The tomato is the most widely grown high tunnel and greenhouse vegetable because of strong consumer demand for its high-value fruits, and because of its ability to utilize the vertical growing space and to maintain production over a relatively long harvest period. Greenhouse tomatoes are grown in a wide range of structures from simple hoop-houses and high tunnels to more expensive greenhouses with heating systems and environmental controls. Regardless, the horticultural principles are the same.
Types and Varieties
Some growers plant field tomato varieties in tunnels, but the majority of varieties used in more expensive structures are those bred specifically for greenhouse production. In general, these varieties are indeterminate, bred for production when pruned to a single stem, and have some tolerance to common greenhouse diseases, primarily leaf mold. Greenhouse beefsteak varieties include: Arbason, Cobra, Geronimo, Lola, Rhapsodie, Rebelski and Trust. Many growers also produce cherry, cluster, paste and heirloom tomatoes in the greenhouse. Rootstock varieties used for grafting include Colossus and Maxifort (very vigorous), or Beaufort and Estamino (less vigorous).
Fertility and Growing Media
To obtain sufficient quantity and quality of yield to justify the expense of greenhouse tomato production, careful attention must be paid to the growing medium. There are different approaches to production: soil vs soilless culture and in-ground vs container culture. With any system, the physical environment of the medium must promote good root growth. Nutrient supply must be optimized and maintained to encourage healthy plants and good fruit production.
The importance of rotation holds true with in-ground systems, but not with soilless or hydroponic systems where the media is entirely replaced each growing season. In the latter, all containers should be sterilized before the next crop. Many growers do not rotate the tomato crop because the greenhouse is designed specifically for that crop; in this case, sanitation is especially important. Remove all plant residues and maintain the house free of all vegetation, including weeds, for several months between crops. Disease resistant varieties or rootstocks may be necessary if rotation is not practiced.
A greenhouse may be placed over high-quality field soil but be sure to avoid soil compaction that can occur during the construction of the greenhouse. Even if the topsoil is worked up after heavy traffic, plants may suffer once roots reach the compacted subsoil. On compacted soils or poor fertility sites, it may be advisable to make raised beds. Field soil in a greenhouse is usually amended with a large volume of organic matter such as well-made compost before growing greenhouse tomatoes. If uncomposted manure is used it should be applied at least 4 months prior to harvest to avoid contamination from human pathogens such as E. coli. Use of chicken manure in a greenhouse can generate excess ammonia that will damage plants. The use of unfinished compost may also provide excess ammonia.
Greenhouse soils amended with compost or manure may or may not require additional fertilization or liming for good plant and root growth. In some cases, fertility levels may be excessive, especially after many years of compost application. Beds should be tilled or remixed each year since nutrient salts tend to strongly accumulate in the top 1-2 inches of soil. Annual soil testing is critical to optimizing soil fertility for good productivity. The soil test should include soluble salts (electrical conductivity) because rain does not leach salts from the greenhouse. If salt levels are excessive, the soil should be leached with several inches of water either by removal of the plastic covering over winter or a long period of sprinkler irrigation. If leaching is necessary, it should be done well before planting. This allows time to retest the soil and apply appropriate nutrients since leaching often reduces nutrient levels. Avoid fertilizers with a high salt index (like potassium chloride) and those high in ammonium forms of N. Peat moss can be used to maintain organic matter and dilute high salt levels without adding additional nutrients or salts. High salt levels can also be diluted by mixing in low fertility field soil.
If a tunnel/greenhouse soil has been heavily amended with compost or manure, the saturated media extract (SME) test is appropriate. Unlike a regular field soil test, it is a water extract that tests the immediate availability of nutrients, including nitrogen, as well as salt levels. Both types of tests are sometimes run at the same time so that both available and reserve levels of nutrients are measured. The SME test is most useful in houses that have not had salts regularly flushed and/or that have been aggressively amended, resulting in relatively high EC levels.
This system may employ pots, plastic bags, troughs 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. These mixtures need to be supplied with additional nutrients after plants are well established to sustain crop growth. Soluble fertilizers can be injected into the irrigation water and adjusted 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' high. A popular program is to use calcium nitrate plus a 7-11-27 or similar liquid fertilizer. These two materials are mixed with water to make separate stock solutions. These should be injected separately, but at the same time with 2 injectors. Follow directions on the 7-11-27 fertilizer label.
With both in-ground and container culture, leaf tissue (foliar) analysis is valuable for determining the nutritional status of tomato plants and the adequacy of a greenhouse fertility program. For accurate results, submit 8 to 10 healthy whole leaves: these should be the first fully expanded leaf from the growing point, usually the 5th or 6th down. Test early in the growing season (5 to 6 weeks after transplanting) so fertility adjustments can be made in a timely fashion. If you are expecting a long season of fruit production, sample again after 3 more weeks.
A steady, sufficient supply of water is essential to good tomato production. Irregular or insufficient 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 Irrigation), although many growers simply feel the soil or medium to assess its water content. 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.
Grafting is a way to manage root diseases and increase plant vigor. In-ground growers, in particular, may benefit from grafting because growing tomatoes in soil and compost rather than in sterile media often leads to problems with root disease. Popular greenhouse varieties, as well as cherry, cluster and even heirlooms, have all been used as scions, but far fewer varieties are available for rootstocks, and these vary in the degree of vigor they confer on the plant.
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 an 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. The scion and rootstock stem diameters must be similar, and grafting is most effective on very small plants. The ideal size is when the stems are about 2 mm in diameter for top grafting and 2 to 3 mm for side grafting. After grafting, keep the plants in a shaded area at about 80 to 85°F and 95% relative humidity while the grafts heal. They should be 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 high 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 from under cover for a few hours early or late in the day. If using plastic domes, prop them open during this time to increase air flow.
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. Vigor can be reduced by allowing the first sucker on the scion to grow into a second fruiting leader. Leaf removal also reduces vigor. Only 10 to 12 fully expanded leaves are needed to capture sunlight to feed a grafted tomato plant in the Northeast. Take care to leave sufficient leaf cover over the fruit to avoid yellow shoulders. Cut or snap leaves cleanly and do not leave stubs that may promote disease.
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. All suckers are removed, ideally when 2 to 3" 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. This, however, may lead to poor air circulation as the plants range all around, setting up conditions for disease outbreak. 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 re-tie the support twine. Remove the lower foliage after harvest of the lower clusters to improve air circulation and allow for lowering the plants; the lower leaves contribute little to the plant at this stage. 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.
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, especially during damp and cloudy weather because the pollen does not release well. A hand-held mechanical vibrator with a probe that just touches the flower cluster is 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 but internal combustion engines release ethylene and carbon monoxide; both can damage tomatoes and the latter is harmful to humans. A well ventilated high tunnel in a steadily breezy location is usually adequate. Some growers use hand-held electric blowers. Blowers for pollination can also hasten the spread of diseases, such as Botrytis, that produce airborne spores. Commercially available hives of bumble bees are widely used to assure effective pollination of greenhouse tomatoes.
High-quality thermostats should be used to monitor greenhouse temperature. Daytime temperature of 75°F to 80°F is ideal; use ventilation and/or shading to keep temperatures below 85°F. Maintain night temperatures of 62°F to 65°F after sunny days and 60°F to 62°F after cloudy days. Rough fruit develops with temperatures lower than 60°F during flower initiation which begins several weeks before flowers appear (seedling stage). For passively ventilated gothic-style high tunnels, consider modifying the peak with a roll-up vent to allow escape of excess heat.
Effective ventilation is needed to manage temperature and humidity; side plus ridge openings are best for passive ventilation; properly sized louvers and fans are needed for optimal mechanical ventilation. Keep relative humidity below 90% to minimize leaf diseases and optimize pollination. 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, air is cool and the soil is warm.
On cool nights, a combination of ventilation and heating is needed to reduce humidity. Ventilation exchanges moist air with drier air from outdoors while heating brings outdoor air up to optimum growing temperature and increases the capacity of the air to hold moisture, avoiding condensation. In greenhouses with vents, turn on the heat and crack the vents open so warmed humid air can escape and be replaced with drier outside air. In houses with fans, they should be operated for a few minutes to cool the house down from day to evening temperatures. During the night, a clock could be set to activate the exhaust fans for 20 to 30 seconds, 2 to 3 times per hour. A relay is needed to lock out the heater until the fans shut off so that both the fans and heating system do not operate at the same time. Otherwise, flue gases will be drawn into the greenhouse. The venting and heating cycle should be done several times after the sun goes down and at sunrise. For some greenhouses it may take a few minutes per air exchange; with passive ventilation, it may take 30 minutes. Heating and venting is effective even if it's cool and raining outside. The relative humidity of air at 50°F/100% RH can be cut in half (50%) when it is heated to 70°F.
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 dew point so does not condense on plant surfaces.
Managing Plant Growth (adapted from NC State University)
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 growth 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" (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.
Combustion gases, which contain ethylene, can enter the greenhouse via faulty heat equipment. Even at very low levels, ethylene can make tomato leaves bend downward or become twisted and contorted (epinasty), and if exposure is ongoing, stems may thicken, branching may increase, and flower buds may abort or develop into malformed fruit. To prevent ethylene injury, hire professionals to perform proper heating system maintenance before the start of the heating season. High ammonium can stimulate plants to produce ethylene. Avoid fertilizers high in ammonium and be sure composts are finished. Composts should always be tested before using in the greenhouse. If growing in containers, be sure they have adequate bottom drainage. Saturated, highly organic media in the bottom of containers can spontaneously generate ethylene in rare cases.
Harvest and Storage
Greenhouse tomatoes are usually picked at the turning stage or later and packed by uniform ripeness and size in single or double layer cartons. Fruits are sensitive to compression injury so pack and display accordingly. For best flavor greenhouse fruits should be fully ripe when harvested, if your markets may allow, but cracking is a risk if fruit are left on the plant too long. Ripe fruit can be held for a couple of days at 45 to 50 °F but flavor and aroma may be reduced compared to storage at room temperature.