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.
The three stages of early development are germination, post-emergence, and transplant. Germination should occur within one week of seeding, post-emergence is generally 5 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 (3 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.
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 equitorial 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.