Varieties

In addition to the greenhouse tomato varieties listed below, many growers are growing cherry, grape and heirloom types in greenhouses.

• Buffalo
• Capello
• Cobra
• Goliath
• Laura
• Monroe
• Trend
• Trust

Planting Schedule and Yield

Late spring/early summer crop: Plants should be started in late January to mid-February for transplanting into the greenhouse, March 1-15. This is intended for a late April to late May start of harvest. Growers utilizing inexpensive fuel may be able to start earlier and aim for an earlier market. Some growers delay transplanting into the greenhouse until early April and begin harvest in late May to early June. This can reduce fuel use by one half or more.

Fall crop: Seed June 20 to July 1 and transplant into the greenhouse during the first week of August for completion of harvest in the first part of December.

An average yield from a well-managed spring/early summer crop should be at least 12 lb/plant (3 lb/sq ft). Generally, the yield from a fall crop is half that of a spring crop, because of the reduced light intensity in the fall. If plants grown for a spring crop are healthy, they can continue to produce though the summer and into the fall.

Plant Density

If plants are permitted to grow to a height of 6 ft or more, space rows 4 ft apart with plants 16” apart. One can use two rows per bed in an A-frame arrangement, but around 4 sq ft/plant should be maintained. Some growers are converting to bag culture, but still utilize similar spacing. Plants set into the greenhouse in April, receive more hours of sunlight than those put in the house earlier. They can therefore be grown at a higher density of 3 to 4 sq ft/plant. Although harvest begins later, the total yield can be similar to an earlier crop planted at a lower density. However, air circulation is reduced at higher densities and the risk of disease increases.

Fertility Management

To obtain sufficient quantity and quality of yield to justify the expense of tomato greenhouse production, careful attention must be paid to the growing medium. There are two basic approaches to production: inground culture or container culture. With either system, the physical environment must promote good root growth; and nutrient supply must be optimized to encourage healthy plants and good fruit production. In addition, the importance of rotation holds true with these systems. In-ground systems should not stay in tomatoes for more than a few years in a row without a break, and container systems should be sterilized and filled with clean medium before the next tomato crop.

In-ground culture: This technique may not be successful if a greenhouse is simply placed over soil managed as it would be in the field. To begin with, substantial compaction often occurs during the construction of the greenhouse structure, and this can limit tomato growth. Even if the topsoil is worked up, plants may suffer once roots reach the compacted subsoil unless steps are taken to minimize equipment traffic. Field soils also typically require significant additions of organic matter and nutrients to produce greenhouse tomatoes. Ofthe the first step is to deeply amend the greenhouse soil with large quantities of well-made compost, before setting plants. Some growers have developed extensive composting operations to “feed” their greenhouses, adding compost on an annual basis. Others purchase well-made compost or well-rotted manure for greenhouse use. Manure should be applied at least 4 months prior to harvest to prevent contamination from human pathogens such as E. coli. 

Soils amended with high quality compost may or may not require additional fertilization for good plant and root growth, In some cases, fertility levels may actually be excessive, expecially after many years of compost application. This can be avoided by yearly soil testing.  This should include a soluble salts test. If salt levels are excessive, the soil should be leached with several inches of water and retested prior to planting the crop. Lime should be applied based on soil test recommendations. Epsom salts may also be required to provide magnesium. In-ground cultures based mostly in mineral soil with low levels of compost amendments will require more extensive fertilization. If a soil has been amended extensively with compost, the saturated media test would be appropriate. Check with the soil testing lab for further information on available tests. This should be based on soil test results. Take care to avoid overfertilization in the greenhouse. Fertilizer salts are not readily leached from the protected environment of the greenhouse, so proper selection and application of fertilizers based on a soil test is crucial. Avoid fertilizers with a high salt index (like potassium chloride) and those high in ammonium forms of N. Soils should be tested for nutrients and salts level before planting. Over time, salts can build up in a greenhouse, regardless of the nutrient source, including compost and manure.

Container culture: This system may employ pots, plastic bags, troughs, trenches or other structures that contain some form of artificial soil mix and allow for adequate drainage. Such mixes are usually soilless, being comprised of peat, vermiculite and/or perlite, lime, fertilizers and wetting agents. There are many brands and formulations available. Be sure to select one that has a proven track record. Water and nutrients are supplied through trickle irrigation. These mixtures need to be supplied with additional nutrients as soon as plants are established to sustain crop growth. Water and nutrients are supplied through trickle irrigation. Soluble nutrients should be supplied every time water is applied. This is somewhat like hydroponic systems, except that the water and nutrients are not recycled. Feed by injecting soluble fertilizers into the irrigation water and adjust to meet the needs of the plants. N-P-K should be supplied in a ratio of 1:1:1.25 until the fourth flower cluster, then the ratio is adjusted to 1.25:1:3 to increase the proportion of N and K. The level of N in solution is usually maintained at around 100 ppm during early growth stages, and gradually increased to 200 ppm by the time the plants are about 3 feet high. A feeding program that works well for many growers is to use a 7-11-27 fertilizer and calcium nitrate (15.5-0-0). These should be applied according to the directions on the 7-11-27 label. Concentrated stock solutions should be mixed in separate containers and applied at the same time using two separate injectors. It is not generally satisfactory to alternate between solutions using a single injector.

Leaf tissue analysis can be helpful in determining the nutritional status of tomato plants and the adequacy of a greenhouse fertility program. For accurate results, submit five to six whole leaves taken from just below clusters with golf ball-size fruit. Contact your land grant university soil testing lab to see if it offers this service.

Irrigation

A steady, sufficient supply of water is essential to good tomato production. Irregular watering can result in blossom-end rot and fruit cracking. Some form of drip irrigation is recommended. In-ground growing systems can utilize tensiometers to monitor soil moisture (see Trickle or Drip Irrigation). Once tomato plants are well grown, they utilize large quantities of water. Irrigation may then be needed more than once a day to maintain a consistent moisture supply. In order to minimize the development of foliar diseases, it is critical to avoid wetting the leaves of the plants when watering. Otherwise, an important benefit of greenhouse production will be lost. Peat-based media should be moist enough so that water drips out when a handful is squeezed.

Growth of Transplants

Essentially the same procedures as for production of field transplants should be followed. However, an additional one to three weeks may be required due to the lower temperature and light intensity which occurs in February and March.

Grafting

Grafting is a way to manage root diseases and increase fruit production. Organic growers in particular can gain from grafting because growing tomatoes in soil and compost rather than in sterile media often leads to problems with root disease. Popular greenuouse varieties such as Trust, Cobra and Buffalo as well as cherry, cluster and even heirlooms have all been used as scions, but far fewer varieties have been used for rootstocks. The two most common rootstocks are Maxifort and Beaufort. Both have tolerance to some common soil-borne diseases. Maxifort results in very vigorous growth while Beaufort leads to a moderate increase in plant vigor.

Two common grafting techniques are top grafting and side grafting. With top grafting, the scion is completely cut off from its roots and placed on top of the rootstock stem. Side grafting involves making a partial cut into the stem of the scion plant and then inserting the cut-off stem of the rootstock into that cut. The seedling is then allowed to retain both sets of roots until the graft with the new rootstock heals, after which the original root is cut from the plant. Top grafting relies on a tiny plastic tube or sleeve to hold the scion and rootstock together until the graft heals. Top grafting is quicker and a bit less complicated to do than side grafting because it requires only a single complete cut through both the root and the shoot portions of the graft. This technique can be used on very small seedlings.

Side grafting takes a little longer, but is preferred by some growers because it is a bit more forgiving. If greenhouse conditions for graft healing are less than ideal, the grafted seedling still has its original set of roots to help during the transition. Side grafting can also be done with seedlings that have become larger than is ideal for top grafting. A small clip, much like and office binder clip, is used to hold side grafted plants together until they heal.

Some growers produce sequential planting of seedlings over several days to assure that they have the right selection of plant sizes to choose from for grafting. There is a narrow range of plant size for grafting. The ideal is when the stems are about 2 mm in diameter for top grafting and 2-3 mm for side grafting. After grafting, keep the plants where it is about 80-85°F and 95% relative humidity while the grafts heal. They should be in a shaded area such as under a bench, and misted enough to maintain relative humidity, but not so much that the leaves are wet all the time. Healing takes about 4 to 5 days for top grafts and 6 to 7 days for side grafts. Placing plastic domes over trays of top-grafted plants appears to enhance success. For a couple of days before setting the grafted plants out, gradually increase their exposure to direct light by pulling them out form under benches or removing any covering for a few hours early or late in the day. If using plastic domes, prop them open during this time.

Because grafted plants are more vigorous, they will produce a lot of vegetative growth at the expense of reproductive growth. In other words: too much foliage and not enough fruit. So you have to take steps to reduce plant vigor. Leaf removal is one way. This may seem counter-intuitive, but apparently only 10 to 12 fully expanded leaves are needed to to do the job of capturing sunlight to feed a grafted tomato plant in the northeast. Another way to suppress vigor is to let them develop two leaders, or main stems, rather than a single stem.

Grafting clips and rootstock varieties are available from: Johnny’s Selected Seeds www.johnnyseeds.com and Hydrogardens www.hydro-gardens.com

Pruning

Greenhouse tomatoes are normally pruned to a single stem and the plant supported by nylon twine tied loosely at the base of each plant row and secured to a wire at least 8’ above the bed. The twine is clipped to or spiraled around the stem as it grows. Prune when the suckers are 2 to 3 inches long as they appear in leaf axils. The plant may be topped when it reaches the supporting wire or, to extend the production season, the plant can be looped over the top wire and allowed to grow 3’ to 4’ down the other side before topping. Topping after the sixth flower cluster or less has shown increases of U.S. #1 fruit. Another way to prolong the season is to untie the support twine from the overhead wire, lower the plants 2’ to 3’, lay the stems down on the ground, move and and re-tie the support twine. This requires that extra twine be retained at the top when initially tying the plants. Remove the lower foliage after harvest of the lower clusters to improve air circulation and allow for lowering the plants. These lower leaves contribute little to the plant at this stage. Leaves provide food for tomaotes lower on the plant. Do not remove leaves until tomatoes below them are harvested. Note: Frequent handling of plants is a primary means of spreading Tobacco Mosaic Virus (TMV) and tomato canker. Wash hands frequently and don’t handle tobacco products.

Pollination

In the field, tomatoes are self-pollinated by the wind. In the greenhouse, the flowers must be lightly shaken to get effective pollination. Daily shaking is necessary, expecially during damp and cloudy weather because the jpollen does not release well. A hand-held mechanical vibrator with a probe that just touches the flower cluster is very effective. Some growers have developed a system of shaking the support wires daily. This may not be adequate for lower clusters. A backpack air blast blower will also provide good pollination and reduce labor costs. Considerable care should be exercised with the latter. Internal combustion engines release ethylene and carbon monoxide; both can cause damage to the tomatoes and the latter is harmful to humans. Be certain there is adequate ventilation using outside air. Electric blowers can also be used. Using blowers for pollination can also hasten the spread of diseases, such as Botrytis, that produce airborne spores. Many growers are purchasing hives of bumble bees for pollination. They do a good job of pollinating tomatoes, but the hives are short-lived and must be replaced once or more depending on the length of the season. Honey bees do not effectively pollinate tomatoes.

Temperature and Humidity

Maintain a day temperature of 70°F to 75°F on sunny days and 60°F to 65°F on cloudy days. For good fruit set, maintain night temperatures of 62°F to 65°F. Rough fruit develops with temperatures lower than 60°F during flower initiation which begins several weeks before flowers appear. Keep humidity below 90% RH, if possible, by heating and ventilating to minimize leaf mold and botrytis. High humidity also reduces water use by the plant (by restricting transpiration) and, thus, contributes to reduced calcium uptake which leads to blossom end rot and/or cracking. Oedema is caused by excessive water uptake and may occur when humidity is high and the soil is warm.

Techniques to reduce high humidity: Warm air holds more moisture than cool air. During warm days, the greenhouse air is more humid. As the air cools in the evening, the moisture-holding capacity drops until the dew point is reached. Water then begins to condense on surfaces. Humidity can be reduced by exhausting the moist air and replacing it with cooler outside air that is drier. Activate the exhaust fans for a few minutes and then heat the greenhouse to raise the air temperature. Then, exhaust the humid air. The cooler, outside air will lower humidity levels as it is warmed in the greenhouse. A relay may be needed to lock out the furnace or boiler until the fan shuts off so that flue gases are not drawn back into the greenhouse. This will help to prevent air pollution damage to sensitive seedlings. Heat and vent two or three times per hour in the evening after the sun goes down and early in the morning at sunrise.

Using horizontal airflow (HAF) can also reduce condensation. HAF fans keep the air moving in the greenhouse, helping to minimize temperature differentials and cold spots where condensation occurs. Air that is moving is continually mixed. The mixed air along the surface does not cool below the dewpoint so does not condense on plant surfaces.

Managing Plant Growth (adapted from NC State Univ.)

Greenhouse tomatoes tend to cycle between being overly vegetative (too much plant growth and too little fruiting) early in the season and being overly generative (too little plant growtn and excessive fruit load) later in the season.  The greenhouse environment can be manipulated to try to balance plant growth. A well-balanced plant has a stem about 3/8 inch (1 cm) thick at a point 6 inches below the growing point. It has dark green leaves, and large, closely spaced, readily setting flower clusters.

Low light and low transpiration tend to promote vegetative growth. In an overly vegetative plant, stems are thicker and fruit set is low. Flowers appear far down from the top of the plants, open slowly and incompletely and are pale yellow. The uppermost leaves are flat, soft, long; light colored, and may have a somewhat mottled appearance. The cluster stem is thin and long. Fruit will be slow to develop, few in number, and may be misshapen.

To steer the plant to a more generative growth pattern the difference between day and night temperatures can be increased by up to 9°F and temperatures reduced more quickly in the early evening when going from day to night set-points. Greenhouse temperatures should be raised, the relative humidity should be lowered and ventilation should be increased. Increasing transpiration reduces turgor pressure and inclines the plant to generative, rather than vegetative growth. CO2 enrichment also encourages generative growth.

In an overly generative plant, stems are thinner (indicating lack of carbohydrates), growth is slow, and trusses are short and horizontal. Dark yellow flowers appear immediately below the top of the plant, and open quickly. Although fruit are large, well shaped, and develop rapidly in an overly generative plant, over the long term, yields will be reduced because growth is reduced at the top. Leaves at the very top of a too generative plant develop slowly resulting in short, dark, strong leaves, which may be curled under.

To correct an overly generative plant, day temperatures are lowered to re-direct assimilate from the already-set fruit to the top of the plant and the developing trusses, but do not lower night temperatures as this will slow down fruit ripening, prolonging the problem of too much assimilate going to the older fruit. Reducing transpiration by raising relative humidity or reducing ventilation also stimulates vegetative growth.

Ethylene Injury

Combustion gases, which contain ethylene, can enter the greenhouse via faulty heat equipment. Very low  ethylene levels can make tomato leaves bend downward (epinasty), and if exposure is ongoing, stems may thicken,branching may increase, and flower buds may abort or develop into malformed fruit. Proper heating system  maintentance done by professionals before the start of the heating season is very important to prevent ethylene injury.  

Harvest and Storage

Greenhouse fruits should be as ripe as possible before being harvested based on market needs, but cracking may increase as fruit are left on the plant longer. See Harvest section of Tomato (Outdoor), for more details.