Growing Tomatoes Hydroponically (Part Two)

By Dr. Merle Jensen

Greenhouses

There are many factors to consider in determining the amount of greenhouse space you will need. Amount of investment capital, training, the type of business, environmental requirements, market, labor requirements and personal preferences must all be evaluated. You should also be aware of factors which are important in choosing a good building site, such as drainage, accessibility, available utilities and amount of sun exposure.

There are companies who sell greenhouse packages which contain everything needed for turnkey operation. Greenhouse tomatoes with indeterminate growth habits are best managed in houses with high roofs. The structural design of a greenhouse must provide protection from wind, rain, heat and cold. The structural supports must be of minimal size to permit maximum light transmission to the crop while still supporting the structure itself, heating and ventilation units and the weight of the crop which is trained to grow up a support system carried by the greenhouse frame.

There are a variety of types of greenhouse covers. Glass is still a common glazing material. Large panes reduce the shading of the crop from the support frame. While shading may seem minimal in traditional greenhouses, it is estimated that every 1% decrease in light will result in a 1% decrease in yield.

Despite the common use of glass as a covering for greenhouses in Northwest Europe, glass remains inflexible, heavy, and expensive. Consequently, the hectarage of glasshouses on a world basis has remained static, (approximately 30,000 ha.) during the last 25 years. In contrast, the quantity of plastic used for greenhouses is increasing rapidly. Since polyethylene sheet film was first developed in England in 1938, it has been used widely in greenhouses because it is easy to work with and inexpensive. Worldwide, there are nearly 300,000 ha. of plastic greenhouses for growing high value horticultural crops.

Several other plastics have also been used for greenhouse glazings. Polyvinylchloride (PVC) has a very high emissivity for long wave radiation (similar to glass), which creates slightly higher air temperatures in the greenhouse during the night. The Japanese consider this improvement in thermal environment a benefit that outweighs the price advantage offered by the less expensive polyethylene (PE). The disadvantage of PVC is its narrow width as compared to PE, which may be manufactured in widths of up to15 meters. The narrow PVC sheets can be heat-welded together to form a large sheet, but this adds to the cost of the glazing material.

The large sheets of PE can be applied as an air-inflated “blanket” over a greenhouse: two sheets of PE are separated by air pressure maintained by a small continuously running fan. This arrangement provides approximately 30-40% heat savings during winter. The double-layer, air-inflated roof has also proven valuable in regions with high winds or typhoons. It offers stability during these conditions, saving the greenhouse and the crop during times when structures covered only with one layer of plastic are often lost and the crops damaged or destroyed. PVC film is not suitable for air-inflated roofs because the air pressure stretches the film and reduces its structural strength. Because PVC film is not photodegradable, as is PE, environmental concerns about disposal may diminish the use of PVC in Japan in favor of PE, which is the predominant cover for greenhouses worldwide.

New materials such as double-skinned panels made of polycarbonate and acrylic are becoming increasingly popular. Unfortunately, their technical merit is offset by high costs, making them affordable only in the industrial nations of the world rather than in developing agricultural communities.

The ideal greenhouse “selective film” should do the following:

1. Transmit the visible light portion of the solar radiation spectrum, the only portion utilized by plants for photosynthesis.
2. Absorb the small amount of ultraviolet radiation in the spectrum and cause some of it to fluoresce into visible light, useful to plants.
3. Reflect or absorb infrared radiation , which plants cannot use and which cause greenhouse interiors to overheat.
4. Minimize costs, and have at least a 10 to 20 year useable life.

Environmental Control Systems

Light

Photosynthesis is the key to good growth and high yields. If photosynthesis is decreased, due to low light conditions, high humidity (which closes stomates and reduces gas exchange), or water stress, then the production of sugars will decline and the fruit quality, shelf life, and size will all diminish.

Because of the critical role of photosynthesis in plant growth, a one-percent decrease in light can translate to a one-percent decrease in yield. Shading from outside topography and trees, the greenhouse structure itself, or taller plants in the greenhouse can significantly reduce the amount of light reaching the crop. Both the greenhouses and the rows of plants in the greenhouse should be oriented north and south so the light is evenly distributed across each plant. Some growers reflect light back into the crop using white floorcoverings or paint. Clean white paint is more reflective than metallic or foil, although there is some indication that foil tends to “confuse” insects and slightly decrease insect pest damage.

During long periods of cloudy weather, tomato leaves become low in sugars, and may become pale and thin. Excess nitrogen at that time can be detrimental.

Some growers prefer to shade tomatoes, while others do not. Theoretically, shading will reduce photosynthesis, and therefore total yield, however, this has not always been shown in controlled studies. In fact, in some studies, total yield was improved using 30% shadecloth. Shading can improve fruit quality, since direct sunlight on fruit can cause yellow or green shoulders, cracking, and russeting. Alternatively, older leaves can be left in place to shade the individual fruit trusses. In areas of high summer temperatures and humidity, shading may be necessary to keep temperatures within a reasonable range. Ultimately, however, the decision to shade or not depends on the location of the greenhouse, the cultivar of tomato grown, the season and the overall management system employed by the grower.

Historically, the greenhouse industry has traditionally measured light in foot-candles and lumens. Foot-candles are the amount of light received on the surface and lumens are the measure of light emitted by a light source. Natural sunlight and artificial light falling on a plant are measured in foot-candles (f.c.) while the light emitted by sources such as the sun and electric lamps are rated in lumens. A clear, sunny day may measure 10,000 f.c. and an overall winter day as low as 500 f.c. To read comfortably requires about 20 f.c. The light of the full moon measures less than 1 f.c. A light meter with a scale in direct foot-candle readings is manufactured by the General Electric Company and is sold by most greenhouse supply companies.

Supplementary artificial light, from cool white, high output fluorescent or high intensity discharge sodium vapor lamps is beneficial to plants when sunlight is unavailable but is not a complete substitution. Intensity of supplementary lighting should be about 800-1000 foot-candles at the plant surface.

Today, plant scientists are primarily interested in that light which is responsible for photosynthesis. The portion of the light band most responsible for photosynthesis measures 400-700 nanometers. This band is often termed the Photosynthetically Active Radiation (PAR). Within this range, intensity is the most critical factor along with light period. Within the PAR region light is measured as the Photosynthetic Photon Flux (PPF) and is expressed in µmol/m2/s. Daily total of PPF, expressed in mol/m2 have been shown to relate to total photosynthesis for the day.

In the southwestern region of the United States, the winter light readings are three times higher than in the northern regions such as the states of New York and Ohio. This is why the greenhouse tomato industry is growing so rapidly in Arizona and surrounding states.

Supplemental lighting is generally not economical for vegetable crops, with the exception of seedling production. However, for backyard or hobby situations, full-spectrum lighting can be effective in increasing yields by increasing the daylength to 18 hours during winter months.

Temperature

Both day and night temperatures influence plant vigor, leaf size, leaf expansion rate, and time to fruit development. Under low night temperatures, the rate of leaf growth is slower, and leaf size is reduced in young plants. Day and night temperatures should be carefully monitored. A general rule of thumb for most horticultural crops is for night temperatures to be approximately 5.5° C (10° F) lower than day temperatures. For tomatoes, day temperatures should be 21° -26° C (70° -79° F) and night temperatures around 16° -18.5° C (61° -65° F), although many new varieties do best with little difference between day and night temperature (check with your seed company for recommended growing temperatures). For seedlings, the temperatures should be constant, 20° -22° C (68° -72° F), then gradually acclimate the plants to the diurnal temperatures before transplanting.

High temps in excess of 30° C to 35° C will cause many different types of damage to the plants, such as inhibition of growth and even death. The physiological nature of heat damage is thought to involve a denaturation of some protein component of plant cells. Fruit abortion may occur at these temperatures as well. Temperatures lower than optimum will alter the plant metabolic systems to slow growth and again hinder fruit set.

Fogging systems can be an alternative to evaporative pad cooling. They depend on absolutely clean water, free of any soluble salt, in order to prevent plugging of the mist nozzles. Like fan and pad cooling, fog cooling is only really efficient in low humidity environments.

In hobby greenhouses, temperatures can be measured easily with a minimum/maximum thermometer. Several thermometers should be placed throughout the greenhouse, and should be calibrated against each other and a quality thermometer at least twice per year. In large commercial operations, computer controlled systems are common. Such systems can provide fully-integrated control of temperature, humidity, irrigation and fertilization, carbon dioxide, light and shade levels.

Air Circulation and Ventilation

Good circulation is necessary for proper cooling, heating, CO2 replenishment, and removal of undesirable gases, such as ethylene. Your circulation system must work together with your heating, cooling, and CO2 systems in order to obtain peak efficiency.

Many different methods of circulating air have been developed. The vent-tube system is used quite a bit, and consists of a fan-jet connected to a perforated plastic tube running the length of the greenhouse at ceiling height. The fan forces air through the tube, which moves the warm air in the roof space downward to displace the cooler air at the floor level. This design is not very efficient. A horizontal airflow system is more efficient, and can move a larger amount of air around the plants. Large fans, hanging above the crop, are set up facing one direction in one section of the greenhouse, and in the opposite direction in the adjacent section of the greenhouse. A more complicated system is a vertical airflow system, which uses fan-jets to move air along the roof, downward at the end walls, then along the floor through the crop. This system provides the best mixing of air and brings warm air down into the plants. Various types of alternative ventilation systems have been proposed, such as up-draft and down-draft chimneys. However, it will be some time before these systems are thoroughly tested and refined.

In the tropics, natural air exchange to the outside of the greenhouse can be achieved simply through the sides of the greenhouse structure. For active or mechanical ventilation, low-pressure propeller blade fans are used for greenhouse ventilation. They are placed on the end of the greenhouse opposite the air intake, which is often covered by evaporative cooling pads and louvers. The cooling pads used in combination with fans (fan and pad cooling) can be made from a number of materials. Most often they are made of a cellulose material, usually aspen wood, or a multi-celled/honeycombed material called “kool-cel”. The ventilation fans for larger greenhouses (100-120 feet in length) are normally sized to allow a maximum air exchange once per minute. Small hobby greenhouses, which have a large greenhouse surface area to floor area ratio, may require an air exchange of up to 2.5 times per minute.

Humidity

In order for a plant to actively grow, it must be allowed to transpire freely during photosynthesis; this means plenty of available water, low to moderate humidity, and good air circulation. Humidity influences calcium uptake and hormonal distribution by controlling transpiration, ion pumping, and stomatal opening and closing. High humidity coupled with low air movement reduces transpirational cooling, and can lead to heat overload for the plant.

People tend to think of humidity in terms of relative humidity, which is the ratio of the amount of water vapor in the air to the amount of water vapor the air could hold at that temperature, expressed as a percent. Plants, on the other hand, perceive humidity in terms of vapor pressure deficit (VPD). VPD is the difference between the vapor pressure in the air and the vapor pressure inside the leaf. Water moves by diffusion from the roots through the plant and out the leaves as transpired vapor, thereby being “pumped” up the plant as the vapor moves from the higher pressure inside the leaf to the lower pressure in the surrounding air. Low VPD (high humidity, greater than 90%) is often responsible for nutrient deficiency symptoms, such as blossom end rot (calcium deficiency) because the plant is not transpiring, therefore it is not drawing water, or nutrients, into the roots. High VPD (low humidity, less than 50%) can also lead to the same symptoms, because water and nutrients are pumped too quickly through the plants, depositing nutrient ions in the leaves rather than properly in the fruit.

Greenhouse humidity can be measured with a sling psychrometer. Other equipment such as a humidistat can measure relative humidity to an accuracy within 4%. Most greenhouse supply companies sell equipment to measure humidity.

Most plants can function adequately in relative humidities of between 55 and 95%, which corresponds to VPD’s of 1.0 to 0.2 kPa. For tomatoes, the ideal humidity should be between 65 and 75% during the night and 80 to 90% during the day. Tomato yields and fruit quality are lower at lower VPDs (higher humidity). Leaf size can also be reduced, and flower and fruit abortion can be significantly increased under high humidity conditions. Glassiness and “gold fleck” in tomato fruit is also attributed to high atmospheric humidity.

Misting and fogging systems are used by some growers to increase humidity and decrease temperatures. However, if used improperly, these systems can greatly increase the incidence of mildews and plant diseases, not to mention corrode metal greenhouse structures.

Propagation

Seeds

Several tomato varieties have been specifically developed for hydroponic production in controlled environments. All varieties have indeterminate morphology; meaning vegetative growth of the plant is continual and does not stop once flowering begins. This creates long tomato “vines” which must be trained up strings hanging from the greenhouse structures to maximize space and manage the crop. Some of the more popular varieties are Apollo, Belmondo, Caruso, Dombito, Larma, Perfecto, Trend and Trust. These are hybrid varieties, and the seed can be rather expensive. This may lead some novice growers to consider germinating seed from mature fruit, but those successive generations will not necessarily have the same characteristics of the parent plants. Some hobbyists prefer to grow successive generations from vegetative cuttings, producing genetic clones from the original plants. This is okay on a small scale, however, the high risk of perpetuating a latent disease or pest problem on a large scale outweighs the cost of new seed.

Starting Media and Nutrients

Any propagation medium must be thoroughly soaked before seeds are sown to assure uniform distribution of moisture. There are many different propagation media available.

Seeding trays can be filled with a soilless mix, such as peat and perlite. Peat pellets are also popular starters. Seedlings grown in a soilless mix may have enough nutrients available to them from the media that they would not need any additional nutrients for the first few weeks of growth, and therefore could be watered with fresh water only. However, seedlings in an inert medium, such as rockwool or oasis, will definitely require nutrient solution at all times.

Rockwool blocks are available in several sizes, and are designed so that seeds can be placed directly into seeding cubes, then, as the plants develop, the cubes can be nested inside larger blocks, for a “pot in a pot” system. This minimizes transplant shock, since the larger block consists of the same material as the germination cube. Oasis horticubes are similar to rockwool cubes in that they are inert, sterile blocks with excellent drainage. Other cubes made of urethane foam and paper fiber are also available.

Tomato seeds should be sown 1/4 to 3/8 inch (0.6 to 1 cm) deep. Sprinkle a thin layer of vermiculite over the seeds or cover the germination cubes or pots with a large piece of clear plastic to conserve moisture at the surface. Avoid the use of plastic if the cubes receive direct sunlight, as the temperature may get too hot for good germination. The plastic must be removed as soon as emergence begins.

Seedling System Design

Overhead watering is the most common method used for germinating seedlings. It is important for the seedlings to be in full sun and at the proper temperature as soon as germination occurs. When watering, the water must be sprinkled uniformly over all seedlings to avoid uneven growth. The plants must be checked often to assure they do not become water stressed.

Flood and drain (ebb and flow) systems can also be very effective for germinating seedlings. Nutrient solution or water floods a shallow tray containing the sown cubes or pots, providing moisture from the bottom, which will diffuse throughout the propagation block by capillary action. Once the blocks are evenly moist, the tray is drained, which allows the cubes or pots to drain and assure aeration of the roots. This process will need to be repeated often throughout the day, but may not need to be done at all during the night. The advantages of this system are even moisture, no physical beating of the leaves and tender plants, and low labor costs (especially if timers are used).

In any event, the temperature of the irrigation solution should be at least 18° C (64° F). Irrigating seedlings with colder water will result in slower growth. During winter months, especially in Northern latitudes, supplemental light may be required for strong growth of seedlings. The lights should operate 14 to 18 hours per day.

Transplanting

The three stages of early development are germination, post-emergence, and transplant. Germination should occur within one week of seeding, post-emergence is generally five to 12 days, and transplanting should be done between 12 and 14 days from seeding. Once true leaves appear (during post-emergence), seedlings should be transferred into larger growing blocks (pots) from the original seedling cubes, then evenly spaced to maximize light to each plant, without any crowding or shading. The transplants must be spaced so as not to touch one another, and may need to be spread several times during their growth. If crowded, the plants will become spindly. A good transplant is one that is as wide as it is tall. If plants are somewhat “leggy”, with long stems, they can be transferred into the larger blocks with their stems bent 180° so the original cube is upside-down inside the larger block, and the main stem forms a “U” shape, emerging vertically upward from the block. Tomato plants readily grow adventitious roots from the stems if given the opportunity, producing a stronger plant with more roots. Adventitious roots will grow from the bent stem inside the block.

Transplanting into the final growing media should be done before any flowering. The final growing media should be properly leached and moistened and be at the proper temperatures before plants are brought in. Plants should be irrigated with nutrient solution immediately after moving.

The spacing of tomatoes in hydroponic systems can be much denser than in soil. As little as two square feet per plant (0.2 square meters per plant) have been used with good yields and quality under high light conditions. Spacing is a function of sunlight, so in areas of lower light wider spacing should be applied.

Indeterminate tomatoes must be trained up support strings immediately after transplanting. The strings should be hung from horizontal wires, which are connected to the frame of the greenhouse. These wires will need to support hundreds of pounds of weight, as each mature plant with fruit may weigh 20 to 30 pounds (7 to 14 kilograms). Additional vertical poles can be added to help support the horizontal wires. The wires and strings should be put in place before any other paraphernalia is brought into the greenhouse, and should be at least 10 feet (three meters) above the ground. The strings should not be re-used, however, a variety of clips are available which can be sterilized and re-used. As the plants grow, the strings are unwound from their hangers and moved along the horizontal wire, effectively “lowering” the plants without breaking them. Mature indeterminate tomato plants may be 40 feet (12 meters) in length, and can grow much more.

Double Cropping

Some growers prefer to grow two crops of tomatoes in the growing media before tearing the system down, cleaning and sterilizing, and starting again. In this management system, young plants would be planted in the media between the older plants, just as the older plants are reaching their maximum economic life span. This effectively overlaps the crops, increasing total annual yield. However, the older plants must still be completely removed to prevent buildup of disease and excessive shading of the new crop, and care must be taken to work around the younger plants. In high light regions of the world, such as deserts and equatorial latitudes, the first crop is generally planted in midsummer and lasts through to the end of the year. The second crop can be planted in January and continue through the end of June. Alternatively, one long crop planted in late summer or fall can be grown until July.

Be sure to read Part Three of Four of this article, Growing Tomatoes Hydroponically by Merle Jenson, in the next issue of Maximum Yield.

Dr. Merle Jensen is Assistant Dean, College of Agriculture, University of Arizona