How to Grow Lettuce: Lactuca Sativa
Although the exact origins of the lettuce are unknown it was certainly known to the Greeks and Romans, and has been grown in Britain since at least the middle of the sixteenth century. It is now the most important leafy salad vegetable. Lettuces are made up of over 95 per cent water, but contribute other essential components of our diet such as Vitamin A.
The Different lettuce cultivars can be grown outside during the summer and under protection during the winter.is a hardy or half-hardy annual.
The majority of lettuce cultivars produce a heart although a few leaf-lettuce/chicken-lettuce cultivars are available. Hearted lettuces are divided into two main groups —types and cos types. Many of the cabbage types have smooth, flattened leaves of a ‘buttery’ texture and are referred to as butterhead types.
Others have large, curled, crisp leaves which wilt more rapidly after cutting and are called crisphead types. Cos lettuce have longer, crisper leaves which have a sweeter flavour but the plants, which withstand drought more readily, take longer to reach maturity. The less popular leaf lettuce does not have a compact head but is just a collection of leaves which can be gradually removed. This ‘cut-and-come-again’ approach is useful and allows space to be used for other crops.
Lettuce grows best on light, rich soils that are able to retain moisture but will do well on any fertile, well-drained. Continuity of supply after outdoor crops have finished can be achieved by growing plants under protection during the autumn, winter and early spring. Soils for the protected crops should, again, be fertile, well-drained and moisture retentive. Watering lettuce during periods of dull, cold winter weather can bring a host of associated problems so it is best to have a soil which contains adequate amounts of well-rotted organic manure or which will retain moisture.
For summer crops, apply a base dressing of a general-purpose fertilizer such as fish and bone meal before sowing and/or transplanting at the rate of 50g per m2 (2 oz per square yard). Be wary of applying fertilizers with too much nitrogen before sowing since this might retard seed germination and the emergence of. For overwintered crops it is best to apply the fertilizer, in the form of a top dressing, just as growth recommences in the spring since too much fertilizer applied while the plants are establishing themselves may lead to soft, lush growth which is easily damaged by severe winter weather. Seed sowing and plant raising Continuous supplies of lettuce from June onwards can be assured by sowing in the open at fortnightly intervals from early April until early August.
Seeds should be sown thinly in 1.5-cm (1-in) deep drills which are 30cm (12in) apart. Thin out the seedlings as soon as they can be handled to 30cm (12in) apart in the rows. These distances apply to cabbage lettuce and the larger cos cultivars. Dwarf cos types can be grown in rows which are 25cm (10in) apart with 15cm (6in) between the plants. Outdoor summer lettuce can be harvested about 8 to 12 weeks after sowing, but the exact time depends on the sowing date. The time that the crop occupies the ground can be reduced by 3 to 4 weeks if you raise plants in seedpans — either in a greenhouse, a frame or even on a light, airy windowsill — and then transplant the seedlings once they have three true leaves. Seed of many lettuce cultivars has a very low germination percentage at temperatures above 21°C/70°F so be careful to keep the seedbed or seed pan cool until the seedlings have emerged. Cover them with a sheet of white polystyrene about 4cm (1-¼ in) thick to insulate against increasing temperature.
Lettuce may also be overwintered outside as young plants which will then mature in May. The easiest way of starting the crop is from seed which should be sown in September, at the spacings indicated for summer crops, and subsequently thinned. Alternatively you could sow the seed in a seedbed / seedpan and plant out during November in sheltered southern and western districts or during February/March in more exposed areas. Special winter hardy cultivars must be used for this purpose.
Protected crops can be grown in greenhouses, frames or cloches (either glass or polythene). In all cases it is best to raise seedlings and transplant. The two main periods for planting are autumn and early spring. Plantings made in mid-September into unheated protection from a mid-August sowing should produce mature lettuce in December while mid-October plantings will be ready in March. Spring plantings—made in mid-February from a mid-January sowing—grow more quickly in the lengthening days and will be ready for cutting in April or May.
Heated forms of protection will allow crops to be grown more quickly but it is only necessary to maintain minimum temperatures of 10°C/50°F. Plants for all protected crops can be raised by sowing thinly in seedpans/seedtrays which are kept in a heated greenhouse or the kitchen until the young seedlings are large enough to handle.
Transplanting and crop management
Protected crops are transplanted with the plants spaced 20 to 25cm (8 to 10in) apart in each direction. Great care must be taken when you are handling the plants during transplanting since bruising of the stem or damage to the root system will allow secondary infections such as grey mould (Botrytis) to enter. Always hold the young seedlings lightly by the leaves. Slugs can be a particular problem on protected lettuce so sprinkle pellets around the plants immediately after planting.
Ventilation must be given freely on bright, sunny days so that soft growth can be avoided, but on no account should air be given during damp, foggy weather. Growth of autumn-planted, spring-maturing, protected lettuce should be slow in the pre-Christmas period. Ideally only a rosette of leaves will have developed by the turn of the year. Plants will then begin to grow rapidly—as will spring-planted crops—provided no checks or competition are encountered. Carefully hoe outfrom around plants, using an onion hoe. Once the outer leaves cover the ground other weed development will be smothered.
Weed control is also of great importance on outdoor crops. Not only do they compete with the lettuce for light, water, plant nutrients and the like but they also act as overwintering hosts for aphids (greenfly).
In order to obtain crisp, well-hearted lettuce the growth—once it begins—must be quick and unchecked. It is at this stage of development that plants need the most water and the importance of moisture-retentive soils and careful irrigation when necessary cannot be over-emphasized.
Lettuce will quickly run to seed or ‘bolt’ in warm weather and any delay in harvesting the mature heads will increase this likelihood.
Since lettuce contain such a high percentage of water they wilt rapidly after cutting. Freshness can be maintained for the maximum period by harvesting heads early in the morning before the plants begin to heat up. If you must pick the plants when warm, freshness will be improved by immediately immersing the lettuce in ice-cold water for 15 minutes. Then shake off all the water and put the lettuce into a refrigerator until needed.
Pests and diseases
Aphids are the most common pest of lettuce. Usually the leaves are infested, causing distortion. Root aphids may also occur.
Slugs can be a persistent nuisance.
Seedlings are liable to be attacked by damping off fungi of which pythium and rhizoctonia are the most common. Downy-mildew produces yellow blotches on the upper surface of leaves with patches of white fungal growth below. Cold, wet weather favours this disease. Grey mould (botrytis) is a secondary infection. Previously damaged plants develop areas of grey, velvety fungal growth. Wilting and complete plant collapse soon follow, accompanied by the production of innumerable spores which may infect other damaged plants. The best method of control is to prevent the primary damage being caused. Occasional plants may develop symptoms of yellowing, dwarfing or distortion which can be caused by aphid-transmitted virus diseases.
Cabbage—Butterheads: ‘Cobham Green’, ‘Suzan’, ‘Fortune’. Crispheads ‘Webb’s Wonderful’, ‘Great Lakes’, ‘Windermere’, ‘Avoncrisp’.
Cos: ‘Lobjoits Green Cos’, ‘Little Gem’ (smaller cultivar), ‘Winter Density’.
Leaf Lettuce: ‘Salad Bowl’.
Overwintered outdoor types: (all hardy butter-head cultivars) ‘Arctic King’, ‘Imperial Winter’, ‘Valdor’.
Protected types: ‘Delta’, ‘Knap’, ‘Kwiek’.