» Optimal daytime and nighttime temperatures for greenhouse tomatoes vary depending on growth stage and light conditions.
» Greenhouse humidity levels affect water and nutrient uptake by tomato plants.
» Tomato watering schedules should reflect the water demands of the tomato plants.
Air temperature affects several aspects of tomato development, including growth, photosynthesis, respiration, and physiological development.1 Temperature also affects relative humidity in that warm air can hold more water than cool air, and relative humidity is expressed as a percentage of the maximum amount of water that the air can hold. Understanding the effects of temperature is necessary to optimize the production of tomatoes in protected culture systems. Thermometers should be located near the center of the greenhouse at blossom height to avoid hot or cool spots near doors, vents, or heaters. For the most accurate measurements, thermostats should be enclosed in a shaded, aspirated box and calibrated periodically.2
In general, the optimal daytime temperature for tomato production is in the range of 70 to 82°F (21 to 27°C), and optimal nighttime temperatures are between 62 and 64°F (17 and 18°C). However, optimal temperatures vary somewhat with the growth stage of the plant. Nutrient deficiency symptoms, primarily from the lack of taking up phosphorus, start to appear at temperatures below 60°F (16°C). At temperatures above 86°F (30°C) lycopene, the red pigment in tomatoes, does not form and fruit do not color properly.2
Temperatures should be held at 77°F (25°C) for seed germination and the first few days after emergence. After that, the temperature should be gradually lowered over a period of two days to 66 to 68°F (19 to 20°C) for both daytime and nighttime. Maintain this temperature until it is time for transplanting seedlings into mineral-wool blocks. At transplanting, set the temperature to 75°F (24°C). Maintain this temperature for one to two days after transplanting and then gradually lower by 2°F per day (1°C per day) until the temperature reaches 66°F (19°C) for both daytime and nighttime.1
When plants produce their first flower clusters, they are transplanted into the production greenhouse. At this point, temperature is used to adjust the balance of plant growth between vegetative (forming leaves and stems) and generative (forming flowers and fruit), as well as the relative strength and weakness of the plants. Lower 24-hour average temperatures result in stronger (thicker stemmed) plants, and higher 24 hour average temperatures result in weaker plants. Constant (day and night) temperatures near 77°F (25°C) promote vegetative growth, while maximum fruit production occurs with nighttime temperatures of 64°F (18°C) and daytime temperatures of 68°F (20°C). Younger plants tend to grow more vegetatively because of their low fruit load. So lower temperatures [a daily average of 66°F (19°C)] promote a shift in the balance towards fruit production. As plants mature they tend to grow more generatively, and slightly warmer temperatures [68 to 72°F (20 to 22°C) daytime and 62 to 66°F (17 to 19°C) nighttime] help the plants maintain some vegetative growth.1
Optimum temperatures also vary with light levels. On days with low light levels (cloudy days) temperatures should be somewhat lower than on days with high light levels (sunny) (Table 1).1
Tomato plants can tolerate a wide range of humidity levels, but for optimum fruit quality humidity levels should not fluctuate greatly in a short amount of time. Production guides for greenhouse tomatoes vary somewhat in their recommendations for optimal relative humidity (RH) levels; anywhere from 60 to 85%.1,2 Higher humidity levels can result in better fruit set, to a point, but high humidity also favors disease development. The optimum relative humidity for pollination is 70%. At RH levels above 80%, pollen grains start to stick together, reducing their dispersal and lowering pollination rates. The stickiness of pollen goes down as humidity levels drop. At RH levels below 60%, the stigma (the female part of the flower that receives pollen) can dry out, also lowering the rates of pollination.2
Maintaining humidity levels near 70% RH can be difficult in areas with humid climates. Relative humidity directly affects the vapor pressure deficit (VPD), which is the difference in water vapor content of the air inside and outside of the tomato leaf. The VPD is a measure of how strongly the water is pulled out of the leaf. The air inside the tomato leaf is saturated (100% RH). If the VPD is too low (high % RH in the air outside of the leaf) transpiration is reduced because water is not evaporating from the leaf. As a result, water does not move through the plant, and some nutrients (such as calcium) are not adequately taken up by the root system. If the VPD is too high (dry air in the greenhouse), plants can transpire a lot of water, which can cause plant stress. The optimal range of VPD is between 4 and 8 mbar (Table 2).2
The volume of water needed by tomato plants for optimal growth and production varies with the season and size of the plants. Young seedlings (transplants) usually need about two fluid ounces (50 ml) per plant per day, while mature plants need two to three quarts (2L) per day.1,2 On average, 90% of the water taken up by the plant is used for transpiration (cooling), and the remaining 10% is used for growth.3 Inadequate amounts of water lead to wilted plants, and wilted plants are not productive. Prolonged water deficits can lead to permanent wilting, which can result in the death of the growing tips of the plant.2
The water should be provided in six to twelve (or more) applications per day, depending on the type of growth medium used. Well-drained media, such as rice hulls, may need more than twelve applications per day. The timing of applications should be determined by the water demand of the plants, with most of the applications occurring during the day. Some nighttime watering may be needed during periods of low humidity. More applications may be needed on sunny days to accommodate the increased rates of transpiration by the plants.3 Enough water should be applied that some drainage occurs from the growth medium bags (10 to 20% of the water applied), and this additional amount of water needs to be added to the total amount of water required.