Greenhouse - Plant Temperatures vs Air Temperatures

If the temperature of your greenhouse plant is lower than the air temperature, it will develop more slowly. The following is a good article on plant temperatures versus air temperatures.

Temperature controls plant development (leaf numbers, time to flower) and height (DIF). The temperature of the plant is determined by air temperature, light intensity, humidity, air movement, media temperature and water status. In the past growers were comfortable measuring just air temperature since it’s easy to monitor and is “close enough” to the real temperature of the plant. However, high fuel bills have made “close enough” not very comfortable anymore.

Measuring plant temperature isn’t easy. The growing point (meristem) is where the plant senses temperature so that’s where we need to measure. But growing points are small and often buried under leaves so they’re hard to reach reliably. During research it’s OK to carefully put small thermocouples into the growing points, but that wouldn’t work in production ranges. For under $100 (more likely less than $60) you can buy a small infrared thermometer that will quickly and accurately measure growing point temperatures. The units are about the size of a small (not a universal) remote control and have a red laser pointer that shows where the thermometer is reading. The closer to the surface the more accurate the reading. It takes no time at all to uncover the meristem, target the laser pointer and get a reading. Infrared thermometers are available from greenhouse suppliers or can be found on the Internet.

It’s fun to shine laser beams around and measure infrared temperatures, but does it really mean anything? Ever have an early New Guinea Impatiens crop that just didn’t grow? They looked healthy, had good roots but just sat in the pot for weeks. Dr. Royal Heins found the temperature of New Guinea growing points could be up to 15 degrees colder than air temperature. The cold growing point was caused by large amounts of water evaporating from the surrounding leaves due to low relative humidity in the house. The solution: increase the relative humidity. The effect: the plants started to grow. How much could you increase your profits if your early New Guinea’s started to grow immediately after planting? A lot more than you invested in the infrared thermometer.

Reprinted from "Air temperature doesn’t always equal plant temperature" by Dean Krauskopf, MSU Greenhouse Agent in the February 17, 2006 edition of the Greenhouse Alert newsletter from Michigan State University.