Winter Lettuce Production

Lettuce is a great crop for year-round, greenhouse production, particularly in northern latitudes. Lettuce is a low-light, low-temperature crop, so supplemental lighting and heating costs can be kept to a minimum. Lettuce is also a very perishable crop, giving local growers who pick fresh and deliver daily a competitive advantage. For example, shipping lettuce from the west coast to the east coast in temperature-controlled containers may cost as much as forty cents per pound, and the longer the lettuce is in transit, the more that may be lost as scrap. On the other hand, local growers who can provide a steady supply of fresh, gourmet-quality lettuce can often charge premium prices or above, while saving money on shipping costs!

Head lettuces, such as iceburg lettuce, are not usually recommended for hydroponics, but there are many leaf lettuces and other leafy vegetables that are well suited for hydroponics applications. Bibb lettuces are the most popular, with fast turn-around times and excellent quality. Romaine takes a little longer to grow, but can command higher prices, especially in years when field production is poor. Spinach is great for hydroponics since there is no sandy grit, and other green vegetables such as bok choi and arugula can be grown for specialty markets. Another emerging market for hydroponics is micro-greens. Micro-greens are baby lettuces, mustards and herbs that are densely sown and harvested in days, and they often bring in premium prices in niche markets. So whether growing for home use or for commercial production, a wide variety of lettuces and leafy vegetables can be successfully grown in hydroponic systems.

By far the most popular hydroponic system for lettuce production in the United States and Europe is the NFT system. NFT stands for Nutrient Film Technique. The lettuce is placed in plastic gullies, usually 2-1/2” to 6” wide, and a thin film of nutrient solution trickles over the bare roots. The nutrient solution is stored in a reservoir, and a submersible pump pumps the solution to the far end of the gullies. The gullies are placed at a slight slope so that the nutrient solution flows down the gullies to be collected at the other end and returned to the reservoir. The system then recirculates the nutrient solution over and over again. Since the nutrient solution is flowing in a thin film, aeration at the root zone is optimal, and since the reservoir is in a closed system, it is possible to capture, clean and re-use the nutrient solution for maximum efficiency. Only a fraction of the water and nutrients required for field production is necessary, and there is little or no runoff of concentrated fertilizer salts to pollute the environment.

No bottom heat is required to germinate lettuce seeds. Lettuces are cool weather crops, so excessive heat can actual delay or prevent germination! Simply cover the tray of rockwool starter cubes or Oasis cubes with a plastic sheet or humidity dome, place under cool fluorescent lights and wait a few days for germination. Once the roots begin to poke out through the bottom of the cubes, they are ready for transplanting into the NFT system.

In northern climates, prolonged periods of short, overcast days can dramatically slow down growth rates of lettuce. Therefore, the judicious use of horticultural grow lights is often recommended. Plants need full spectrum light, with both the blue and red ends of the spectrum for vigorous growth. During the vegetative growth stage, plants prefer the blue end of the spectrum. Blue light at the 460 nanometer wavelength greatly influences “phototropism”, the tendency of plants to lean toward the light. If plants receive enough blue light in the 460 nm range, plants tend to have a more compact growth habit. Also, blue is responsible for chlorophyll production more than other wavelengths. Metal halide (MH) grow lights are full spectrum lamps, rich in the blue end of the spectrum, and they are well suited for lettuce production. Advances in LED technology are also proving beneficial for indoor lettuce production.

Some growers like to warm their nutrient solution in the winter, especially in NFT systems where water is continuously circulating over the roots. By holding the root temperature at about 75 degrees F, it is often possible to lower air temperatures without sacrificing quality or yield. Since fuel costs for greenhouse temperature control can become significant in the winter, heating the nutrient can save production costs. Just make sure that the nutrient temperature does not exceed 78 degrees. The warmer the water, the less dissolved oxygen the water can hold, setting up anaerobic conditions that can promote root rot.

Since lettuce is a vegetative crop, a well-balanced “grow formula” is all that is necessary to produce a high-quality crop. Grow formulas are proportionately higher in nitrogen than bloom formulas, usually with an N-P-K ratio of about 3-1-5. The nitrate form of nitrogen is usually preferred, with no more than about 3-10% of the available nitrogen in the ammonium form. Ammonium ions are rapidly taken up and utilized by the lettuce plants for fast vegetative growth, but too much ammonium nitrogen results in “rank” growth, producing soft, weak tissue with poor shelf life. Nitrate nitrogen is utilized more slowly by the plant, producing stronger, healthier lettuce plants.

Unlike tomatoes and other fruiting crops which can handle high levels of nutrient salts, lettuce has a low salt tolerance. Therefore, when growing lettuce it is beneficial to use a clean, slightly soft water source with relatively low levels of minerals and very low levels of sodium. During the summer months, the total EC of the nutrient solution should be maintained at 10 cf or below. During the winter months, with lower light levels, the EC should be raised a little to help prevent stretching, but the grower should still try not to exceed a total cf of 12. By starting with raw water with a 2-4 cf and adding only a mild nutrient formula, a good balance can be maintained between fast growth and top quality.

Amino acid blends may also be beneficial in a complete nutrition program for lettuce. Amino acids are intermediate chelators, improving the availability of calcium and other important cations. When phosphoric acid is added to hard water, the bicarbonates are burned off as carbon dioxide and water, but the calcium ions react to form calcium phosphate. Calcium phosphate is what bone is made of, and it is 95% water insoluble. So calcium phosphate locks up both calcium and phosphate, making them unavailable to the plant. When amino acids are added, however, the calcium remains available. Primary amino acid chelators, such as glutamic acid and glycine, attach to the calcium ions like a claw, preventing them from reacting with the phosphorous. At the same time, glutamic acid and glycine stimulate root cells to open up calcium ion channels, taking up calcium thousands to millions of times faster than simple osmosis! Inside the plant, the calcium is used to strengthen the cell walls and help protect the lettuce from temperature stress, pathogen attack and common physiological diseases such as tip burn.

By Harley Smith