Genetics: The tomato (Lycopersicon esculentum) is one of the most popular crops to grow hydroponically. Those grown in greenhouses and controlled environments have been bred specifically for these environments. The four most popular types of tomato cultivars grown hydroponically are: 1) beefsteak, 2) tomatoes-on-the-vine, 3) cherry or cocktail, and 4) grape. The relationship between yield and sweetness (brix) is inversely related for these cultivars; beefsteak and tomatoes-on-vine have greater yields than cherry and grape tomatoes, but the lower-yielding varieties are sweeter compared to the larger fruits. Many of the tomato cultivars developed for hydroponic greenhouse production also are bred to have resistance to pathogens including bacterial speck, powdery mildew, fusarium and verticillium wilts, leaf mold, root-knot nematodes; and viruses such as tomato mosaic, tomato spotted wilt and tomato yellow leaf curl.
Production systems: As a high-wire vine crop, stone wool or coconut slabs are the most popular system for producing tomatoes (Fig. 2). For both stone wool and coconut coir, different products are available that vary in their capacity to dry down and rewet, which can be used to manipulate substrate moisture and steer crops between vegetative and reproductive (or generative) states. Dutch buckets filled with perlite or lightweight expanded clay aggregate (LECA) are also commonly used for tomato production, though the labor involved with washing the containers between crops limits their use to smaller facilities. Tomatoes can be grown in nutrient-film technique channels, though the larger root systems necessitate larger channels and can impede achieving the target flow rate of nutrient solution. Therefore, NFT systems should be avoided for commercial production.
Propagation and young plant production: Tomato plants are propagated from seed. Seed are initially sown in small propagation cubes (~50 to 200 seedlings per flat). With temperatures between 72 to 75° F, seeds will germinate within a few days. Depending on the time of year, providing supplemental light to avoid unwanted stretch is beneficial. Seedlings should be fertilized with a solution around 1.0 mS/cm after cotyledons have unfolded. Too much fertilizer can result in excessive growth. In addition to seed propagation, transplants may also be propagated vegetatively using grafting. First, seedlings of cultivars to be used as rootstocks and scions (shoots) are grown. Once the second true leaf has unfolded, seedlings are ready to graft. Both rootstocks and scions are cut at a 45-degree angle; rootstocks should be cut below cotyledons, while scions should be cut wherever the stem diameter matches that of the rootstock. After cuts are made, a grafting clip or tube is placed around the rootstock and the scion is placed in the clip that will hold tissue together. Once clips are placed on the grafted plant, they are placed in an environment conducive to healing for four days to one week, then exposed to ~100 µmol·m–2·s–1, with high humidity (= 95%) the whole time. Both grafted and non-grafted plants are transplanted into four-inch blocks of rockwool or coconut coir to grow more before transplanting into systems. Both fertilizer concentration and light intensity can increase after transplanting into blocks. Tomato plants are trained in a lean-and-lower system and training can begin during young plant production. Stakes are placed into substrate blocks at a 45-degree angle and the stems of young plants can be trained to these. This introduces an angle to the stem, so when stems are lowered later in production, the stem is not cracked and plant damage is avoided.
Nutrient solution: Once blocks with young tomato plants are transplanted into slabs or Dutch buckets, the strength of the fertilizer solution can be increased again while plants are establishing, and again once plants have established. A nutrient solution around 2.5 mS/cm can be used for mature plants in production, though this will need to be adjusted based on environmental conditions and plant uptake of nutrients. By monitoring the EC of the solution being provided to the crop and the EC of the leachate, you can determine how to adjust fertilizer strength. One important consideration is that as plants transition from flowering to fruit production, the nutrient solution composition should be adjusted in addition to EC. Developing fruits increase the demand for potassium and calcium, so the proportion of these nutrients in the fertilizer should increase as well.
Temperature: Recommended temperatures for producing tomatoes are a 75° F during the day and 65° F at night. Temperatures should not go below 54° F to avoid excessive delays in growth. Similarly, day temperatures should not exceed 86° F, as temperatures above this can cause flowers to abort and decrease yield.
Light: Tomatoes require high light (> 20 mol·m–2·d–1) to maximize productivity and there are several ways in which this can be achieved. First, glass is the preferred glazing material for greenhouses used for tomato production since it has the highest light transmission of any material (up to 93% ambient light transmitted). Also, keeping the glazing material clean can help maximize light transmission. Once transmission of ambient light is maximized, the only way to increase light in the greenhouse is to use supplemental lighting. Both high-pressure sodium lights and light-emitting diodes (LEDs) can be used to provide supplemental light to tomatoes, with each technology having distinct benefits and applications. For overhead lighting, HPS lamps may be a better choice than LEDs for tomato production. When used above a tomato crop, the radiant heat from HPS lamps can promote transpiration and, therefore, calcium uptake; ultimately, this can help reduce the incidence of blossom end rot during low-light times of the year. Alternatively, LEDs are most useful for inter-canopy lighting (Fig. 1) and promoting photosynthesis in functional leaves that are simply shaded by other plants; the lack of radiant heat in LED light allows for them to be placed close to plant foliage without burning the foliage of the canopy interior. A combination of overhead HPS and inter-canopy LED lighting can be a successful combination for increasing productivity, though the investment in lights and expense in running them must be offset by increased yield to make it economically viable.
CO2: Mature tomato plants can consume carbon dioxide (CO2) very quickly due to the large amount of vegetative growth. This is especially true in the late fall, winter and early spring when ventilation and replenishment of CO2 in the greenhouse from outside air is limited. Therefore, during these times supplemental CO2 should be provided to plants. Maintaining a CO2 concentration of ~1,000 ppm from liquid injection or burners can enhance growth and, ultimately, yield.