Lettuce are well suited to growing in hydroponics, virtually all round if you use the appropriate variety and manage it properly.
Growing conditions
Growth needs to be fast and at an even rate (will mature in 40 – 85 days depending on variety).
Shading may be needed in hot conditions.
The root zone should never overheat.
Most varieties prefer temperatures between 12 and 20 degrees C.
Temperatures over 27 degrees C. can affect quality and cause flower stalk development.
Nutrient requirements
Requires a moderate, standard solution at a uniform rate (too strong or too weak can lead to irregular growth).
NPK early in the season 9:1:16
As growth progresses reduce nitrogen slowly to reach 15% lower nitrogen at harvest.
Tolerates high levels of boron, but only has a moderate requirement.
Molybdenum, copper and manganese are more important than most other minor nutrients.
pH 6.0 to 7.0.
Yield has been reduced by EC levels of 2.0 or higher. Lettuce responds to more frequent irrigations with a lower concentration nutrient solution.
Some growers have used foliar feeds to increase head weight and extend shelf life of lettuces.
Suitable systems
Vertical columns have been used in Poland.
NFT has been used in England, Japan, and Australia.
Modified NFT with gravel in gullies is very successful.
Coarse aggregate culture gives excellent results in any medium 10 cm deep.
Rockwool gives excellent results and has been used commercially.
Media with a greater cation exchange capacity are not suitable.
Planting
Germinate seed in vermiculite, perlite or rockwool propagation blocks, then transplant into system at 6 to 8 leaf stage.
Sow direct into aggregate beds then thin out.
Some growers buy in seedlings. This accelerates the crop turnover time and reduces labour, but there may be tradeoffs in quality, availability, or varietal choices.
Special cultural techniques
Remove marked or damaged outer leaves.
Problems
Irregular burst of growth can cause decreased quality or quantity of produce.
Rapidly-grown lettuces are relatively free of disease.
Excess water or poor aeration commonly causes yellow or rotting of the lower (outer) leaves.
Some will burn on leaf tips if exposed to too much sunlight.
Pests include aphis, flea beetles, crickets, springtails, leaf hoppers, caterpillars, whitefly, slugs and snails.
Aphis is of particular concern because it transmits viral diseases.
Other disease problems include damping off (pythium), sclerotinia, downy mildew, powdery mildew, botrytis, rhizoctonia, and anthracnose.
Harvest and post harvest
Lettuce are ready to pick after four weeks in summer.
Rockwool grown plants can be harvested with roots intact, still in the rockwool – this significantly improves the keeping quality.
Damaged or marked leaves should be removed at harvest.
Lettuce should be cooled rapidly after harvest, in order to maximise shelf life.
Store at between 2 and 4 degrees C under high humidity.
Varieties
Hydroponics is especially suited to the production of the small, fancy leaf type lettuces. Commercial growers will find it necessary to source varieties for different seasons.
Mignonette can be grown all year (with the aid of a greenhouse in cooler climates).
Some varieties are slower bolting, reacting more slowly to higher temperatures. These are most suited to grow in warmer months.