hydroponic education lettuce

8/10/2019 Hydroponic Education Lettuce

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HYDROPONIC LETTUCE PRODUCTION

2013 Harley Smith

All rights reserved.

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, givinglocal growers who pick fresh and deliver daily a competitive advantage. For example, to

ship 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, themore 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, butthere 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 timesand 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 ismicro-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 lettucesand leafy vegetables can be successfully grown in hydroponic systems.

Lettuce Growing Systems

Around the world, different styles of hydroponic systems are used for growing lettuce. In

Japan, for example, lettuce is produced in warehouses under artificial light, growing

crops from seed to harvest in only 30 days. The lettuce is grown aeroponically in A-frames with nutrient solution sprayed on the bare roots. Computers automate the entire

growing process, even the speed that the air flows over the crop, and manual labor is only

used for sowing and harvesting. Workers wear gloves, hats and masks during harvestingand the produce is so clean that it doesnt even have to be washed before delivery to local

restaurants. Since the lettuce is grown indoors in filtered air, the produce can be certified

as bacteria free, allaying health concerns over potential bacterial contaminants.

In Canada, some large commercial facilities use a raft system for producing lettuce. The

lettuce is grown in Styrofoam boards that float in a pool of nutrient solution held in wide

channels. The rafts of lettuce start at one end of the channel and are pushed down the

channel as the lettuce grows. The lettuce is transplanted several times along the way,allowing for wider spacing of the growing lettuce in the rafts. Finally, it exits the channel

at the far end for packaging and shipping. The system is very efficient since there are no


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rows between the channels, but extra care must be taken to insure proper aeration, disease

control and nutrient solution management in the deep pools.

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 solutiontrickles over the bare roots. The nutrient solution is stored in a reservoir, and asubmersible 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 solutionover 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 norunoff of concentrated fertilizer salts to pollute the environment.

Propagation

Lettuce is usually propagated in rockwool starter cubes or Oasis cubes. A sheet of 98

one-inch rockwool starter cubes fits perfectly in a standard nursery tray. Before planting,

the sheets of rockwool are soaked overnight in rockwool conditioning solution with a pHof about 5.5. The conditioning solution neutralizes the lime used during the

manufacturing process, and it provides a small amount of vitamins, minerals and root

promoters to help the lettuce get off to a good head start. After conditioning therockwool, the excess solution is drained so that the starter cubes are not sitting in a pool

of water, and one lettuce seed is placed into each starter cube. Lettuce seeds are quite

small, so commercial growers sometimes use seeds that are coated with clay. The clay

coating makes it easier to use automated seeding machines, and it holds a little moisturefor good germination rates. Most hobbyists, however, simply plant the lettuce seeds

directly into the moist rockwool starter cubes or into pre-soaked Oasis foam cubes.

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 cubes with a plastic sheet or humidity dome, place under cool fluorescent lightsand 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.

Most NFT systems use net pots to hold the seedlings. The roots grow through thebottoms of the net pots directly into the flow of nutrient solution. So make sure that there

are enough roots coming out of the net pots to touch the flow of nutrients. Sometimes

capillary matting is used to ensure that the newly planted roots remain moist. Capillary

matting is simply a piece of felt or other absorbent material that insures even moisteningof the roots. After initial rooting, however, capillary matting makes little or no difference

to the growth of the plant, and it can actually become a hindrance when harvesting lettuce

roots on. Another option is to place the net pots into an aeroponic system for a fewdays before planting into the NFT system. The net pots are suspended in air, with


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nutrient solution sprayed directly onto the roots. Roots develop rapidly during this

transition phase, and the use of capillary matting can be avoided. The lettuce plants only

need enough root development to easily reach the nutrient flow in the troughs. It is notnecessary to wait for excessively long roots to develop. The excess roots will simply die

back and new roots will develop in the NFT channels.

NFT System Design and Setup

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 theNFT 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 dontdam 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 channelsare only 4to 6 in length, so there is littledepletion 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 isalso 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 side of the stand should be about one inch higher than the other side. In longersystems, 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 millimetersdeep. Therefore, it is important to keep the width of the stands level so that the nutrient

flows evenly and doesnt trickledown 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 15gallons 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 verticaldistance the water must be pumped; the higher the head height, the lower the total volume

of water being pumped. Also make sure that the emitter tubing is wide enough to allow

adequate flow. Since water pressure is not as important as water flow rate in an NFT

system, it is often better to increase the size of the pipes and tubing instead of increasingthe capacity of the pump!

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 fullgrown 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 shadeout light they also restrict proper air flow and promote fungal disease. Since plant


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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 thenursery areas and finishing areas, scheduling can be planned to provide just-in-time

deliveries without wasting valuable resources.

Nutrient Solution Management

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 nitrogenthan 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 rankgrowth, 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 bemaintained 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 goodbalance can be maintained between fast growth and top quality.

If hard water from a well is your only option, a special hard water nutrient formula may

need to be used. To compensate for the high levels of calcium and magnesium carbonatein the hard water (usually over 150 ppm), hard water formulas have lower levels of

calcium and magnesium than standard formulas, and the magnesium is provided as its

nitrate salt instead of as magnesium sulfate. Hard water is often very high in sulfur,giving it that rotten eggsmell, so adding more sulfate to the nutrient solution can be

detrimental. Since sulfur is taken up slowly by the plants, sulfate levels tend to build up

quickly in the nutrient solution, reducing the availability of other essential elements.

Hard water nutrients are formulated to be more acidic than standard formulas. Hard

water buffers pH more than standard water, so using a more acidic nutrient formula

reduces the amount of acid that has to be added to reach the target pH of 5.8 to 6.4. Thetype of acid used to lower pH is also important, especially in hard water. For commercial

lettuce growers, nitric acid is the acid of choice. Nitric acid burns off the bicarbonates as

carbon dioxide and water, and it doesnt form insoluble lime scale in the process.

Unfortunately, nitric acid is a very aggressive chemical and extreme caution must betaken when handling it! Even the smallest spills can result in severe burns, and breathing

the fumes can permanently damage the lungs! For the hobbyist, using a mild phosphoric

acid to lower pH is a much safer choice. Although gloves and eye protection are


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recommended, there is little chance of serious injury when using dilute phosphoric acid to

reduce pH.

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 areburned off, but the calcium ions react to form calcium phosphate. Calcium phosphate iswhat 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 chelators, such as glutamic acidand 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 andhelp protect the lettuce from temperature stress, pathogen attack and common

physiological diseases such as tip burn.

When a complete nutrition program is followed for lettuce production, including both

mineral and organic components, 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. An objective measurement of how well your plants are taking upwater and minerals can be obtained by using a brix refractometer, an instrument that

measures the sugars and dissolved solids in the sap. But many experienced chefs and

produce buyers can detect superior quality simply by looking at it. Once your nutrientsolution management program is developed and fine-tuned, a foundation can be laid for a

continual harvest of gourmet-quality produce.

Environmental Control

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, the stomata in the leaf cells close and transpiration is reduced. Calcium uptakebecomes 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 moisture from accumulating around the leaf surfaces and closing the

stomata. So if temperature and humidity levels are within reasonable limits, oscillatingfans 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 temperatureand relative humidity in the room. Once relative humidity reaches a preset threshold,


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blower fans are actuated and the humid air is exhausted from the greenhouse or grow

room. Since warm air holds more moisture than cool air, it is often desirable to purge the

humid air before cooler night temperatures set in. Many advanced computer controls forgreenhouses include a purge cycle. The temperature of the zone is first heated by a few

degrees to allow the air to hold more moisture, and then the humid air is purged from the

room. When the air cools, the relatively dry air does not condense moisture on the plants.

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. Ingreenhouses, shade cloth is often used in the summer to keep the lettuce cool, but in some

climates it may be necessary to install cooling pads such as swamp coolers. Swamp

coolers, also know as evaporative coolers, blow air through a honeycomb-structured wall,

over-flowing with a trickle of water. As the water evaporates, heat is removed and thegreenhouse is cooled.

Winter poses another set of problems. 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 lightat 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 forchlorophyll 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.

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 withoutsacrificing quality or yield. Since fuel costs for greenhouse temperature control can

become significant in the winter, heating the nutrient can save production costs.

Value-Added Products

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


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