HOW TO GROW HYDROPONIC LETTUCE
By Harley Smith
Lettuce is a great crop for year-round, hydroponic 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 refrigerated containers could cost more than 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, 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.
Lettuce is usually propagated in rockwool starter plugs. A sheet of 200 one-inch rockwool starter plugs fits perfectly in a standard nursery tray. Before planting, the sheets of rockwool are soaked in a conditioning solution with a pH of about 5.5. The conditioning solution neutralizes the lime used during the manufacturing process, and lowers the pH of the rockwool to about 6.2. After conditioning the rockwool, the excess solution is drained so that the starter plugs are not sitting in a pool of water, and one lettuce seed is placed into each plug. Lettuce seeds are quite small, so it’s sometimes difficult to put only one seed in each hole. If several seedlings pop up in a plug, simply prick out the extras and leave the best plant to grow on.
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 starter plugs with a plastic humidity dome, place the tray near a window or under cool fluorescent lights, and wait a few days for germination. Once the roots begin to poke out through the bottom of the plugs, they are ready for transplanting into the NFT system.
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 a manifold at 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 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.
Care should be taken when designing and building an NFT system to make sure that the proper combination of water and aeration are delivered to the roots. The bottoms of the NFT troughs should be flat so that no puddling of the nutrient solution occurs. Even a little sagging or depression in the troughs can cause aeration problems. Flat bottoms also insure that the roots fan out in the nutrient solution so that there is plenty of surface area for air exchange. The troughs should also be wide enough so that the growing roots don’t dam up the flow. Areas of stagnant water can reduce aeration and promote root disease.
Attention should also be placed on gulley length. In a small hobby system, NFT channels are only 4’ to 6’ in length, so there is little depletion of dissolved oxygen and nutrients over the length of the gulley. Much longer gullies are used in commercial production. Gulley length should never exceed 60 feet. If the gullies are too long, plants will deplete the oxygen along its length and the plants at the far end of the rows will suffer. Slope is also important for nutrient flow and aeration. When building stands for the NFT system, make sure that there is enough slope for a good gravity feed. In a four foot NFT system, one end of the stand should be about one or two inches higher than the other end. In longer systems, a slope of 50:1 or 75:1 should be adequate.
Flow rate and distribution is also important to successful lettuce growing. In NFT, the nutrient solution flows over the roots in a film not more than a couple of millimeters deep. Therefore, it is important to keep the width of the stands level so that the nutrient flows evenly and doesn’t trickle down only one side of the gulley. Flow rates vary according to the width and slope of the gullies, but generally speaking, a flow rate of 15 gallons per hour is adequate for about 50 feet of 4-inch NFT channel. When calculating pump size, make sure that you take into account the height of the NFT stands. Centrifugal pumps rate gallons-per-hour according to the “head height” or vertical distance the water must be pumped; the higher the head height, the lower the total volume of water being pumped.
The spacing between lettuce plants depends on the size of the plants being grown. Generally speaking, spacing plants on 7 inch centers is adequate for finishing most full-grown lettuce plants. Nursery gullies with closer spacing between plants are sometimes used to start lettuce plants and conserve growing space. Just make sure that there is adequate space to keep the leaves from overlapping. Overlapping leaves not only shade out light; they also restrict proper air flow and promote fungal disease. Since plant density is a factor of how many mature heads of lettuce are to be harvested and delivered per day, cycle times are more important for calculating space requirements than total amount of plants produced. By properly calculating how much time plants spend in the nursery areas and finishing areas, scheduling can be planned to provide just-in-time deliveries without wasting valuable resources.
Nutrient Solution Management for Lettuce
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 may be raised a little, 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.
pH control is also important for lettuce production. A pH test kit should be used to check the pH of the nutrient solution daily, and pH adjustment chemicals should be used to keep the pH in the proper range. A slightly acidic pH range of 5.8-6.4 is recommended, with 6.0 as a good target pH. If the pH is too high (alkaline), use a pH Lower solution containing a mild acid. If the pH is too acidic, use a pH Raise solution containing a mild base such as potassium hydroxide. Although gloves and eye protection are always recommended, there is little chance of serious injury when using mild pH adjustment solutions.
For best results, the nutrient solution should be drained and refreshed periodically. When growing indoors under lights, replace the nutrient solution every ten days to two weeks. In between reservoir changes, check the pH and EC daily and make adjustments as necessary. If the EC gets too high, especially on hot days, simply top off with water. Just make sure that all of the nutrient solution is completely replaced from time to time. If you never drain the reservoir and just keep adding water and nutrients, the nutrient solution will get out of balance over time and the plants could begin to suffer from nutrient deficiencies or toxicities. When a complete nutrition program is followed for lettuce production, it is possible to produce a consistently superior product. Color, texture and flavor can be enhanced, with the added benefit of improved vitamin and mineral content.
Winter growing presents a unique set of challenges. 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, even in a greenhouse. Plants need full spectrum light, with both the blue and red ends of the spectrum for vigorous growth. Metal halide (MH) grow lights are full spectrum lamps, but they are particularly rich in the blue end of the spectrum. During the vegetative growth stage, plants prefer the blue end of the spectrum; so metal halide lights are well suited for lettuce production. Blue light greatly influences “phototropism”, the tendency of plants to lean toward the light. So if lettuce receives enough blue light, the plants will tend to have a more compact growth habit. Also, blue is responsible for chlorophyll and pigment production more than other wavelengths, so lettuce plants grown under MH lamps will typically have better color.
When growing lettuce indoors with no sunlight, a 400-watt MH lamp will cover about a 4’ X 4’ area. Keep the light about one to two feet above the garden and raise the light a little as the plants grow. Ratcheting light hangers make raising and lowering lights easy. A mechanical timer can be used to control the day length. For best results, use a heavy duty 15-amp timer, and set the timer to provide at least 12 hours per day of continuous light.
Temperature and humidity control are also important to successful lettuce production. Tip burn is caused by calcium deficiency, but high humidity can limit calcium uptake, even when there is plenty of water-soluble calcium in the nutrient solution. Calcium is an immobile element, and must be taken up in the transpiration stream. Under high relative humidity, transpiration is reduced. Calcium uptake becomes insufficient to reach the newly-developing cells, and tip burn symptoms begin to appear. Once the cell walls begin to weaken and collapse, the plant also becomes more susceptible to fungal attacks, and since fungi thrives in humid conditions, disease can easily spread.
Ventilation is the first line of defense. Simply installing horizontal fans for air movement can help keep excess moisture from forming on the leaf surfaces. So if temperature and humidity levels are within reasonable limits, between 40 and 60%, oscillating fans may be enough to keep tip burn at bay. If temperature and humidity rise above acceptable levels, however, it may be necessary to install exhaust fans to remove the warm, moist air from the room. Thermostats and humidistats monitor the temperature and relative humidity in the room. Once relative humidity reaches a preset threshold, blower fans are actuated and the humid air is exhausted from the greenhouse or grow room.
Temperature control is also important. Lettuce is a cool weather crop, so long days and hot summer temperatures can cause the crop to “bolt” or go to flower. Once the lettuce starts to bolt, the quality rapidly declines and the lettuce can become bitter. If too much heat becomes a problem, try using air-cooled reflectors on your lights. The reflectors have a 6” duct that allows you to add a squirrel cage fan to the light. More than 50% of the heat of the light can be exhausted from the light fixture before entering the room.
For optimal results, the temperature of the nutrient solution should also be monitored. The ideal water temperature is between 68 and 75 degrees F. If the water temperature gets too high, the nutrient solution will not hold enough dissolved oxygen, but if the temperature is too low, the metabolism of the roots is slowed down and plant growth can stall. Some growers like to warm their nutrient solution in the winter, especially in NFT systems where water is continuously circulating over the roots. By maintaining the root temperature at about 75 degrees F with a submersible heater, 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 energy costs.
Fancy hydroponic lettuces and leafy vegetables can provide valuable products for niche markets. For example, some leaf lettuces grown in NFT systems can be harvested roots-on and sold as “living lettuce”. Special clam shell packaging is already available for individual heads, and the advantages of gourmet quality, guaranteed freshness and extended shelf life allow the lettuce to be sold at premium prices.
Many of the bagged salad mixes on the market today are grown hydroponically. The lettuce blends are thoroughly washed, cooled and vacuum packed to provide a convenient ready-to-serve product. Sometimes the blends are supplemented with fancy vegetables, red oakleaf lettuces and micro-greens to add color and texture to the mix. By growing and marketing a unique blend of savory herbs and greens to meet local market demand, and by providing hard-to-find produce for specialty and ethnic markets, repeat customers can be cultivated.
After-market products, such as “private-labeled” salad dressings can add value to locally-produced lettuces. Fresh salad dressings with expiration dates can be delivered along with the fresh produce, adding to the gourmet perception of your business. To further extend profits, many of the fresh herbs included in the dressings can be grown alongside your main lettuce crop, and even the trimmings from the herbs can be processed and utilized. Many growers have developed highly profitable businesses from products regarded as after-thoughts!
For questions about growing lettuce hydroponically, please contact:
Sue or Harley N. Smith
111 E. Knight St.
Eaton Rapids, MI 48827
(517) 763-8165 (Harley’s cell phone)
(517) 712-8097 (Sue’s cell phone)
(517) 441-9317 (office)