Tomato, Greenhouse and High Tunnel

Introduction

Tomato is the most widely grown high tunnel and greenhouse vegetable crop 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. In New England, tomato yield and quality are usually greatly improved when growing in protected culture compared to the field. Greenhouse tomatoes are grown in a wide range of structures from simple hoop-houses and high tunnels to more expensive greenhouses with permanent foundations and sophisticated 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 larger, more expensive structures are those bred specifically for greenhouse production. In general, these varieties are indeterminate, bred for sustained production over many months when pruned to a single stem, and have some tolerance to common greenhouse diseases such as leaf mold. Greenhouse beefsteak varieties include: Geronimo, Rebelski and Trust. Field varieties commonly grown in tunnels include BHN 589Big Dena and Big Beef. Many growers also produce cherry, cluster and heirloom tomatoes in greenhouses and tunnels. Rootstock varieties used for grafting include Maxifort (very vigorous) 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 benefit of crop 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 in-ground tomato crops from year to year because the greenhouse is designed specifically for that crop; in this case, sanitation is especially important. Remove all plant residues, twine, clips, mulch, etc. and maintain the house free of all vegetation, including weeds, for several weeks or months in-between tomato crops. Use of disease resistant varieties and rootstocks is also desirable.

In-Ground Culture

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 initially 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. 

Annual soil testing is critical to optimizing soil fertility for good productivity. Greenhouse soils amended with compost or manure may still require additional fertilization or liming for good plant and root growth. Often, additional N and K are needed annually but P levels may be excessive, especially after many years of compost application. Beds should be deeply tilled each year since nutrient salts tend to accumulate in the top 1-2 inches of soil. The soil should be tested for soluble salts (electrical conductivity) because rain does not leach salts from the greenhouse. If salt levels are excessive, the soil should be intensively irrigated, or the plastic covering should be removed over winter, to leach salts down into the soil profile. 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 use of the saturated media extract (SME) potting soil test as well as a regular field soil test (modified Morgan’s extract) is recommended, in order to measure water-soluble as well as reserve nutrient levels. The SME also tests for available nitrogen (as nitrate and ammonium) as well as salt levels. The combination of these two tests is offered as the “long-term high tunnel test” by the University of Maine Agricultural Testing Lab.

Fertilization should be based on soil test results and yield goals. Yields may vary from about 1 lb./sq. ft. to 5 lb./sq. ft., depending on the duration of the crop, cultivar, and crop management.  

N APPLICATION RATE BASED ON YIELD GOAL
  Yield Goal (lb/Acre) Yield (lb/ft2) Yield lb/stem (4 ft2)      Approx. plant height   N need lb/acre 
@ 90% recovery
Total N need
 lb/1,000 ft2
Low Yield Goal 40,000 1 4 8' 100 2.3
Medium Yield Goal 80,000 2 8 12' 200 4.6
Good Yield Goal 120,000 3 12 16' 300 6.9
High Yield Goal 160,000 4 16 20' 400 9.2

 

P205 application rate based on modified Morgan’s soil test result and yield goal
  Low
(<40 lb/A =
<20 ppm)
Medium
(40-80 lb/A =
20-40 ppm)
High/optimum
(80-160 lb/A =
40-80 ppm)
Excessive
(> 160 lb/A =
> 80 ppm)
  lb/acre lb/1000 ft2 lb/acre lb/1000 ft2 lb/acre lb/1000 ft2 lb/acre lb/1000 ft2
Low Yield Goal 180 4.1 120 2.8 60 1.4 0 0
Medium Yield Goal 240 5.5 160 3.7 80 1.8 0 0
Good Yield Goal 300 6.9 200 4.6 100 2.3 0 0
High Yield Goal 360 8.3 140 5.5 120 2.8 0 0

 

K20 application rate based on modified Morgan’s soil test result and yield goals
  Low
(<400 lb/A =
<200 ppm K)
Medium
(400-800 lb/A =
200-400 ppm K)
High/optimum
(800-1200 lb/A =
400-600 ppm K)
Excessive
(>1200 lb/A =
>600 ppm K)
  lb/acre lb/1000 ft2 lb/acre lb/1000 ft2 lb/acre lb/1000 ft2 lb/acre lb/1000 ft2
Low Yield Goal 300 6.9 200 4.6 100 2.2 0 0
Medium Yield Goal 450 10.3 300 6.9 150 3.4 0 0
Good Yield Goal 600 13.8 400 9.2 200 4.6 0 0
High Yield Goal 750 17.2 600 13.8 300 6.9 0 0

Container Culture

This system typically employs ‘grow bags’ that contain artificial soil mix and allow for adequate drainage. Such mixes are similar to potting soils, 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.

Leaf analysis. 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 15 to 20 recently matured whole leaves: these should be the 3rd or 4th leaf from the growing point, the first one to be at a 90-degree angle to the stem. Test early in the growing season so fertility adjustments can be made in a timely fashion. 

Irrigation

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. Sufficient lines should be in place to completely wet the area that roots are expected to explore. In many tunnels this requires 3 or 4 drip lines per bed. An insufficient number of drip lines will leave areas of soil dry, therefore limiting root growth and nutrient uptake. 

In-ground growing systems can utilize sensors places at several locations and depths to monitor soil moisture, although many growers simply feel the soil or growing 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. 

Transplants

Healthy transplants are key to a healthy crop. If staring your own plants, provide optimal conditions (light, heat, nutrients, water) to produce strong, stocky plants. Scout seedlings often for pests, and use sticky cards, indicator plants and other tools to monitor for infestations. If purchasing plants, inspect them carefully to be sure that they are not infected with disease, such as powdery mildew, or harbor insect pest, such as thrips. Close examination with a hand lens is essential. A period of quarantine and observation in a single area is advisable before setting plants out into tunnel(s). 

Transplants with large root systems tend to take off faster, yield earlier, and cope with environmental stresses better than those with smaller roots. Thus, small pots, at least 2.5 inches in diameter, are preferable to cell trays for the last weeks of transplant production. The optimum germination temperature for tomato seed is 75°F, and the optimum temperature range for growth of the transplant is 60-70°F. Seeds should be sown approximately 5-7 weeks before transplanting. 

Excessive watering, nitrogen, temperature or low light will cause excessive “leggy” growth. A good tomato transplant should be stocky. Tomato transplants can be conditioned or “hardened off” starting about 10 days before transplanting by taking plants from the starter greenhouse and exposing them to outside temperatures (over 50°F) and wind for a few hours each day. Tomatoes can be transplanted into tunnel or greenhouse soil once soil temperatures reach 60°F at a 2-inch depth. For early production, row covers, raised beds, plastic mulch and (back-up) heat should be considered to promote growth and/or reduce risk of chilling injury.  

Each leader on tomato plants should occupy about 3.5 to 6 ft2 of area. Using a high planting density can lead to increased yields, but will also reduce air movement, which can promote foliar disease, especially in susceptible cultivars and in tunnels with inadequate ventilation. 

Grafting

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 rather than in artificial media often leads to problems with root disease. Any tomato variety can be used as the scion (top) 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 clusters to avoid sun scald. Cut or snap off suckers cleanly; do not leave stubs on the stem that may promote disease.

Mulching

For early tomato production, black, clear or IRT (infrared transmitting) mulch can be applied to increase soil temperatures, reduce weed emergence and soil evaporation. For maximum effectiveness, black plastic mulch should have good contact between the mulch and the surface of the bed for effective transfer of heat. Embossed plastic mulch will fit tightly over the bed. Clear plastic will increase soil temperatures significantly more than black plastic, but weeds will emerge under the clear film. White plastic (white on black or white) will significantly lower soil temperatures and can be used for late summer or fall high tunnel tomato production. Organic mulches such as straw, hay or compost can be used for high tunnel tomatoes. Organic mulches create a favorable environment for many beneficial insects while increasing organic matter. However, some organic mulches (straw or hay) can significantly lower soil temperature and thus would not be effective for warming the soil in the spring. Compost can increase soil temperatures, but not as effectively as black plastic mulch. Organic mulches can be applied when the soil temperatures have increased. 

Pruning

Greenhouse tomatoes are normally pruned to a single stem and the plant is 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 they are only 2 to 3" long. 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 a dense canopy with poor air circulation, setting up conditions for disease.  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.

Pollination

Tomato plants do not need a specific day length in order to flower. The flowers are self- pollinated but require vibration by shaking, wind, or insects to assure good pollination. The optimum temperatures for pollination are 68-75°F (night) and 60-90°F (day). Air temperature below 55°F or over 95°F can cause flowers to drop from the plant. High humidity can also adversely affect pollination. Daily shaking of flowers, especially during damp and cloudy weather when pollen does not release well, can be achieved with a hand-held mechanical vibrator with a probe that just touches the flower cluster. Some growers shake the plant support wires daily, but this may not be adequate for lower clusters. A backpack blower will also provide good pollination and reduce labor costs; electric units are preferable to internal combustion engines which release ethylene and carbon monoxide; both can damage tomatoes and the latter is harmful to humans. Commercially available hives of bumble bees are widely used to assure effective pollination of greenhouse tomatoes. A well-ventilated high tunnel in a steadily breezy location is usually adequate, once outside temperatures are warm enough to open the tunnel during the day. 

Temperature

High-quality thermostats should be used to manage 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 installing a ridge vent or gable vents in the top of end walls to promote ventilation and cooling. Roll-up sides alone are often inadequate for good ventilation, especially in locations with little wind, and once the tomato canopy is dense. If mechanically ventilating, fans and louvers should be properly sized to move air through and out of the greenhouse or tunnel even when the tomato canopy is fully mature. Root-zone heating can be beneficial to early-season production, as it is difficult to warm the soil from above, whether the air is heated passively or with a heating system. 

Humidity

Effective ventilation is also needed to manage humidity. Keeping relative humidity below 90% will help avoid foliar diseases and optimize pollination. Excess humidity also restricts transpiration and water flow through the plant which restricts calcium uptake and can lead to blossom end rot and/or fruit cracking. Edema is caused by excessive water uptake when soils are warm and moist but humidity is high, so leaf stomates are closed and leaf cells ‘burst’. Cool air can exacerbate this condition.

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.

Horizontal air flow (HAF) fans keep the air moving in the greenhouse, helping to mix it, thus minimize temperature and CO2 differentials. This can reduce condensation in colder areas.  Air that is moving is continually mixed along the surface of leaves is less likely to cool below the dew point so does not condense on plant surfaces. In a 30’ x 100’ greenhouse, four fans each with at least 1600 cfm output are needed to keep the air mass moving. Fans should be mounted above the canopy, about ¼ of the width of the greenhouse from the sidewall. On each side, the first fan should be located 10’ to 15’ from the endwall to catch air coming around the corner, the next fan should be located 30’ to 50’ away, but no closer than 50’ from the endwall, to keep the air moving.

For more information on environmental controls, see the section on High Tunnels.

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.

Ethylene Injury

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.