Vegetables that have a determinate growth pattern are ideal for hydroponic production. Here we look at how to grow lettuce in a recirculating system.

Leafy and other determinate vegetables are usually small plants with small root systems. If they are grown hydroponically, production generally takes place in systems such as the gravel flow system.

An important advantage of hydroponic vegetable growing is that leaves and fruit are free of dirt. And, as leafy crops do not grow tall, greenhouses don’t need to be as high as for indeterminate growers.

A variety of determinate vegetables can be grown successfully in hydropo¬nic systems, but field production’s often cheaper. Before starting, therefore, do a thorough market study and plan well – for quality, continuity and the correct mass and class.

Find out beforehand what the markets expect and require.

Establish where the produce will be marketed and plant only those cultivars customers prefer.

LETTUCE

• Lettuce is rich in calcium, iron and vitamin A, and the nutrient value is high, because it is eaten uncooked.
• Producing lettuce hydroponically, in a controlled environment, produces a clean, uniform and superior quality crop throughout the year.
• Lettuce has a shallow root system, and lends itself to nutrient film production systems.
• In South Africa the gravel flow technique (GFT) is the most widely used system for its cultivation.

CLIMATIC REQUIREMENTS

• Lettuce is a cool-weather crop and, if grown under shade net and in tunnels, is not generally affected by winter cold and frost.
• The only exception is fully matured crisphead lettuce, which sometimes suffers frost damage if not harvested in time.
• If the heads start to decay, the plants must be removed as soon as possible to avoid spreading diseases to other plants.
• The optimum temperatures are 17°C to 27°C (daytime) and 3°C to 12°C (night).
• If the night temperature falls below 3°C, the growth rate will be retarded.
• If temperatures of 28°C and higher prevail, most of the cultivars will form small, inferior, loose heads, and bolt (form seed stalks).
• They will have a bitter taste.
• High temperatures can also cause a high incidence of tip burn, a physiological disorder.

TYPES AND CULTIVARS

Many cultivars of each lettuce type are available from the various seed companies. Here are some examples:

Crisphead or Iceberg lettuce (Lactuca sativa L var capitata): Crisphead has a large, firm head and the leaves are usually wider than their length. The outside leaves are dark green with either a dull or bright cast, and the inner leaves are whitish to creamy yellow. Choose a cultivar that produces a solid heart and is easy to handle.

Cos (Romana) lettuce (Lactuca sativa L var romana L): Cos lettuce develops long, loaf-shaped heads that are either closed at the top or open. The leaves are long and quite narrow and have a coarse, crisp texture. The outer leaves are dark green and the inner ones are yellowish.

Leaf lettuce (Lactuca sativa L): These vary in size and colour, with leaves characteristically forming a bunch, or rosette. Leaves can be long or broad, round, spatulate or lobed, frilled, smooth dark or light green, and with, or without, a red tinge. Because of its open growth habit, leaf lettuce has fewer bleached leaves than the crisp types. It has a higher vitamin and mineral content than crisp lettuce and, because the leaves are more exposed to sunlight, there’s a much higher proportion of green leaves.

Butterhead lettuce (Lactuca sativa L var capitata L): Butter lettuce can easily be identified by its smooth, oily, soft-textured leaves forming a soft head which is smaller than the crisphead types. The leaves are broader, shorter and wrinkled. The outer leaves can be light to dark green, and the inner leaves are creamy yellow. Some cultivars have a red pigmentation. Choose a variety that’s easy to harvest, pack and handle.

SEEDS

• Lettuce seed lots contain approximately 800 000 to 1 million seeds per kg.
• Seed has a poor shelf life, so the producer should take care to use fresh seed, or insist on a recent germination test by an independent laboratory.
• Use only fungicide-treated seed.
• Nowadays, seed is palletised with various types of coating.
• Palletised seed germinates rapidly (even in high temperatures) and precision planting is possible.

TRANSPLANTING

• Choose healthy, strong young plants (3 cm to 4 cm tall) for transplanting.
• Water the plants well beforehand.
• Remove some of the medium by washing it off in a bucket; this helps to prevent too much of it remaining in the gravel after harvesting.
• It’s very important that the roots of seedlings make contact with the film of nutrient solution at the bottom of the hydroline.
• The film is usually 3 mm to 4 mm thick, but you have to adjust it to 5 mm to 6 mm for a day or 2 after transplanting to allow roots to become established.

MANIPULATING GROWTH

• The aim of producing lettuce should be to deliver as many marketable heads in the shortest time possible.
• Remember that the number of leaves formed per unit area increases with an increase in temperature.
• Lettuce needs light and it has been found that plants grown under conditions of low light and high temperature produce long, narrow leaves and head formation is delayed.

Also read:
Hydroponic farming: The basics – popular systems
Hydroponic farming: How to produce vegetable seedlings

• This article was written by Erika van den Heever, Martin Maboko and Silence Chiloane and first appeared in Farming SA.