Temperature and Humidity Management PDF Print E-mail
Written by Matthew Blanchard, Ph.D. Technical Services Specialist   
Friday, 28 October 2011 02:36

Temperature influences many processes during plant development, including seed germination, flowering time and plant habit. Temperature management varies depending on the crop stage. Humidity is another important factor to monitor and control during production. High relative humidity reduces plant transpiration and also increases the incidence of disease. Knowledge of how temperature and humidity influence crop production and how to manage them will help improve efficiency and quality. Here are some tips for managing temperature and humidity in the greenhouse.

Temperature Management

Monitor environmental controls. Proper temperature and humidity management in the greenhouse begin with a reliable climate-control system. Never assume that everything is working properly. Check historical climate data at least once a week to determine whether your system is delivering the desired temperature and that the greenhouse isn't cycling too frequently between heating and cooling.

Don’t forget about media temperature. Many growers monitor and control air temperature, but they sometimes overlook media temperature. This is especially important during seed germination and while cuttings are rooting. The optimum media temperature for germination or rooting varies by species. In general, at cool media temperatures, germination percentages decrease and rooting times increase. Some crops such as New Guinea impatiens, need to be grown at warm air temperatures, will benefit from using bottom heat during finishing. As media heats, warm air rises and warms the plant canopy.

Don’t cheat on heat. The average daily temperature (ADT) controls how fast or slow a plant develops. Crops respond differently to changes in ADT. For example, lowering ADT from 65° F to 60° F delays flowering of some plants by more than two weeks. When managing temperature, know how different crops will respond to adjusting set points.

Group crops. Crops have different temperature responses, so be sure to grow crops with similar temperature requirements in the same greenhouse section

Know how to use DIF. Most growers know that DIF, the difference between day and night temperatures, influences plant height. The greater the DIF, the stronger the response. Lowering the night temperature will help to save money on heating costs, but remember that a positive DIF will promote stem elongation. Delivering a negative DIF is a great strategy to suppress stem elongation, but it may not be economical in locations where outside temperatures are cold at night. Although many crops respond to DIF, others, respond very little or not at all. As mentioned above, plants develop in response to the 24-hour ADT; therefore, DIF typically has no effect on crop timing. For example, crop timing is similar if plants are grown at a constant temperature of 68° F (20° C) or a day/night of 73° F/63° F (23° C/17° C) if the day and night are each 12 hours long.

Consider morning temperature drop. An alternative to negative DIF to suppress stem elongation is to deliver a 5-15° F (3-8° C) morning temperature drop. Because plant elongation is greatest during the first daylight hours, lowering temperatures about 30 minutes before sunrise and throughout the first few hours of the day will reduce the elongation rate. Generally, the greater the drop in temperature, the greater the inhibition of stem extension. Because a temperature drop will decrease the 24-hour ADT, increase the afternoon and/or night temperature to offset the cool morning temperatures.

Pay attention to plant temperature. The actual plant's temperature controls how fast or slow it develops. A plant's temperature is influenced by many factors, and depending on conditions, plants can be several degrees warmer or cooler than the surrounding air temperature. For example, when using HID lamps for supplemental lighting, plant temperature can be warmer than air temperature. During cold and clear winter nights, crops grown without screens extended overhead can be several degrees cooler than the air temperature because of radiative cooling. Knowing the actual temperature of a crop will help improve scheduling. The best tool to measure plant temperature is a hand-held IR thermometer.

Humidity Management

Air's moisture-holding capacity is related to temperature. Warm air holds more moisture than cool air. As temperature decreases, relative humidity increases. When transpiration is low because of high humidity, nutrients such as boron and calcium that move via transpiration can become deficient in young shoot tips. Here are some cultural and environmental tips to reduce high humidity in the greenhouse.

Keep the greenhouse dry. Avoid irrigating late in the day and allow surfaces to dry before night. Provide sufficient floor drainage to minimize water puddling. In general, relative humidity above 85-90 percent should be avoided during finish production.

Provide good air movement around plants. Operate horizontal air flow fans, increase plant spacing and use benches with holes (e.g., expanded metal). A recommended air velocity around plants is approximately 0.5 to 2 feet per second (0.2 to 0.6 meters per second). Providing bottom heat will also help increase air movement because warm air rises and creates circulation around plants.

Provide adequate ventilation. Ventilation will allow moist greenhouse air to be exchanged with dryer outside air. This can be challenging during winter in cold climates, when greenhouses are sealed up tight, but opening the vents as little as an inch will allow humid greenhouse air to escape. During winter, you'll have to compensate for ventilation by heating to raise the temperature of the cool, dry air brought in from outside.

Watch for condensation. When temperatures cool down to the dew point, condensation can occur on greenhouse glazing and plant leaves. On greenhouses glazed with poly, consider choosing plastics with anticondensation films. Anticondensation sprays also can be applied to the greenhouse glazing to reduce dripping water.