Lettuce is an excellent crop for greenhouse production throughout the year, especially in colder areas in the north. Lettuce is a crop with low light and low temperature, so additional lighting and heating costs can be minimized.
On the other hand, local producers who can provide a constant supply of fresh and gourmet-quality lettuce can often charge higher or higher prices, saving money on shipping costs.
What types of lettuce grow best in winter?
Head lettuces are generally not recommended for hydroponic crops, but there are many lettuces and other leafy vegetables that are suitable for hydroponic applications.
Bibb lettuces are the most popular with fast response times and excellent quality.
Romaine lettuce takes a little longer to grow, but it can get higher prices, especially in the years when production in the field is low.
Spinach is excellent for hydroponic cultivation since there is no sand, and other green vegetables such as bok choy and arugula can be grown in specialized markets.
Another emerging market is microgreens. Microgreens are baby lettuces, mustards, and herbs that are planted and densely harvested in days, and often bring premium prices in niche markets.
Whether for domestic cultivation or commercial production, a wide variety of lettuce and leafy vegetables can be successfully grown in hydroponic systems.
The best hydroponic system for lettuce cultivation
By far the most popular hydroponic system for lettuce production in the United States and Europe is the nutrient film technique (NFT) system.
The lettuce is placed in polypropylene tubes where a thin layer of the nutrient solution runs over the bare roots of the plants.
The nutrient solution is stored in a tank and a submerged pump pumps the solution to the opposite end of the tubes.
The tubes are placed with a slight inclination so that the nutrient solution flows into the tube and the nutrient solution is collected at the other end and returned to the reservoir.
The system recirculates the nutrient solution repeatedly. Since the nutrient solution flows in a thin film, oxygenation in the root zone is ideal and, since the reservoir is in a closed system, it is possible to capture, clean and reuse the nutrient solution for maximum efficiency.
Only a fraction of the water and nutrients needed for production in the field are needed, and there is little or no runoff from concentrated fertilizer salts to pollute the environment.
Germinating lettuce seeds
No heat is needed at the bottom to germinate lettuce seeds. Lettuces are cold-weather crops, so excessive heat can actually delay or prevent germination.
Simply cover a tray of starter cubes with a plastic foil or moisture dome, place it under fluorescent lights and wait a few days for it to germinate.
Once the roots begin to appear at the bottom of the cubes, they are ready to be transplanted to the NFT system.
The best artificial lighting for lettuce
In northern climates, prolonged periods of short, cloudy days can dramatically reduce lettuce growth rates. Therefore, judicious use of horticultural lights is often recommended. Plants need full-spectrum light, with the blue and red ends of the spectrum for vigorous growth. With 10 years of experience in the horticulture lighting system, VANQ has worked out a cultivation-proved light recipe for lettuce cultivation.
With a clear focus on the deep blue 460nm and hyper red 660nm spectrum, this recipe focusses on vegetative growth phases and propagation.
The recipe delivers shorter, compact plants with a good biomass ratio – the extra energy in the blue 460nm bandwidth also promotes stomata opening. Altogether a good balance between both photon levels guarantees a good mix between vegetative growth and biomass production.
Ideal temperature for winter lettuce cultivation
Some producers like to heat their nutrient solution in winter, especially in NFT systems, where water continuously circulates through the roots. By keeping the root temperature at approximately 24 ° C, it is often possible to lower the air temperature without sacrificing quality or performance.
As fuel costs for greenhouse temperature control can become significant in winter, heating the nutrient solution can save production costs. Just make sure that the temperature of the nutrients does not exceed 30 ° C.
The hotter the water, the less dissolved oxygen it can retain, establishing anaerobic conditions that can promote root rot.
The right nutrients for winter lettuce cultivation
Because lettuce is a vegetative crop, a well-balanced crop formula is all that is required to produce a high-quality crop.
The culture formulas are proportionally higher in nitrogen than the flowering formulas, generally with an NPK ratio of approximately 3-1-5. The nitrate form of nitrogen is generally preferred, with no more than about 3-10% of the available nitrogen in the form of ammonia.
Ammonium ions are rapidly absorbed and used by lettuce plants for rapid vegetative growth, but excess ammonia nitrogen produces a “range” growth, producing soft and weak tissues with little validity.
Nitrogen is used more slowly by the plant, producing stronger and healthier lettuce plants.
Amino acid blends can also be beneficial in a complete lettuce nutrition program. Amino acids are intermediate chelators, improving the availability of calcium and other important cations for plants.
When phosphoric acid is added to hard water, bicarbonates burn like carbon dioxide and water, but calcium ions react to form calcium phosphate.
The bone is composed of calcium phosphate and is 95% insoluble in water; Therefore, calcium phosphate traps calcium and phosphate, making them unavailable to the plant.
When amino acids are added, however, calcium remains available. Primary amino acid chelators, such as glutamic acid and glycine, adhere to calcium ions like a claw, preventing them from reacting with phosphorus.
At the same time, glutamic acid and glycine stimulate root cells to open calcium ion channels, consuming calcium thousands to millions of times faster than simple osmosis.
Inside the plant, calcium is used to strengthen cell walls and help protect lettuce from temperature stress, pathogen attacks and common physiological diseases such as tip burns.
Water quality and CE
Unlike tomatoes and other fruit crops, which can handle high levels of nutritional salts, lettuce has a low salt tolerance; therefore, it is beneficial to use a source of clean and slightly soft water with relatively low mineral levels and low sodium levels.
During the summer months, the total EC of the nutrient solution should be maintained at 10 cf or less. During the winter months, with lower light levels, the EC should be slightly increased to help prevent stretching, but the producer should still try not to exceed a total of 12 cf.
Starting with 2-4 cf of raw water and adding only a mild nutritional formula, you can maintain a good balance between rapid growth and superior quality.
Winter hydroponic lettuce?
When a complete nutrition program is followed for the production of lettuce, including mineral and organic components, it is possible to produce a consistently superior product.
The color, texture, and taste can be improved, with the additional benefit of improving the content of vitamins and minerals.
Objective measurement of how well your plants are absorbing water and minerals can be obtained using a Brix refractometer, an instrument that measures sugars and solids dissolved in the sap.
But many experienced chefs and product buyers can detect superior quality just by looking at it. For example, the best romaine lettuce has a thin gold ribbon at the edges of the leaves and should have complex flavors, which include sweet, bitter and spicy tones.
Once your nutritional solutions management program is developed and adjusted, you can lay the groundwork for a continuous harvest of quality gourmet products.