Hydroponic Culture of Tomatoes

Before discussing the sowing of the seeds we must talk about appropriate varieties for hydroponic culture. While you may grow any garden variety, you may achieve higher yields by the use of special “greenhouse” varieties. Greenhouse varieties are “staking” or correctly termed “indeterminate” (Photo1).

That is, they continue to grow upwards and must be supported vertically by stakes or strings. On the other hand, many garden varieties are called “bush” or “determinate” varieties because they grow many shoots spreading outward, but do not continue growing vertically. Of course, you may purchase garden “staking” varieties, but these are more suitable to outdoor culture rather than indoor hydroponic culture. Outdoor varieties tolerate greater fluctuations between night and day temperatures than the greenhouse varieties that have a relatively narrow temperature range differential, usually about 10 degrees F. Your hydroponic indoor garden will have controlled temperatures from your central heating and cooling system of your home.

In addition, the choice of variety or “cultivar” depends upon the type of tomato we wish to have. Do you want cherry, beefsteak, cluster or pear tomatoes? Each type has many varieties. From my experience I recommend the following varieties for hydroponics in these categories:

Beefsteak: Trust, Quest, Match, Blitz, Geronimo
Cherry: Favorita, Conchita
Cocktail: Picolino (red), Flavorino (plum)
Truss (Cluster): Locarno (yellow), DRK 902 (orange), Tradiro (red), Ambiance (red)

Truss or cluster tomatoes are about half the size of beefsteak tomatoes and are normally harvested all together on the vine (TOV-tomatoes-on-vine) or cluster of fruit. Of course, when growing these tomatoes for your own use there is no need to harvest them all at the same time. Pick them when they are fully red or the color of the variety. Your final choice of variety will depend upon the results you get using a number of varieties under your specific conditions. You can start with those recommended above and through trials can determine which ones yield best and produce the most flavorful fruit for your needs.

Sow seeds in small 1 1/2” x 1 1/2” x 1 1/2” rockwool cubes (Photo 2). You can use other cubes of peat (“Jiffy cubes”) or “Oasis Horticubes,” but, in my opinion the rockwool cubes are superior. They have better drainage, oxygenation and structural integrity. They do not break apart during transplanting. When the tomato seedlings are 7 to 8 days old break the cubes apart and lay each one on the side separating them to about 28 per tray. This additional spacing and laying them on their sides will permit the plants to bend up and become sturdier (Photo 3).

Tomatoes are transplanted to 3” rockwool blocks after 14 to 18 days. They are placed on their sides into the rockwool blocks so that roots will form along the stems. Grow for 2 to 3 weeks longer before transplanting to the final growing system. In this way, the seedlings may be grown closely under supplementary lighting until they reach about 8 inches in height. You must be sure to space them apart as they grow so that their leaves are not overlapping to keep the plants from getting tall “leggy.” The growing cubes and blocks must be placed in meshed trays that have been sterilized with a 10% bleach solution prior to placing the seedlings into the trays. Soak the rockwool cubes and blocks well with water prior to sowing or transplanting to prevent any dry spots in the medium. If you grow healthy seedlings you will get healthy plants. It is important to know that a healthy seedling is as wide as high. Poor plants that are leggy will never yield to their full potential.

Selection of a hydroponic system depends upon your budget and what you believe is simplest for your conditions. My choice is the use of Bato buckets of perlite. Coarse perlite is a good substrate in providing adequate oxygenation through rapid drainage. It is very stable in structure, not releasing extraneous elements. The Bato buckets are placed on a 1 1/2″ to 2” diameter drainage pipe to circulate the nutrient solution back to the nutrient reservoir. Two tomato plants are set into each pot with a drip line to each plant (Photo 4).

Place the drip line on a stake into the rockwool block at one edge, not close to the base of the plant as that may cause disease problems. Perlite culture requires more frequent irrigation cycles than rockwool slabs, the second alternative.

Rockwool slabs come in various sizes. Those most suitable for tomatoes measure 36” x 3” x 8” (Photo 5). They are wrapped with polyethylene so require that you place drainage holes in them. However, before cutting drainage holes you must soak the slabs for about 24 hours to completely moisten them. This can be done by placing several drip lines into the top of the slab. Generally, it is best to place the slabs on special drainage-return channels to recirculate the nutrient solution. Up to 5 to 6 plants may be placed in each slab. After the slabs have been soaked slit three angled drainage cuts of 2 inches in length at the bottom edge between plant locations. Then cut “X” openings in the top plastic at the plant sites. Place the seedling with its rockwool block directly on top of the slab at the cut holes. Place a drip line at each plant about 1-inch away from the base of the transplant.

Whether we use perlite or rockwool culture we must install a drip irrigation system consisting of nutrient tanks, pumps, plumbing and irrigation lines. Please refer to my book “Hydroponic Food Production” for more information on the layout and equipment needed for such a drip system.

The irrigation cycles should be automatically timed with an irrigation controller. Irrigate sufficient to get at least 25% leachate (drainage) during any given cycle. This will keep the nutrient solution concentration stable in the rockwool or perlite substrate.

Tomatoes require between 3.5 and 4.0 square feet of floor area per plant. Space the rows and plants within the rows to obtain that area per plant. If you are using Bato buckets with perlite, locate the pots alternating on each side of the drain pipe at 16-inch centers along the pipe and 6 feet between rows. With two plants per pot the area per plant will be about 4.0 square feet. With rockwool slabs place the slabs in a single row separated by 7 inches between the ends. The single rows should be 6 feet apart. By locating 5 plants/slab the area per plant is 4.2 square feet. It is very important to have the correct spacing per plant to prevent lack of light resulting in “leggy” plants that will not yield highly.

It is very important to train your plants correctly in order to maximize your production. As mentioned above, we use staking varieties of tomatoes, so they must be trained vertically. As soon as the seedlings are transplanted into their final growing system, rockwool or perlite, etc. training must begin. Support the plants from strings attached to overhead wires that can be attached to hooks in the ceiling or use a supporting frame above to tie the strings. Use “Tomahooks” with string wound on them to provide the support string for the plants (Photo 6).

These metal hooks are suspended from the overhead support wire. The string is unwound until it reaches the base of the plant where it is attached to the plant under a strong leaf by a plastic plant clip (Photo 7).

The Tomahooks permit the plants to be “lowered and leaned”, as they grow up approaching the ceiling. Lower the plants about 1 foot every week as you remove suckers and lower leaves up to the ripening fruit. Do not remove more than 3 to 4 leaves at any given time or the plants may undergo stress. Removing the leaves permits good air flow at the base of the plants and therefore will reduce disease infection. By pushing the plants along the support wire you lean them over so that the upper part of the plant bearing fruit is suspended above the pots or slabs. The end plants will be positioned to the row on the other side as the plants are supported in two rows. The plants are trained in a V-cordon configuration whereby alternate plants are tied to strings on the opposite support wire. This gives the plants more even spacing as they grow vertically. Attach a plant clip below a strong leaf about every foot of the stem (Photo 8).

Train each plant to a single stem by removing the “suckers” or side shoots that form at each leaf axil (Photo 9). Break the suckers off by hand when they are about 1 to 2 inches long. Do not permit them to grow larger or they will take food away from the fruit formation making the plant vegetative. To see these procedures in detail please refer to my book “Hydroponic Food Production.”

Tomato flowers must be pollinated every day to get “fruit set”. Fruit set is the formation of small fruit on the clusters that will expand into tomatoes within 6 weeks or so. Pollination in a commercial greenhouse is achieved by use of bumble bees. This is not feasible for in your home. The best way is to use an electric toothbrush. Simply place the toothbrush behind the truss vibrating it for 3 to 4 seconds. Do this in the late morning or early afternoon when humidity levels in the air will be lower than the early morning. Flowers are receptive when their sepals bend back (Photo 10).

You can tap the flowers with your hand and see the fine pollen fall. Placing a piece of black paper behind will permit you to better see the flow of pollen. Without pollination fruit cannot form. In some cases you can use a fruit set hormone, but often its use results in soft fruit due to inadequate pollination. Pollination is an important part of the everyday training of your plants in order to get tomato production (Photo 11).

Tomatoes take about 100 days from seeding to first fruit harvest. After that they will continue to produce fruit, but yields will fall once the plants reach 10 to 11 months in age. With indoor growing in your home it may be beneficial to grow two crops annually instead of the conventional one per year. This may be determined by the productivity of the plants. Once they stop bearing fruit well or the size of fruit is becoming small, it would be better to change the crop to get more vigorous plants. Two crops a year will give you a cropping period of about 5 months each. Start the seedlings for the new crop 5 weeks prior to your anticipated time of removing the old crop to reduce the crop turn around period to two months.

Hydroponics offers a great advantage over soil growing through providing the plants optimum nutrition. However, at the same time you must be careful not to make mistakes in your formulation or weighing of fertilizer salts or the plants may be damaged. Below is a general formulation for tomatoes as a starting point for your plants. You need to refine it and adjust it as you gain growing experience. Tomato formulations are at three levels for the different stages of plant growth for commercial growers; however, I think that one general formula is sufficient for small-scale hobby units. Table 1 is a general formula that indicates weights for a 10 U.S. gallon nutrient tank. Since the weights of every element remain in ratios, you can adjust for larger tanks by using the increased ratio. For example, if you need 31 grams of calcium nitrate in a 10-gallon tank, but you prefer to use a 15 gallon tank, simply multiply the 31 grams by 15/10 = 46.5 grams. The weights of the microelements are very small, so to avoid weighing inaccuracies, make up a concentrated stock solution and store it in a clean container that is closed and kept in the dark to prevent algae growth. Add a small amount of the liquid stock solution to your nutrient tank when making up the solution. Table 2 gives the weights for a stock solution of 300 times normal strength made up in a 10-gallon container.

Note: Add 2 grams of iron chelate (FeDTPA) to the 10 gallon tank of Table 1 above. This will give 5 ppm of iron.

For a 10 gallon nutrient tank add: 10 x 1/300 = 0.0333 U.S. gallons of stock solution. Convert to liters: 0.0333 x 3.785 = 0.1262 liters or 126 milliliters (ml). You will need a 100 ml graduated cylinder to measure this volume of solution. Weigh the compounds for Table 1 using a triple-beam balance. You may purchase these from a scientific laboratory supply company such as Fisher Scientific.

pH

The acid or basic property of the nutrient solution is measured by pH. The pH of the nutrient solution must be maintained between 6.0 and 6.3 for tomatoes. Monitor the pH with a pH meter or indicator paper. Since pH meters are somewhat meticulous in maintaining their calibration, I prefer on a small scale to use pH indicator paper such as the “Merck color pHast Indicator strips” (Photo 12).

To adjust the pH, add an acid such as sulfuric (battery) acid to lower the pH or a base as potassium hydroxide, sodium hydroxide or baking soda to increase the pH. Wear gloves and protective goggles when handling these substances. Always add acid to water, never the opposite.

EC (Electrical Conductivity)

The concentration of the nutrient solution is measured by its electrical conductivity (EC), which is its ability to conduct electricity. That property of the solution is a function of the elements in the solution which conducts the electricity. We use an electrical conductivity meter to detect the level of total dissolved solutes in the solution (Photo 13).

This is expressed on a scale of millimhos (mMhos) or milliSiemens (mS). The EC does not differentiate among the different elements present in the solution so is only a general indicator of the overall concentration of all elements present. However, an EC meter is essential to monitor the nutrient solution.

Generally, a nutrient solution may have an EC of from 1.5 to 3.0 mS, depending upon the concentrations of each element present. Record the EC when you make up your nutrient solution and then monitor it daily recording it in a log book so you can see changes taking place. Over time the EC will start to fall as the elements in the nutrient solution are taken up by the plants. As the EC falls below an acceptable percentage, you may change the solution or add a percentage of all the elements according to the percentage of decline in the EC. It is best to eventually, perhaps every 3 to 4 weeks, to change the nutrient solution as the plants take up the elements at different rates and therefore, some may be in deficiency while the EC still indicates there are adequate total solutes in the solution. We want to prevent any possible nutrient deficiency from occurring in the plants, otherwise, their yields will be reduced. Hydroponics is a precise science, but with experience and following procedures carefully, the benefits over soil will more than offset the technical aspects that you need to apply.