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Lettuce is a cool season crop that grows best within a temperature range of 12 °C to 20 °C. It does not suffer from light frosts and winter cold except near maturity.
Severe frost before harvest can scorch leaves and heads. Temperatures above 27 °C affect head development and plant edible quality and also promote premature seed stalk development. High temperatures also inhibit germination and can cause a high incidence of tip burn.
The crop has high moisture requirements and not more than 50% of the available water in the root zone should be depleted before irrigation.
Apply phosphorus and potassium fertilizers prior to planting. Nitrogen fertilization should be kept to a minimum prior to planting.
Apply nitrogen as a side-dress application once the crop is established. Frequent, light watering will cause the leaves to develop rapidly, resulting in high-quality lettuce.
Watering lettuce early in the morning will reduce the number of hours of leaf wetness and limit foliar diseases. Overwatering, especially in heavy, tight soils, can lead to diseases, soft growth and scalding or burning of the leaf margins.
Fertilize three weeks after transplanting. Lettuce prefers soil that is high in humus, with plenty of compost and a steady supply of nitrogen to keep if growing fast. Use organic alfalfa meal or a slow-release fertilizer.
Weeds are controlled mechanically, manually or chemically. Mechanical weed control can only be practiced before planting because of close spacings.
Weeds are removed by hand hoeing or pulling between plants in the rows. Chemical control can be achieved through the application of propyzamide shortly after sowing, which can last 12 months and longer in the soil.
Like most crops, lettuce is vulnerable to pests. These vary by region and time of year. Fortunately, growing plants off the ground is one of the best ways to avoid pests.
Aphids are small, soft-bodied insects that are most commonly green and black in colour, but may also be gray, brown, pink, red, yellow, or lavender. They tend to feed on tender, young growth causing it to appear puckered or deformed.
There are numerous methods of aphid control, including botanical sprays such as insecticidal soap, pyrethrum, rotenone, and horticultural oils. Beneficial insects such as ladybugs will also help to eradicate the pest.
Other major pests of lettuce are Cabbage LoopersCucumber Beetles.
HARVESTING AND STORAGE
Leaf lettuce may be cut whenever it is large enough to use. Cutting every other plant at ground level will give the remaining plants more space for growth.
Leaf lettuce reaches a maximum size (6 to 12 ounces) in 50 to 60 days.
Butterhead varieties form small, loose heads that weigh from 4 to 8 ounces at harvest (60 to 70 days). The innermost leaves, which tend to blanch themselves, are a delicacy.
Cos varieties have an upright growth habit and form a large head. To store lettuce, wash, drip dry and place it in a plastic bag in the refrigerator.
Lettuce keeps best at 32 degrees F and at high (95 percent) humidity.