Temperature variation within a greenhouse
In the production of greenhouse crops, plants are grown on the floor, on benches, or overhead in hanging containers. In each of these locations, the temperature and light environment can be considerably different. Growers are most familiar with the temperature variation that can exist horizontally within a greenhouse. For example, flowering can be delayed in crops grown adjacent to a cold sidewall compared to crops grown in the center of the greenhouse (Photo1).
Photo 1. The effect of a cold
exterior side wall on flowering
Temperature variation can also occur vertically within a greenhouse, from floor to ceiling. With the increased need for energy-efficient production, growers have asked us how much temperature varies between the floor, the bench, and overhead. We didn’t have any data, so we conducted an experiment to help answer this question by measuring the air temperature at different vertical positions above the floor in one of our research greenhouses at MSU. In the center of a greenhouse section, we placed a thermocouple at four different heights above the concrete floor: 0 ft 6 in, 3 ft 6 in (bench level), 6 ft 6 in, and 9 ft 6 in. Each of the thermocouples was inserted into a 2-inch diameter white rigid PVC pipe that was positioned vertically in the greenhouse (Figure 1). At each thermocouple location, small holes were drilled into the sidewall of the PVC pipe and a 2-inch diameter fan forced air across the thermocouple. The thermocouples were connected to a data logger that measured the temperature every 10 seconds and recorded hourly averages. We collected data with horizontal air flow (HAF) fans off or on for 8 days each.
Figure 1. Experimental setup used to measure air temperature
at four different heights above the greenhouse floor. At each
height, air temperature was measured with a thermocouple that
was aspirated with a 2-inch diameter fan.
As expected, the data collected indicate that the average air temperature within the greenhouse increased as the distance above the floor increased. However, the magnitude of temperature variation depended on the time of day and on the operation of HAF fans (Figure 2). The greatest temperature variation occurred during the day and there was generally little difference with and without HAF fans operating during this period. For example, during the 8-day period with HAF fans on, the air temperature near the floor during the day was on average 1.2 °F (0.7 °C) cooler than at bench height, and 3.0 °F (1.7 °C) cooler than 9 ft 6 in above the floor.
Figure 2. Measured air temperature at bench height (3 ft 6 in) and at 3 different heights above the greenhouse floor with or without the operation of horizontal air flow (HAF) fans. Data was averaged for 8 days with HAF fans on and 8 days with them off. The daylength was extended with high-pressure sodium lamps to provide a 16-hour photoperiod (from 6:00am to 10:00pm).
The use of HAF fans was most important during the night. For example, during the 8-day period, the average air temperature at night near the floor without HAF fans was 1.2 (0.7 °C) cooler than the temperature at bench height, and 3.3 °F (1.9 °C) cooler than 9 ft 6 in above the floor. When the HAF fans were operating during the night, the air temperature near the floor was similar to bench height and only 1.9 °F (1.0 °C) cooler than overhead.
This study provides an example of how much temperature variation can exist vertically in a greenhouse during the day and night with and without HAF fans. The magnitude of temperature variation will vary from one greenhouse to another and is dependent on the size and structure of the greenhouse, HAF fan characteristics, humidity, etcetera. These results also demonstrate the importance of using HAF fans especially during the night to mix warm air overhead with cool air on the floor. Are HAF fans installed and working properly in your greenhouse? For more information on the benefits and installation of HAF fans, read John Bartok’s article, available as a pdf file through the MSU Greenhouse Energy Resource website.
Remember that plant temperature, not air temperature, controls how fast or slow plants develop. Plants can be several degrees warmer than the air under high-intensity lighting and when it is sunny. In contrast, plants can be several degrees cooler than the air during the night, especially when humidity is low. Plant temperature can be easily measured with a hand-held infrared thermometer. This practice can give you an idea of how plant temperature varies from the air.
In conclusion, we encourage all growers to position their covered, aspirated temperature sensors at crop height. As our data shows, temperature measured just 3 feet above a crop can be a little warmer than at crop level. Thus, a temperature sensor mounted above a crop can lead you to believe that crops are growing at a warmer temperature than they actually are.