Using temperature to maximize crop quality and profitability is a critical part of growing in greenhouses. It was also the topic discussed by John Erwin, Ph.D., of the Plant Science & Landscape Architecture Department with the University of Maryland at Cultivate’22.

“We don’t appreciate temperature as much as an industry as we should,” Erwin said. Temperature affects plants’ development rates, stem elongation, photosynthesis, flowering and even their roots.

When it comes to the development rate, the speed at which plants grow depends on their average daily temperature. Ideally, this is between 50º and 86º F. Over 86º, the rate slows, which extends growing length. Erwin said this is happening to more and more growers throughout the U.S. as daily temperatures slowly rise. (Temps over 86º affect insects too, he noted – they don’t like it very hot either.)

Growers should remember that a leaf’s surface temperature is five to seven degrees higher than the air on a sunny day. “Everything you do should be based on keeping leaf temperature the same,” Erwin said. He added that many growers ask about the opposite issue, though – during the cold season, should they not heat as much?

“Reducing the temperature by one degree is not always cheaper, because plants will be sitting in the greenhouse longer,” he replied. Some notable examples included petunia “Purple Wave,” which sees development delayed by 3.3 days when the average daily temperature is reduced by 1º; blue salvia “Strata” is delayed by 3.1 days; petunia “Avalanche Pink,” 2.5 days; and angelonia “Angelface White,” 2.4 days.

Erwin said the estimated heating cost per square foot to produce a “Purple Wave” petunia crop in Boulder, CO, increased from 32 cents to 40 cents per square foot when the average temperatures were dropped from 68º to 63º.

Some plants may even see chilling injury if they’re too cold, usually expressed as leaf yellowing. Erwin suggested grouping plants that tolerate similar temperatures to reduce heating costs as much as possible.

Plants that require a high temperature (68º – 78º) greenhouse include angelonia, petunia, tomato, coleus, zinnia, gomphrena, cosmos, celosia, peppers, basil and many grasses. Those that are better with a moderate temperature between 63º and 72º include calibracoa, impatiens, snapdragon, verbena, some petunias, portulaca and sunflowers. On the cooler side (55º – 65º) are pansy, viola, nemesia, dahlia, dianthus and alyssum.

Erwin said a new plug should always be considered for a warm temperature greenhouse. “It’s quite common cuttings are stored at improper temperatures and that impacts the rootability of the cuttings,” he explained. The optimal temperature for the rooting of cuttings and roots is a 72º – 76º growing medium temperature – but the top of the plant is what influences its growth rate, not the root temperature.

The temperature in your greenhouse can impact your plants in a few key ways.

Plant stem elongation increases as the difference between day and night temperature increases. Erwin explained that plants grow the most at the end of the night and the beginning of the day. To reduce elongation, do a temperature drop at those times (around 4 or 5 a.m.). This is especially notable since approximately 80% of heating costs accrue at night.

When it comes to photosynthesis, plants can only absorb so much light. In addition, if it gets too hot, photosynthesis slows down. Just two to three hours of high temperature exposure can reduce photosynthesis in some plants for days.

“These periodic high temperatures we’re seeing throughout the country are impacting photosynthesis way more than we realize,” Erwin said. “The worst thing you can do is let it get hot and then pull up the covers,” as that will trap heat inside. “It’s better to have an open roof greenhouse.”

He noted that one of the easiest things growers can do to improve crop quality is simply to add carbon dioxide – no lights, no other equipment needed. “And outside air is free,” he added.

Flowering is one of the most heat-sensitive things plants do. Erwin emphasized to never let night temperatures on short day plants (like poinsettia, gomphrena, zinnia, cosmos, mina vine, etc.) get warmer than 72º – 74º in the middle of the night. It will mess them up.

“A lot of plants have an optimal temperature for flowering but none benefit from going above 75º,” he said. And with the way the climate’s been shifting, “we need to develop heat-tolerant varieties quickly.”

Flowering is also affected by dormancy. A number of floriculture crops from the North make their flowers in August and September of the previous year. Those flowers go dormant for winter, and to break dormancy for continued flower development, plants need to either receive six to eight weeks of sustained cold temperatures (at least 42º – 46º) or be sprayed with a plant growth regulator plus receive lighting. That cold period causes eventual flower initiation, in a process called vernalization.

The temperature of roots should also be a factor in how warm your greenhouse is. Erwin said media temps often get much too hot. Root death occurs at 122º – but it may be mistaken for root rot.

He suggested cooling growing media with shading, by using white or reflective pots, grouping plants together or growing on gravel. “You’ll kill less plants and plants will get bigger faster,” he said of these techniques.

by Courtney Llewellyn