Greenhouse tomato production has attracted much attention in recent years, partly because of a new wave of interest in "alternative crops." The attraction is based on the idea that greenhouse tomatoes may be more profitable than the more conventional agronomic or horticultural crops. The popularity may also be due to misconceptions about how easily this crop can be grown.
While the value of greenhouse tomatoes is high on a per unit basis, the costs are also high. The following points are outlined to clear up misconceptions you might have
· Greenhouse tomatoes have unique cultural requirements, unlike crops such as soybeans and cotton, and not even similar to other field vegetable crops.
· The time necessary to grow greenhouse tomatoes is much greater on a per unit basis than any field vegetable crop. The many weekly cultural practices (pruning, wrapping, pollinating, spraying, and others) add up to a significant amount of time. The estimated average labor requirement per greenhouse (or bay) is 20 person-hours per week (for an average 24- by 96-foot greenhouse).
· Greenhouse tomatoes need regular attention. Unlike many field crops that can be planted, sprayed on a fixed schedule, and then harvested after so many days have elapsed, tomatoes must be examined daily.
· The greenhouse environment is not a sterile one. There is a common misconception that crops grown in greenhouses do not have insects and diseases. Just the opposite is true. While a greenhouse environment is excellent for growing tomatoes (and other vegetables), it is even better for propagating insect pests and disease organisms. Because of the higher temperature, higher relative humidity, and lush, green foliage, insects and diseases are constant threats once introduced into a greenhouse. Therefore, weekly sprays with both insecticides and fungicides are standard practice.
When growing greenhouse tomatoes, it is important to use the proper planting density. Greenhouse tomatoes need four square feet per plant or 25,000 plants per ha. To determine how many plants can be grown in your greenhouse, multiply the length in feet by the width and then divide by four. For a 24- by 96-foot greenhouse, about 576 plants can be grown. For a 30- by 96 -foot greenhouse, 720 plants will fit.
Note: If you will use some of the floor space for other purposes (for example, storage, packing, grading), subtract this area from the total before dividing by four.
Using a higher planting density will cause the yield per plant to decrease, while the yield per greenhouse will stay about the same. This is due primarily to plants' shading each other.
The first step in raising any crop is to choose the best variety. Growing a variety that is not the best choice or using seed that are not of the best quality reduces your potential for success at the outset. It is smart to start off with the greatest potential rather than limiting yourself by using inferior seed, even if it saves a few dollars.
Hybrid tomato seed is expensive. It now costs 10 to 25 cents per seed, depending on the variety and quantity that you buy. This cost reflects the laborious process of hand pollination required to produce the hybrid seed. Although this seems rather expensive, it is still one of the lowest costs of production. After the heating, labor, and fertilizer costs are incurred, the extra expense of using the finest seed is relatively small.
Hundreds of tomato varieties are available on the market, but only a few are acceptable for greenhouse production. If you plan to grow tomatoes in a greenhouse, you need to use a greenhouse variety. These are almost exclusively Dutch hybrid varieties, bred in Holland specifically for greenhouse production. Field varieties are typically adapted to higher light and lower humidity conditions and probably would not yield well in the greenhouse. A glass or plastic greenhouse has about 20 percent less light than outdoors, and many field types do not tolerate this reduction.
Base variety selection on these criteria:
· size of fruit desired
· disease resistance
· lack of physiological problems, i.e., cracking, catfacing, blossom-end rot
· yield uniformity of fruit size
· market demand
However, the market preference is for a red tomato. The varieties most worth considering at the time of this printing are Trust, Match, Switch, and Caruso.
Tropic cannot be recommended to commercial growers because of its lack of size uniformity, intolerance to high nitrogen fertilizer, and lack of resistance to Tobacco Mosaic Virus (TMV) and other diseases; however, Tropic is fine for a hobby greenhouse.
Jumbo may be the largest fruited variety available, but it lacks resistance to TMV and some other diseases, is not tolerant to higher levels of nitrogen fertilizer, and is not as uniform in size as some of the other varieties.
You can buy seed by the piece with lower costs per unit for larger quantities and higher costs for smaller quantities. Tomato seeds are very small; one-fifth of an ounce contains about 1,200 seeds. If you have a 2-bay greenhouse (4,500 square feet) with about 550 plants per bay, this is enough seed. Always plant a few extra seeds (10 - 20 percent), since germination will not be 100 percent. This also gives you the opportunity to discard any plants that do not meet your high quality standards. Store extra seed in unopened containers or in zip-locked bags in the freezer.
For best production, prune tomato plants to a single stem by removing all lateral shoots, commonly referred to as "suckers." One sucker will form at the point where each leaf originates from the main stem, just above the leaf petiole (stem). Allowing all suckers to grow and bear fruit would increase the total number of fruit, but they would be small and of poor quality. It is better to have one main stem that bears fruit, as this will produce larger, more uniform, and higher quality fruit.
Removing suckers once per week will keep them under control. It is advisable to leave one or two of the smallest suckers at the top of the plant. Then, if the plant becomes damaged and the terminal breaks off, one of these suckers can be allowed to grow and become the new terminal. Generally, remove any sucker longer than 1 inch.
Rig a support system of wires above the crop. Use 9-gauge or 3/32-inch galvanized wire or stronger (a 100-pound box contains about 1,700 linear feet). These wires should run parallel to the direction of the rows and 7 feet off the ground. Cut strings 14 feet long, so there is enough slack to allow the plant to be leaned and dropped when it reaches the height of the wire. Tie one string loosely (not a slipknot) or clip it to the bottom of each plant, throw it over the wire, and then tie it to the wire with a slip knot. Plastic clips are commercially available.
When the plant reaches the wire height, it should be leaned and dropped. Hold the string securely with your left hand just above the plant; loosen the knot with your right hand. Simultaneously let the plant down about 2 feet below the wire and slide the string to the right. The plant must be leaned over while it is dropped; otherwise the stem will probably break. Always lean in the same direction. Do not lean some to the right and some to left or they will shade each other. Lower all plants to the same height so they don't shade each other. Repeat this operation each time plants grow higher than the wire. This is another reason the two-crop-per-year system is preferable -- there is less labor needed for leaning and dropping the plants.
As you prune the plant to one main stem, wrap it around the support string. You can prune and wrap in one operation, doing both to a plant before moving on to the next plant. Always wrap in the same direction -- if you start clockwise, continue clockwise; otherwise, when the plant gets heavy with fruit, it may slip down the string and break. Some growers prefer to use plastic clips to secure the plant to the string, either in combination with wrapping or to replace wrapping.
Cluster pruning will also improve size and uniformity. This involves removing small fruit from some clusters, leaving three, four, or five of the best ones. Remove misshaped or deformed fruit first. Otherwise, remove the smallest fruit, which is usually the last one formed on each cluster.
In hybrids, pollination of the female flower part must occur before fruit will set. Any activity or inactivity that prevents thorough pollination reduces the number of fruit set per plant. Several problems can result from poor pollination: off-shaped fruit if seeds do not develop uniformly throughout the fruit, smaller fruit, and fruit that are rough (ridged) along the tops. Pollination can be prevented by various stresses such as cold or hot temperatures, drought, high humidity, nutrient deficiencies, nutrient toxicities, as well as lack of pollen transfer.
In the greenhouse, wind is not strong enough to shake the flowers sufficiently to transfer the pollen. Even though the greenhouse is ventilated with fans, on cooler days when the fans are not operating, the air is relatively motionless.
The optimum temperature for pollination is within the range 17 to 22 °C. Optimum relative humidity is 70 percent. Above 80 percent relative humidity, pollen grains stick together and are not dispersed well. With relative humidity less than 60 percent for extended periods, the stigma may dry out so that pollen grains will not stick to it. With ideal conditions, fertilization occurs 48 hours after pollination. Serious greenhouse tomato growers should use an electric pollinator to ensure good fruit set. (What is a "serious" grower? one who grows tomatoes for a profit?)
If you have 10,000 square feet or more under one roof, seriously consider using bumblebees for pollination. Purchase your bumblebee hives from commercial suppliers (see list at end of this publication). Use an electric pollinator if you have one or two gutter-connected bays, or more bays that are not under one roof. For ranges between 5,000 and 10,000 square feet under one roof, you need to compare the cost of the beehives to the cost of labor to achieve pollination. Generally, a hive will last for about 8 weeks, at which time; replace it with a new hive. It is a good idea to allow for some overlapped time between the old and new hives.
Note: Even if you use bumblebees, you will still need an electric pollinator to pollinate the first few flowers that open. Introduce hives when 50 percent of plants have open blooms.
Questions about Pollination
How often should I pollinate?
Pollinate every other day or three times per week. Pollinating less often is taking a chance on reducing fruit set but more often is very likely a waste of time.
Does the time of day matter?
Yes. The best time for pollinating is when the relative humidity is between 60 and 70 percent. Since this is difficult to control, find when the relative humidity is at its daily low point. If the amount of moisture in the air stays constant, the relative humidity decreases as the temperature increases, because warm air can hold more moisture than cool air. The warmest time of day is usually mid-day. This is why the best time to pollinate is generally between 11 a.m. and 2 p.m.
How does cloudy weather affect pollination?
In cloudy weather, the relative humidity is high. In such conditions, pollination is not as effective because pollen tends to stick together in clumps rather than dispersing as individual grains. It is important to stick to the schedule of every other day because if the cloudy days turn into a cloudy week without pollinating, fruit set and quality will certainly be decreased.
What if it is cloudy for a long period of time?
As mentioned, pollination in very wet conditions is not as effective as in dry weather. One technique you can try is to dry the air before pollinating. Turn on the heating system for 30 to 60 minutes before pollinating. Ventilate to maintain the temperature requirements. This will dry the flowers and the air, improving pollen transfer.
Is the expense of an electric pollinator really necessary?
If you want to get maximum yield, the answer is certainly "yes." If you would be happy with a lower yield, then it is not so important. In an experiment at the University of Southwestern Louisiana Center for Greenhouse Research, pollinating with an electric air blower resulted in a 7 percent decrease in yield, while not pollinating resulted in a 21 percent decrease in yield, compared to using an electric pollinator. You can purchase a pollinator from most greenhouse suppliers.
A day temperature of 20 to 25 °C is optimum, while night temperature of 17 to 22 °C is optimum for greenhouse tomatoes. During cloudy weather, a temperature closer to the lower end of these ranges is preferred, while in sunny weather, temperatures closer to the higher end are better. Below 15 °C, nutrient deficiencies may occur because plants cannot absorb some elements at cool temperatures. The first sign of cool temperature stress is purpling of the leaves, indicating lack of phosphorus uptake (even though there may be adequate phosphorus in the nutrient solution). One or two nights of 56 or 58 °F temperature can cause a considerable number of rough fruit several weeks later when fruit exposed to the cold temperature reach full size. You should maintain a minimum temperature of 18-22 °C. Ideally, the thermostat should be located at blossom height rather than above the tops of plants.
Avoid temperatures over 36 °C if at all possible. At 34 °C, lycopene (the pigment that makes tomatoes red) no longer develops.
Locate thermostats near the center of the house for good temperature control. Also, enclose the thermostat in an aspirated box, or shade it so that it indicates the air temperature correctly. If you let the sun shine directly on the thermostat, it will read a higher temperature than the air surrounding it.
The optimum relative humidity for greenhouse tomatoes is 60-70 percent.
· The most common materials are poly- propylene, polyethylene, polyester, and saran, although cheesecloth and tobacco netting can also be used.
· A maximum range to consider for tomatoes is 30 percent.
Irrigation should be controlled automatically with the use of time clocks or electronic controllers. The volume of water will vary depending on the season and the size of plants. New transplants need only about 2 ounces (50 ml) per plant per day. At maturity on sunny days, however, plants may need up to 3 quarts (2.7 liters or 2,700 ml) of water per plant per day. Generally, 2 quarts per plant per day are adequate for fully grown or almost fully grown plants. Monitor plants closely, especially for the first couple of weeks following transplanting, so that the volume of water can be increased as needed. Water should be delivered to each plant. This is usually done with "spaghetti tubing" that carries water from main lines to the base of each plant.
Each watering should include fertilizer, therefore, the process is more appropriately referred to as "fertigation." Most growers use from 6 to 12 waterings per day once plants are established. In a medium that drains extremely well, such as rice hulls, 12 or more waterings per day may be needed to keep plants from drying out between waterings. In pine bark, six waterings per day are usually adequate. The important point is that plants should receive enough water so they do not wilt. A wilting plant is not growing. If the permanent wilting point is reached because of a prolonged period without water, the growing point may be killed. To be certain that plants receive enough water, allow enough so some drainage from the bags (10-20 percent) is apparent after each watering.
Be aware that a prolonged cloudy period followed by bright sun may cause severe wilting. Be ready to increase the amount of water in this situation.
It is a good idea to check the nutrient solution pH daily. The optimum pH range for the nutrient solution is 5.6 to 5.8. If the pH of the solution is too high, caused by alkaline water, add an acid in small quantities to lower the pH to within this range. The choices of materials to use are sulfuric acid (H2SO4), nitric acid (HNO3), or phosphoric acid (H3PO4). You can buy sulfuric acid, the least expensive, from an auto supply store as battery acid. However, there is an advantage to using phosphoric or nitric acid, since they supply nutrients in addition to lowering the pH. (Phosphoric acid supplies phosphorus, and nitric acid supplies nitrogen.) Although they are more expensive than sulfuric acid, they are preferred for this reason. Phosphoric acid and nitric acid are inexpensive sources of these elements when compared to other fertilizers.
The subject of fertility is probably among the most confusing for growers of greenhouse tomatoes; however, it is important to production. The keys to a successful nutrition program include the following:
· Use fertilizer designed specifically for greenhouse tomatoes.
· Know how much of each fertilizer element is needed.
· Know how much is being applied.
· Check the electrical conductivity and pH levels.
· Look for signs that plants may be deficient or have an excess of a nutrient.
· Monitor plant nutrient status by periodically taking samples for tissue analysis.
General guidelines for amount of fertilizer to use
Stage of Growth
Total dissolved solids (TDS)
Germination to first true leaf fully expanded
First true leaf to third true leaf fully expanded
Third leaf to transplant
Transplant to second cluster set
Second cluster to topping
How do you know what is the right amount of fertilizer? In addition to following the directions on the bag and taking regular foliar analyses, the plant also gives an indication. If tops of plants "ball up" with dense, curling-undergrowth, the nutrient solution is a little high in nitrogen. Another sign of having nitrogen too high is when the clusters of flowers end in leaves or shoot growth (these can be pruned off). This condition will not necessarily decrease yield unless nitrogen is excessively high.
If stem diameter is extremely small and plants are spindly, fertilizer concentration is too low. Other signs include faded or yellowed foliage, decreased vigor, blossoms that don't set fruit, and yield reduction.
It is a good idea to have tomato leaf tissue analyzed periodically to determine if the plants are receiving the best levels of nutrients. This technique can be used to "troubleshoot" problems with unhealthy looking plants, or as a monthly check on nutrient levels. Save these monthly checks so you can refer to them in diagnosing problems that might occur.
Recommended levels of elements in tomato leaf tissue
5- 25 ppm