GREENHOUSE TOMATO PRODUCTION PRACTICES

Bob Anderson, Extension Horticulture Specialist, University of Kentucky, Lexington KY 40546

Tomatoes have been grown in greenhouses for nearly 100 years. For this reason, there are many techniques for growing tomatoes in a greenhouse and there is more written about greenhouse tomatoes than any other greenhouse crop. Additionally, hydroponics was developed for greenhouse tomato production, so there can be many complicated steps to the production of tomato fruit in a greenhouse. There is no single BEST way to grow greenhouse tomatoes, many ways are successful. An individual grower must experience tomato production, in order to determine the best and most economic techniques in his or her greenhouse. This publication is a general summary of greenhouse tomato practices. It is highly recommended that the reader obtain the publications listed below to get a more thorough view of greenhouse tomato production, hydroponics and the production of other vegetables in the greenhouse.

CONTENTS

• General Aspects of Tomato Production
• Recommendations for a Fall Tomato Crop
• Additional Sources of Information on Greenhouse Tomatoes
• Suppliers for Greenhouse Vegetable Production

General Aspects of Tomato Production

LIGHT AND TEMPERATURE CONTROL -- Tomatoes are a warm season vegetable crop. They grow best under conditions of high light and warm temperatures (summer conditions). Low light in a fall or winter greenhouse, when it is less than 15% of summer light levels, greatly reduces fruit yield when heating costs are highest (Table 1). For this reason, it is difficult to recommend that a greenhouse operator should grow and harvest fruit from December 15 to February 15. Even with the problem of low light and high energy costs, winter greenhouse tomatoes are common in southern Canada and Europe. These greenhouses may use expensive supplementary HID lighting (street lamps) and are able to sell their tomatoes at very high prices because their market is willing to pay a high price for high quality winter tomatoes. Greenhouse tomatoes are not very common in the U.S. or Kentucky, because consumers seem unwilling to pay a high price for winter tomatoes. If you have a market willing to pay for greenhouse tomatoes, then tomatoes may be a successful crop in your winter greenhouse.

Table 1. Relative percentages of heat costs and percent of light, compared to summer months, for fall, winter and spring months in Kentucky.

Many greenhouses have been built in Kentucky to grow tobacco transplants. Based on many years of experience, tomato production is most successful in the spring. Excellent light, moderate heating costs and good prices annually demonstrate this is the best time for greenhouse tomato production. Yet, tobacco growers have tobacco transplants in their greenhouse in the spring. Low winter light and high heating costs create a problem for winter production, so trials were completed at the University of Kentucky in 1995 and 1996 to evaluate late summer and fall production for greenhouse tomatoes. Results of these trials are presented throughout this publication.

Tomato plants grow best when the night temperature is maintained at 60-62 F. Temperatures below 60 will prevent normal pollination and fruit development. This is especially true for standard greenhouse varieties, less so for field varieties, so the grower must be sure that thermostats control heaters properly. In warm or hot outdoor conditions, tomato greenhouses must be ventilated to keep temperatures below 95 F. High temperatures not only effect the leaves and fruit, but increased soil temperatures also reduce root growth.

PLANT SUPPORT -- Plants must be tied or clipped with tomato clips to a string or twine suspended from a strong overhead cable. This starts as soon as they are about 10 inches high and continues throughout production. A separate support system must be built inside the greenhouse from pipe, etc. to support the crop. You must remember that each plant may weigh 10 to 15 pounds when it is loaded with fruit so the support system must be quite strong. Typical greenhouse structures are usually NOT strong enough to support a tomato crop, consult the greenhouse manufacturer for details; greenhouses designed to support a tomato crop are available from some manufacturers. Actively growing tomatoes will have to be clipped to the support string or twined around the string every 6-10 days.

WATER, FERTILIZER AND GROWING MEDIA -- Tomato production requires that plant nutrition is monitored carefully and regularly. Tomatoes require a well drained growing medium, regular watering and regular applications of fertilizer. The application of water is typically done with a trickle irrigation system composed of distribution lines with drip tubes or spray stakes. Drip tubes or spray stakes are placed at the base of each plant. Tomato plants use a great deal of water, especially in warm weather, so the use of a timeclock to control the irrigation system is highly recommended and relatively inexpensive.

Many types of fertilizer have been used for tomatoes. Generally, the fertilizer is moderate in nitrogen and high in phosphorus, potassium, calcium and magnesium. A grower must be sure that calcium and magnesium are included in the fertilizer program. Normal plant and fruit growth requires these nutrients to be present in the correct amounts. A number of companies - Cropking Inc., Hydro-Gardens, Inc., Totalgro Plant Foods are listed below - have excellent fertilizer mixes for tomatoes. The fertilizer typically comes in two parts, calcium nitrate and a complete fertilizer (without calcium). This is because calcium nitrate is not compatible with other fertilizers in the concentrated form. With two fertilizers, the irrigation system has two injectors, each to inject a specific amount of each type of fertilizer at each watering. In our 1995 and 1996 tomato trials at UK, we successfully used Scotts (Peters) MiracleGro Excel CalMag, 15-5-15, fertilizer as a single fertilizer from a single injector. This fertilizer combines all nutrients in the same mix; others have reported of the successful use of this fertilizer for greenhouse tomatoes.

Many growing media can be used successfully for greenhouse tomatoes. Good field soil in the greenhouse floor, packaged commercial growing media composed primarily of composted bark, peat or coir, perlite alone, peat-lite mixes, rock wool slabs, straw bales, uniform river gravel, a thin layer of irrigation water in a plastic tube (NFT - nutrient film technique), controlled water table irrigation system, etc., will work if the grower correctly manages fertilizer and watering. Rock wool slabs seem to be the most economical and are used in most greenhouse tomatoes in Europe and Canada. Perlite is commonly used in the U.S and seems to be the most economical. Commercial growing media are also used because they are quite flexible. The media can be placed into pots or simply left in the bag and the bag is laid on the floor of the greenhouse. Each plant requires ½ to 1 cubic foot of growing medium. A number of growing media - peat based, bark based, coir based, soil based and muck peat based potting soil - were used in the greenhouse tomato trials at UK in 1995. There were no significant differences in fruit yield between these mixes with the variety `Solarset.' Tobacco growers involved with the same project used the same medium they used for tobacco transplants with success.

Table 2. Tomato fruit yield from the determinate variety `Solar Set' grown in six different growing media in greenhouse tomato trials in the fall of 1995.

TOMATO VARIETY SELECTION -- Tomato variety selection is difficult. Many cultivars have been selected for greenhouse production in Europe, Canada and the U.S. The best varieties in the best greenhouses produce 35 to 45 pounds of fruit per plant in 10-12 months. Most of that production occurs in spring and summer. However, good garden varieties will perform well when grown as a spring or fall crop in greenhouses in Kentucky. Growers should also consider cherry or salad sized tomatoes depending on local market interest. Optimum fruit production will occur from greenhouse tomato cultivars but new growers can learn details of production on good garden cultivars.

The 1995 UK greenhouse tomato trials involved the use of determinate field tomato varieties. We chose these varieties because they produce only 4-6 clusters of fruit. We hoped all fruit would be harvested from October 15 to December 15 in order to reduce the typical high heat costs and low light problems of winter production. Observations in the fall of 1994 demonstrated that the determinate varieties 'Solar Set,' 'Mt. Spring' and 'Sunbeam' might be successful.

Table 3. Tomato fruit yield from four determinate varieties and one indeterminate variety (Greenhouse 761) from greenhouse trials in the fall of 1995. Seed were sown July 6, 1995 or July 18 (for Solar Set-18) and transplanted August 15. Fruit were harvested from October 20 to December 20, 1995.

The average total weight of fruit harvested from each plant ranged from 6.1 to 7.8 pounds (Table 3), but was quite variable between plants ranging from 4 to 13 pounds per plant. Approximately 11 fruit were harvested from each plant, as an overall average, and the average fruit weighed about 10 ounces. There were no statistical differences between the yields of the five varieties in the UK trials in 1995, but we felt `Solarset' was the best variety for fall performance.

The 1996 UK greenhouse tomato trials compared determinate field tomato varieties with indeterminate garden varieties and indeterminate commercial greenhouse varieties. Two of the garden varieties selected had small fruit size so potential yields of these specialty salad tomatoes could be determined.

Table 4. Tomato fruit yield from five determinate varieties and nine indeterminate varieties (four garden varieties and five commercial greenhouse varieties) from greenhouse trials in the fall of 1996. Seed were sown July 15, 1996 and transplanted August 20. Fruit were harvested from October 20 to January 8, 1997.

The average total weight of fruit harvested from the determinate varieties was quite similar in both years, but fruit size decreased and fruit number per plant increased in 1996 (Table 4). Yields of 'Better Boy' and 'Celebrity' and the commercial indeterminate varieties had significantly higher yields than the determinate varieties. The average total weight of fruit and average fruit number per plant was greater but average fruit weight was variable. However, harvest was delayed 1 to 2 weeks on the greenhouse varieties when compared to the determinate varieties. The small fruited varieties performed reasonably well. The determinate varieties grew to a height of 3 to 4 feet while the indeterminate varieties, topped after the 6th cluster, grew to 6 feet tall. The general yield increase in 1996 could be attributed to better weather in November and December compared to 1995. Ten pounds per plant was the general goal of this demonstration project when it was initiated and this goal was met with standard greenhouse varities. Thus, it is appropriate that greenhouse operators choose commercial greenhouse tomato varieties, e.g. 'Trust,' 'Caruso,' etc. for best yields for fall tomato production.

FLOWER POLLINATION -- Tomato flowers must be pollinated in order to get fruit set and fruit development. Traditionally, flower clusters are shaken manually with a tomato flower pollinator as soon as the yellow petals open. Pollination must be done every day, seven days a week, usually between 9 a.m. and 1 p.m. Bumblebees are available from insect companies for pollination as well. Simply purchase a box or hive of bees and place them in the greenhouse when tomato flowers open. The bees do a very good job, just be careful to protect the bees from pesticide applications. In the 1995 and 1996 UK trials, we deliberately did not make a special effort to pollinate the flowers by hand or with purchased bees. We left the greenhouse sidewalls open during the day and feral (native) bees and the wind were responsible for pollination and the fruit yields described above. Additionally, we did not use pesticides while flowers were open. We cannot recommend that all pollination can be done this way, but it was reasonably successful during our trials.

PRUNING, SUCKERING -- Tomato plants in a greenhouse are pruned to a single stem. All lateral branches or suckers must be removed when they are one to three inches long. This allows for maximum air circulation and simplifies pest control problems. Suckering must be done regularly, plants should be checked at least once per week.

PEST CONTROL -- Insects and diseases can be a big problem because so few pesticides are labeled for greenhouse vegetables. Remember, the pesticide label must identify tomatoes and greenhouse applications for it to be legal in the greenhouse (see ENTFACT-36, Controls for Greenhouse Vegetable Insect Pests and PAT-4, Greenhouse Pesticides and Pesticide Safety). The cleared pesticides have low toxicity, so they MUST be applied thoroughly and regularly. Start a regular disease and insect control program after the plants have been set for a week and continue this at 7 to 10 day intervals for the life of the crop. Use sticky yellow cards to monitor the pest population in the greenhouse. Do not wait until your plants are infested to start spraying. All tomato growers should learn about biological control insects and consider using them from the beginning of the crop.

MARKETING -- The market for fall greenhouse tomatoes is quite good. Tomatoes can be sold directly from the greenhouse at retail prices or sold to wholesale distributors, supermarkets or restaurants at wholesale prices. Be sure to contact tomato buyers early in the season so they will know you will have fruit and so you may learn how they want the tomatoes packaged for their use. We hope that greenhouse tomatoes will always receive at least $1.00 per pound.

ECONOMICS -- Success in greenhouse tomatoes depends completely on fruit yield. Yields of 4-5 pounds per plant are probably break-even for annual costs. However, you should keep good records through the crop, so you can honestly evaluate your costs and returns. One thousand plants in a greenhouse (4000 square feet) can produce approximately 7,000 to 10,000 pounds of fruit from October 15 to January 1 and thus return approximately $ 7,000. to $10,000. to the greenhouse operator, based on our data from trials in 1995 and 1996.

Recommendations for a Fall Tomato Crop

These practices were utilized during the fall of 1995 when 12 tobacco farmers in 6 counties cooperated with UK to trial fall tomato production.

1. 1. If you plan to use side wall ventilation, it is best to have the rows of plants run across the greenhouse (perpendicular to the sidewalls) to allow for best air movement. Recognize that roll-up sidewalls are quite energy inefficient and may increase your heating costs dramatically. If you will use fan ventilation with no sidewall ventilation, the rows can be arranged parallel with the greenhouse sidewalls.
2. 2. Determine the type of structure that you will build to support the tomato crop based on the orientation of the plant rows.
3. 3. Install the trickle irrigation system, the injector(s) and timeclock that it works properly. Obtain a conductivity meter to regularly check the fertilizer concentration (conductivity) of the fertilizer solution used for irrigation. If you plan to use soil or your own formulated soil mix, have this mixture tested before you plan to use it. Tests should be completed 3-6 weeks before plants will be planted to allow enough time to follow recommendations received. Bring samples to your county extension agent who will send the sample to the University of Kentucky Soil Testing Laboratory. Be sure to label the sample "Greenhouse Test," fill out the soil test form for greenhouse soils and include information on previous crops, fertilizer applied and crops you plan to grow.
4. 4. Make sure that the greenhouse and its heating, ventilating and air circulation systems are ready for use.
5. 5. Be sure the necessary supplies are on hand, e.g., pots, fertilizer, insecticides, fungicides, string, labels, marketing containers and spraying equipment.
6. 6. Tomato seed should be sown July 5 - July 15 for a fall crop. Growing medium temperature should be maintained at 70 -75 F during germination. Seedlings can be transplanted to pots or cell flats about 2 weeks later. Seedlings will tolerate the high temperatures in summer and may require water twice a day. Apply fertilizer, 50 ppm, about a week after transplanting and again just before the plants will be transplanted into the bags in the greenhouse.
7. 7. Plant six to eight week old plants into growing media bags August 10-20. Allow 4 sq. ft. of floor space per plant. Use the row spacing most convenient to you. In the 1995 trials, we planted 4 plants in two rows in a standard three cubic foot bag of growing medium. The growing medium bags should be end to end in rows and these rows should be on 4½ to 5 foot centers. Thoroughly water each plant daily until the roots are established in the soil. Summer greenhouse temperatures can cause soil temperatures in the bags to reach 100 F and damage the root system. Consider an external shade (saran or tobacco canvas) for the greenhouse, and internal shade (tobacco canvas) over the support structure or an extra layer of black plastic over the growing medium bags to keep the media as cool as possible. Water the plants thoroughly 1-2 times per day.
8. 8. Begin to fertilize the tomato plants immediately after transplanting and continue every day until early December. Use Peters (Scotts-Sierra) Excel CalMag, 15-5-15, to fertilize at 100 ppm for the first two weeks. In September, use 150-175 ppm fertilizer twice per day. As soon as the first fruit form, increase to 200-300 ppm fertilizer with every watering. As the last fruit are ripening in November and December, the fertilizer rate can be reduced to 100 ppm with every watering. Growing medium samples should be taken in early September and early October to be sure the nutritional status is appropriate.
9. 9. Plants grow rapidly in September producing large leaves and lateral branches. Large individual leaves indicate that the plants are growing normally and a good indicator of the quality of plant care.
10. 10. The first flowers will appear in mid September. Be sure the first flower clusters are pollinated properly, they are a significant part of the fruit yield for the fall. Do not use pesticides that will kill bees if you use bees for pollination.
11. 11. Sucker and tie-up the plants weekly to make sure the plants are properly supported. Be extremely observant of the location of lateral branches or suckers. It is easy to mistakenly remove the main stem rather than a sucker. If you remove the main stem, you could cut fruit yields in half. Watch for aphids, whiteflies, beet armyworms and caterpillars of all types in the plant canopy. Do not allow the plants to wilt at any time. Fruit will be damaged if the plants do not get high amounts of water and fertilizer regularly.
12. 12. Monitor greenhouse temperatures closely. Night temperatures should not fall below 60 F while pollination is occurring. When the first cold days occur in October, allow the greenhouse to stay on the warm side rather than the cool side; tomato plants show cold damage quite easily. Be sure the air circulation system inside the greenhouse is working properly. Cool rainy weather in October and November creates an excellent environment for foliar diseases on tomato.
13. 13. Harvest ripe fruit every 3-5 days, wash and package appropriately for your market.
14. 14. Allow the greenhouse to freeze during January to kill all insects and diseases so they will not have an impact on your spring tobacco crop.

Additional Sources of Information on Greenhouse Tomatoes

• American Greenhouse Vegetable Growers Association. P.O. Box 25058, Colorado Springs CO 80936. Annual educational meeting and trade show; Newsletter;
• Florida Greenhouse Vegetable Production Handbook, Vol. 1, 2, 3. 1991. Publications, IFAS Bldg. 664, Gainesville FL 32611-0011. (about $20.00);
• Greenhouse Tomatoes, Lettuce, and Cucumbers 1979. S.H. Wittwer and S. Honma. Michigan State University Press, East Lansing MI 48824. (about $35.00);
• Hydroponic Food Production. 1995. Howard M. Resh. 5th Edition. Woodbridge Press, Santa Barbara, CA 93160. (about $45.00)
• http://res.agr.ca/harrow -- Greenhouse Processing Crops Research Centre, Harrow Ontario CANADA
• http://www.ces.msstate.edu/~ricks/ — Greenhouse Vegetable Publications at Mississippi State University
• http://www. teleport.com/~tomalex -- Growing Edge Magazine
• http://www.growroom.com -- Hydroponics at Grow Room

Suppliers for Greenhouse Vegetable Production

• Agro Dynamics, 12 Elkins Road, East Brunswick NJ 08816; 908-257-4000
• Bruinsma Seeds, P.O. Box 1463, High River, Alberta, CANADA; 403-652-4768
• Cropking Inc., Box 310, Medina OH 44258; 216-725-5656; http//www.cropking.com
• De Ruiter Seeds, Inc., P.O. Box 20228, Columbus OH 43220; 614-459-1498
• Hydro-Gardens, Inc., Box 9707, Colorado Springs CO 80932; 800-634-6362; http://www.hydro-gardens.com
• Totalgro Plant Foods, P.O. Box 805, Winnsboro LA 71295; 800-433-3055.

The 1995-1996 Fall Greenhouse Tomato Trials received a Program Enhancement Grant from the Cooperative Extension Service in the College of Agriculture at the University of Kentucky. Wenwei Jia, Dave Spalding, Mary Hoelterhoff, Darrell Slone, Janet Pfeiffer and Dave Lowry contributed to its completion.

Trade names used in this publication are used for identification purposes only. No specific recommendation is implied to the exclusion of other similar products.