Lettuce (Lactuca sativa)

Introduction: Lettuce (Lactuca sativa) is the fourth most important vegetable crop grown hydroponically in greenhouses proceeded by tomatoes, European cucumbers and peppers. Although greenhouse production of lettuce is very small in comparison to field grown, it has a specific market niche as a gourmet, high-quality item. Being clean and free of soil, it is particularly popular in restaurants. It is generally packaged in plastic bags or rigid plastic clamshell containers to display it as an attractive product on the supermarket shelves. The choice of packaging depends upon the marketplace. For the restaurant trade it is better to use plastic bags for lettuce without roots. With clamshell containers that present the product as “living lettuce” in a supermarket keep the roots in their growing cubes attached. When leaving the roots on use a special clamshell container with a depression in the bottom to contain the roots and growing cube (photo 1). The majority of hydroponic lettuce is bibb or European buttercrunch types. It can be marketed as “Hydro-Bibb” lettuce. Bibb lettuce has a soft head, not the dense compact head of “iceberg” or head lettuces. Some novelty leaf lettuces are grown hydroponically, but accounts for only a small portion of greenhouse lettuce. Varieties include: “Romaine” (“Parris Island”-green and “Freckles”-red), “Oakleaf” (“Cocarde”, “Berenice”- green; “Oscarde”, “Dano”- red), red curly varieties such as “Lolla Rosa”, “Ruby”, “Red Sails”, “New Red Fire”, “Brunia”, to mention a few (photos 2-3). During the longer days of summer months these lettuce will mature within 40 to 48 days. Seedlings are grown for 12- to 18-days as transplants and then 28 to 32 days to maturity.   Bibb Varieties: Since the majority of greenhouse hydroponic lettuce is of the bibb type, culture of this lettuce is specifically discussed. Some of the most suitable European bibb varieties include: “Deci-minor”, “Ostinata”, “Salina”, “Vegas”, and “Rex.” Choose your variety according to climatic conditions, especially light and temperature, potential disease infection and market. Ostinata, Vegas and Rex are suited to higher temperatures and are resistant to tip-burn. Lettuce is a cool season crop, so prefers cooler temperatures. It bolts rapidly at higher temperatures, especially if they exceed 80 to 85 F. “Bolting” is the lettuce shoots up to go to seed (photo 4). Bolting makes the lettuce non marketable. Salina is more tolerant to Pythium fungal root-rot infection, but is not resistant to it. It is often used during summer months, especially in warmer regions such as Florida. There are many different varieties available through different seed companies, so you should carry out a number of varietal trials to determine those that grow best under your specific conditions. We have found at Cuisinart Resort & Spa Hydroponic Farm that “Rex” is by far the most resistant to high temperatures of our tropical conditions and resists “bolting” under stress. However, we must harvest about 26 to 30 days after transplanting 18-day-old seedlings to prevent bolting. Any extended time after that causes rapid bolting. Temperature must also be taken into consideration when choosing a hydroponic system for growing lettuce. The raft culture system is generally better for higher temperatures as the root temperatures may be chilled below 70 F to slow bolting under high air temperatures of 85 to 90 F. STARTING THE PLANTS: Seedling Production: The growing system to which the lettuce seedlings will be transplanted determines what method of sowing is best. Substrates used include: rockwool cubes, Oasis cubes, and multipacks (celled trays) with a peatlite or vermiculite medium. The rockwool and Oasis cubes are available in dimensions of 1” x 1” x 1 1⁄2” and are held together in multicubes to fit a standard 10 1⁄2” x 21” flat or mesh tray (photo 5). The rockwool cubes come as 200 per sheet and the Oasis cubes as 162 per sheet. The small size of lettuce seed makes it difficult to sow by hand. To make this task easier you may purchase “pelletized” lettuce seed. It is more expensive, but is a lot easier to sow. Each seed is encapsulated in an inert clay-like material making it about 1/8” in diameter. This encapsulation not only makes sowing easier, but permits it to be done with automatic seeders. The clay coating also retains moisture and therefore reduces any potential desiccation should an irrigation cycle be missed. If you have thousands of seeds to sow daily you must go to an automatic seeding system which would include the use of a vacuum seeder that will sow an entire tray in a number of seconds. Such an automatic seeder is the “Vandana Tubeless Seeder” by Growing Systems, Inc. (photo 6). Templates for the seeder are available that match the number of cells in a growing tray or cubes in a multicube sheet. The company can also make custom templates to fit numerous tray configurations. Germination: Lettuce seed loses viability quickly with age, so be careful that all seed is dated. The seed must be stored in a refrigerator providing a cold, dry atmosphere. Under such conditions it can be stored for up to 6 months or longer. You can easily test its viability by placing 100 seeds between wet paper towels, place them at 65 F and count the seeds that germinate after 48 hours. This number that germinated divided by 100 will give you the percentage viability. Pelletized seed is more expensive than raw seed and does not keep as long, do no not purchase more than you can use within 6 months. Check the germination percentage on the package of seeds and use the germination test described above to determine how much overseeding is needed. For example, if the seed was tested as 90% germination, them you must overseed by 100/90 = 1.1 times. If you use vermiculite or perlite in multi-celled trays, sow the seeds first then moisten the substrate. However, if you use a peatlite medium you must moisten it before putting it into the trays, or it will retain dry pockets that will reduce germination. In most cases you will use rockwool or Oasis cubes. They also must be thoroughly soaked with a dilute nutrient solution of 0.5mS EC prior to sowing. For rockwool, use a pH of 5.2-5.4 to lower the pH of the rockwool that initially is 7.5 or greater. After germination use a solution of EC 1.5 mS. Lettuce requires cool temperatures to germinate. The seeded trays or cubes can be stacked and placed in a cooler at a temperature of 40 F (4.5 C). for 1 to 2 days to allow imbibition and initiation of germination. Once the seeds crack and begin to grow, immediately place them in the greenhouse at temperatures between 60 and 65 F (15-18 C). Seedlings: Keep the seedlings at temperatures from 64 to 70 F (18 – 21 C) during the day and 55 to 61 F (13 to 16 C) at night in the greenhouse. Carbon dioxide enrichment should be maintained at 1000 ppm during the day. The optimum pH of the nutrient solution is between 5.5 and 6.0 and the EC should be from 1.0 to 2.3 mS depending upon the light. Lower the EC during bright, sunny days. Maintain relative humidity (RH) from 60 to 80 percent. In northerly latitudes during the winter months supplementary artificial lighting is applied for 14 to 16 hours during cloudy days and to extend the day length. In high solar light regions sunlight can be reduced in the seedling area of the greenhouse by a 35% to 40% shade curtain. High temperatures can cause burn on the tips and margins of the leaves (tip burn). As mentioned earlier some varieties have resistance to tip burn. Seedlings are generally grown 14 to 21 days before transplanting to the hydroponic production area of the greenhouse. A lettuce operation, regardless of its size, must produce lettuce daily for the market. Planting schedules are altered according to the length of the growing period (longer in the winter months under shorter days). To achieve continuous production, the operation must include daily sowing of seed, transplanting, harvesting and clean up of the growing system. Transplanting: The lettuce seedlings are transplanted when they reach the 2- to 3-leaf stage (from 14 to 21 days) (photo 7). Be careful not to set trays or seedling cubes with plants on non-sterile surfaces such as the floor as this may introduce diseases such as Pythium. Multicubes are broken apart into individual cubes during transplanting. The cube with its seedling is placed into the hydroponic system of NFT channels or raft culture boards depending upon the system used. Be careful not to damage plant roots during transplanting as such damage predisposes the plant to disease infection. It is better to transplant in the late afternoon to avoid the plants getting stressed during the heat of the day under high solar conditions. The transplant will start to adjust to the new location during the night and roots will begin to grow into the solution below. When transplanting position the base of the plants so that they touch the flow of nutrient solution below. In this way, the solution will be absorbed by the base of the cube to keep it moist. Within several days the roots will extend out of the cubes into the solution and the plants will grow vigorously. PRODUCTION SYSTEMS: 1. Nutrient Film Technique (NFT): As lettuce is a short-term crop, maturing within 30 days or less it can be grown in water culture systems without suffering from oxygen deficit as can occur with many other long-term crops such as, tomatoes, cucumbers and peppers. All water cultures such as NFT and raft culture do not use a substrate apart from the cubes for starting the seedlings. These water culture systems are true hydroponics according to the definition of hydroponics which comes from the word “hydro-ponos” meaning “water working.” Low profile crops like lettuce, spinach, watercress and some herbs grow well in water culture. Nutrient film technique (NFT) is the most popular culture for growing lettuce. Special growing channels or gullies may be purchased or 2” diameter PVC pipes cut with holes at 6- to 7-inches apart will serve as the growing channels. The principle of NFT is to continuously re-circulate a thin layer of nutrient solution past the plant roots providing nutrients and oxygen. The floor of the greenhouse is covered with a weed mat barrier or a concrete slab poured to prevent weed growth and kept the floor clean. Benching to support the channels is placed on top of the weed mat or concrete floor. The channels are supported on a table made of galvanized steel piping to keep the channels at waist height to facilitate working on the plants and keeping them clean (photo 8). Inlet lines to the one end provide the nutrient solution (photo 9) and the pipes, which are sloped about 2 percent, drain to a collection pipe (photo 10) that returns the solution to a cistern tank. The piping for irrigation and return lines is above the floor level and attached to the benching. Gutter NFT: Plastic gutters specifically designed for this system are available commercially such as “Boxsell” and “Suregrow” NFT troughs. Both of these companies are based in Australia. NFT channels are also available from American Hydroponics (www.amhydro.com). These U.S. manufactured gullies are available in 12 ft. lengths with 1 3⁄4” holes on 8” centers. They also have a nursery gully that has 1 3⁄4” holes on 2 1⁄4” centers in the standard length of 12 feet. These gullies are 2” high by 4” wide. The nursery gully is used to transplant seedlings from the cubes to the gullies and hold them for about 10- to 14-days longer before transplanting to the final production gullies with the holes at 8” centers. The reason for this is to save time in the final production gullies. The growing cycle in the production gullies can be reduced by another 10 to 14 days through this second transplanting, therefore, the lettuce would be ready to harvest within 3 weeks from the final transplanting to the production gullies. However, this same principle of saving production space may be achieved by simply transplanting once into the final production gullies and keeping the gullies with the young plants spaced together (photo 11). Once the lettuce grows for 10 to 14 days from transplanting, simply space the channels apart to their final spacing of 6” between the gutters (photo 9). While some channels are available in lengths greater than 12 ft., nutrient and temperature gradients occur along the lengths that could cause lack of oxygen and Pythium infection due to the higher solution temperature. Ideally, the nutrient solution temperature should be maintained between 65 and 70 F. to provide adequate oxygenation and reduce fungal activity. This can be done with a chiller refrigeration unit in the cistern. However, if the gullies are very long the solution will heat up as it travels the length of the channel and may well exceed the optimum temperatures. The most practical length before returning the solution to the cistern is from 12 to 15 feet. You must take into consideration the greenhouse temperatures under which the lettuce is growing. In cooler climates where high temperatures are not a problem, channels could be longer. To get longer sections you can also have the catchment gutter in the center and slope the growing NFT channels from both sides to the central catchment return. The channels should have a minimum slope of 2% back to the catchment pipe. Higher slopes up to 10% have been used in some operations without any detrimental effects on the plants. In fact, one would expect that such greater slopes would be advantageous to plant growth as the nutrient solution flows faster past the roots giving better oxygenation and maintaining lower root temperatures. Such a system is present at “The Land” pavilion greenhouses at Epcot in Disney World in Orlando, Florida (photos 12-14). The optimum oxygen level for plant roots is 7 ppm. In most cases that level is hard to achieve, so levels from 4 to 6 ppm are considered adequate. These optimum levels of oxygen can be reached by use of an air pump and air stones in the cistern along with the returning solution falling from the inlet pipe into the cistern. A catchment pipe returns the solution to the cistern for chilling and sterilization before being returned to the inlet ends of the channels. Nutrient solution is pumped from the cistern through a 100-mesh and a 200-mesh filter in the main line before entering a header. The filters prevent clogging of the trickle feed lines attached to the upper end of each channel (photo 15). The solution flows through the channel to the catchment pipe at the lower end (photo 16). The catchment pipe directs the solution back to the cistern via a main return pipe. A filter or collection screen should be placed at the end of the return pipe to collect any debris from the growing channels before the solution falls into the cistern. In a re-circulating system incorporate methods of sterilization between the tank (cistern) and the mains feeding the growing channels to eliminate diseases. All such equipment must be installed downstream from the pump. An ultraviolet sterilizer will eliminate bacteria and some fungi. Ozone sterilizers are helpful in killing many fungi spores. A third component, a hot water sterilizer, heats the solution to kill the remaining pathogens. After heating the solution it must be cooled. The sterilization of the solution can result in changing the nutrient formulation by breaking down chelates such as iron. As a result, the addition of some nutrients may be necessary after the sterilization process. The electrical conductivity (EC) and pH are monitored after this sterilization procedure and adjustments made to the nutrient solution. To determine the changes in the nutrient solution that are occurring send nutrient solution samples to a laboratory for atomic absorption analyses. If you do this periodically during the crop you can relate the changes that occur with the level of EC. While this is not entirely accurate it will serve as a guide for adding those nutrients that are rapidly depleted. If the channel length is limited to 12 feet you can harvest by removing one at a time and placing them at the end of the bench (photo 17). Cut the lettuce at the crown with a sharp knife. The channels are then soaked for several hours in a centrally located sterilization vat containing a 10% sodium hypochlorite (bleach) solution. Be sure that the sterilization vat is not in the production area of the greenhouse as volatile fumes may injure the growing lettuce. Allow the channels to dry after removing them from the vat before re-using them for transplanting. After sterilization the channels are placed back onto the benches, connected to the inlet lines and seedlings transplanted into them. Initially, after transplanting the channels are positioned tightly together (photo 11). They are spaced several times in accordion fashion over the following weeks as the plants grow (photo 9). This technique saves bench space while the plants are still young. Some NFT troughs have ridges on the bottom to conduct the nutrient solution along the center of the trough so that the solution will contact the roots of the seedlings (photo 18). The cover of the trough may be easily lifted to inspect the health of the plant roots (photo 19). Healthy plant roots will be white without any brown coloration. If the roots begin to brown, you can expect fungal infection by Pythium. This can be a result of high solution temperatures and/or lack of oxygenation. PVC Pipe NFT: An alternative to using the commercial NFT gutters is to make your own system from 2-inch (5-cm) diameter PVC pipe. Cut round holes of the correct size to fit the growing cubes you are using spaced at 7 inches (18 cm) along the pipe. Be careful when drilling the holes that they align in the same position that will be the top of the gutter. The remainder of the system is set up as described for the other NFT channels using benching and an irrigation system. An inlet header and catchment pipe circulates the solution to and from the gutters to return it to the cistern (photos 20, 21). These systems will grow uniform, high-quality lettuce (photo 22).   NUTRIENT SOLUTION: A complete nutrient formulation provides all essential elements to the plants. Lettuce seedlings are fed a half-strength solution until they are transplanted. A half-strength solution contains about one-half of the concentration of macroelements, but the full concentration of microelements. The specific formulation to use is dependent upon temperature, daylength and sunlight. For example, during summer conditions with high sunlight and long days the plants can be forced to grow faster by use of higher nitrogen levels. Under low light levels the potassium and nitrogen should be reduced. A typical nutrient formulation has been taken from my book “Hydroponic Food Production”. This formulation can be used as a basis, which may be optimized for your specific conditions with experience. Ca: 180-200 ppm Mg: 40-50 ppm K: 210 ppm P: 50 ppm Ammonium-N: 15 ppm Nitrate-N: 165 ppm Fe: 3-5 ppm Mn: 0.5 ppm Cu: 0.1 ppm Zn: 0.1 ppm B: 0.5 ppm Mo: 0.05 ppm Use the best grade, highly soluble fertilizers of highest purity available. The optimum pH for lettuce is between 5.5 and 5.8. The EC of most lettuce formulations will be between 1.5 and 2.0 mS. The management of the nutrient solution is key to successful hydroponic growing. The availability of elements to the plants is dependent upon correct pH and the concentration and ratios of these nutrients in the solution. Some of the microelements such as, iron, zinc and manganese can be stabilized through the use of “chelate” forms. The best chelate for iron is FeDTPA that is more stable than FeEDTA. NUTRITIONAL & ENVIRONMENTAL DISORDERS, PESTS & DISEASES: Most of the pests that attack other crops will also infest lettuce. Thrips, whiteflies and larvae from moths and butterflies are the most common. Treat them as for other crops with biological agents. The worst disease of lettuce is Pythium. The most common nutritional disorder is tip burn. It is caused by excessive water loss from the leaves accompanied by inadequate water uptake by the roots. Some growers claim that high relative humidity (RH) in excess of 70% within the head of the lettuce will cause tip burn. Keep the air RH about 60% to prevent it. Tip burn causes necrotic areas on the leaf margin. It may also be caused by low calcium. Control is through proper nutrition, adequate water supply and healthy roots. Sufficient oxygen in the nutrient solution is important to maintain healthy roots as was discussed earlier. Check that the EC is not too high. Avoid excessive temperature fluctuations. Keep day temperatures under 80 F. Lettuce needs 16 hours of daylight. During winter months in the northern latitudes supplementary artificial lighting is beneficial to shorten the cropping period. Metal halide (MH) lighting is best for lettuce as it is a leafy crop. Optimum lighting conditions provide a minimum of 12 to 17 moles per 24-hour period. The light level should be from 400 to 800 micro moles. The important part is that the plants do receive at least 12 moles of energy per day. With higher intensity this amount of light energy may be supplied in less than 16 hours.