Whether you are growing in a greenhouse, high tunnel or operate a hydroponic greenhouse, you have experienced firsthand how temperature variation impacts plant growth. Growers know all too well that temperatures need to be regulated for best crop growth and development.

Doctor Krishna Nemali of Purdue University researched this concept with a greenhouse temperature control study. An associate professor of horticulture in Purdue’s Department of Horticulture & Landscape Architecture, Nemali teaches as well as conducts hydroponic research.

His recent webinar “Temperature Management in Greenhouses” included the impact of temperature variations on hydroponic-grown greenhouse crops; however, the structural temperature control segment applied to all types of greenhouse production.

During his research project, Nemali recorded and evaluated greenhouse air temperatures for hydroponic crops during their growing cycle along with real-time outdoor temperature data. With crop inputs and growing conditions the same, the study evaluated lettuce produced under various temperatures.

Seeds were sown at the same time and the control group’s temperature was kept at 50º F both day and night. Some of the seeds in that group did not germinate at all; those that did experienced very slow growth.

“When temperature goes below 50º, plant growth stops,” Nemali said. Research shows that the ideal temperature range for lettuce is 68º – 78º.

Impact of Temperature Extremes

“Both high and low temperatures affect plant growth and development,” Nemali continued. He explained that while some cold tolerant varieties can grow normally at lower air temperatures (58º to 63º), most plants require a range between 68º and 78º. Negative effects of air temperatures below optimal range are more prevalent in cold sensitive species, causing slow growth and delayed flowering. In many cases, these plants with a “base temperature” that varies between 36º and 53º continue to survive, yet simply stop growing.

At the other end of the spectrum, plants exposed to high temperature extremes experience heat damage that can include excess growth, damage to the final stage of leaf development and flower drop.

Part of the discussion covered different methods of maintaining optimal greenhouse temperatures during the heat of summer. Both passive and mechanical types of greenhouse cooling were discussed.

Dr. Krishna Nemali is studying how a range of temperatures reached while growing indoors can affect crops. Photo courtesy of Nemali Lab/Purdue University

Greenhouse Cooling

“As plants transpire, they absorb heat from the sun to evaporate water from inside of the leaves and cool their surface. The leaves absorb the heat and the greenhouse becomes more humid, but cooler,” Nemali said.

This process of plant transpiration explains why a greenhouse full of plants is easier to cool than one that is empty or even partially full. He added that other studies in their own greenhouses have shown that one square foot of actively growing lettuce plants can transpire about one-half to three-quarters of a pound of water each day.

Other methods of cooling include both natural and mechanical ventilation systems and shade cloth. Less effective in larger gutter-connected greenhouses, sidewall ventilation is more effective in individual hoop houses and high tunnels. It can incorporate sidewall vents of three to six feet that are placed slightly above plant height, allowing outside and inside air to mix before reaching plant material. Open vent areas should be approximately 15% to 20% of the greenhouse’s ground area for effective cooling.

Roof vents are another option, with noticeable improvement observed when roof venting and sidewall vents were combined. Open roof designs are becoming more popular, with those that open more than 50% of the roof area eliminating the need for sidewall venting.

“For effective [forced air] cooling, an air volume approximately 1.5 times the volume of the greenhouse should be ventilated every minute,” Nemali said, adding that exhaust fans do best when placed on the leeward side of buildings and that, ideally, fans should be spaced no more than 25 feet apart. It is also recommended to leave an open space of at least four to five times the fan’s diameter in front of each ventilation fan.

A more expensive method of cooling is fogging. It consists of high pressure spraying of fine water droplets into the air. The water droplets absorb the heat in the air and evaporate, decreasing the air temperature and increasing humidity at the same time. One disadvantage of this method is that the larger water droplets can condense, causing pest problems. Besides the investment of equipment and cost of operating it, good water quality is required. Not many greenhouses opt for this method of cooling.

When it comes to shade cloth, Nemali suggested using it as a last resort. “The best way to use shade cloth is to put it outside the greenhouse,” he said.  A wide range of shade cloth is available, with the 35% and 50% shading the most popular.

Whether your operation uses greenhouses or high tunnels, controlling temperatures is a challenge during the heat of summer as well as the cold months of winter.

by Gail March Yerke