start off under water-soaked conditions and later turn brown, before developing soft leaf rot and death of affected tissue. The disease is favoured by hot and humid weather conditions and the rot can worsen after harvesting, in transit and in the market.
Spray Green Cop® 500WP 50g/20L, a copper-based products which suppress the activity of the pathogen.
Lettuce Mosaic; Early infections result in vein clearing followed by mottling, curling of the leaves, and increased marginal frilliness. Infected mature plants are yellow and stunted and cannot be harvested.
This viral disease is transmitted by the green peach aphid.
Control vectors (aphids) using Kingcode Elite® 50EC 10ml/20L or Lexus® 247SC 8ml/20L or Pentagon® 50EC 10ml/20L or Loyalty® 700WDG 5g/20L or Emerald® 200SL 10ml/20L
Remarks
- In all foliar sprays, mix the chemical with Integra® 3ml/20L, which is a sticker, spreader and penetrant which increases the efficacy of the product.
- Optimizer® is an organic bio-stimulant which is essential for plant growth and stress management. It can be applied at all or any growth and development phase of the crop.
- Post-harvest interval (PHI) and re-entry interval (REI) for all the applied chemicals should be observed.
- All basal fertilizers should be mixed with Humipower® at a rate of 1kg of Humipower® into 50Kg of fertilizer.
- Lettuce should always be kept under shade after harvest to avoid wilting.
- Soil analysis is recommended in order to determine the soil fertility level.
- The demand for lettuce in the market is relatively high. Some of the market opportunities for the produce include institutions like hospitals, supermarkets and open-air markets, among others.