February 2014 Newsletter (vol.26)   >>   Keep it in Control



When growing plants in a greenhouse, a grower should be in control of all the needs of a plant. Controlling the water and the light is on the top of that list, but what is equally important is controlling the climate that the plants are grown in. A climate’s temperature and humidity contribute many things to the development and health of a plant, and when optimized, it could help reduce the need of chemicals.


The temperature at which a plant will thrive differs by crop and variety. Temperature affects many functions of the plants such as photosynthesis, flowering, fertilizer uptake and growth. First you need to decide what type of crop you are growing, is it a cool season or a warm season crop? Cool season crops thrive in cooler temperatures that range between 55° and 65° Fahrenheit, while warm season crops thrive in temperatures ranging from 65° to 75° Fahrenheit. Growers can use temperature to manipulate plants by implementing a DIFF program. DIFF is the difference in daytime temperature and nighttime temperature, and growers who use this program can control the size of plants with minimal use of plant regulators.


To determine your DIFF, subtract the nighttime temperature (which is usually the daily low) from the daytime temperature (usually the daily high). A positive DIFF promotes a longer distance between internodes, which will create a taller, slightly leggier, plant. A negative or zero DIFF will promote a shorter distance between internodes creating a more compact plant. To see the effects of this practice, DIFF needs to be incorporated into the daily growing regime. You can achieve your desired DIFF by using a controller that maintains your preferred greenhouse climate.


Along with temperature, having the right relative humidity levels in a greenhouse is important for plant health. Growers should strive to keep relative humidity levels between 25% and 75% for optimal growing conditions. There are meters available to help find what the humidity level is in your greenhouse as well as meters that can be attached to a controller. High levels of relative humidity can promote fungal infections due to the fact that fungi thrive in a moist environment. Increased levels can also reduce the rate of evaporation, keeping the water on the leaves from evaporating at an optimal time. Climates with high levels of humidity prevent mineral transportation within plant tissue, as well as slowing water movement and decreasing the uptake of any nutrition.


High levels of humidity in the greenhouse usually occur in the cold winter months when greenhouses are sealed and ventilation is minimized to reduce heat loss. Cold weather can promote condensation on greenhouse glazing, which in turn drips onto the plants below allowing for fungal activity to occur. Along with this, moist humid air is harder to heat than drier air. Without bringing in the drier outside air the humidity will continue to climb every time irrigation occurs and when plants transpire, making it harder to heat. This too creates an increased chance that fungal problems might arise. To combat this, it is recommended to bring some outside air in for a brief period of time, which can be done at the end of each day to create your desired level of DIFF.


While higher levels of relative humidity can create challenges, you can also run into problems when levels are too low. Some pests, such as spider mites, thrive in an arid environment. Arid/drier environments also increase the rate in which plants transpire, creating a need for increased watering’s. This will not only increase labor, but also water usage and fertilizer. Another symptom of low level humidity is stunted plants and leaf tip burn due to the fact that plants photosynthesize more at higher levels of humidity. This occurs because plant stomas open more as humidity increases, taking in more CO2 for photosynthesis to occur. Hot, windy summer temperatures can also contribute to dry conditions making your humidity battle seem never-ending. An easy way to combat a dry, low level of relative humidity environment is to install evaporative cooling and fogging systems in your greenhouse.


It is important to remember, climate is just as big of a factor as water and light. Making sure your greenhouse is at the right temperature to facilitate the best growing environment for cool or warm season plants is key. Using a DIFF program to control plant height can assist in reducing the amount of growth regulating chemicals needed. Maintaining the proper amount of relative humidity will also ensure healthy plants that require fewer pesticides and less maintenance. Ultimately, having a greenhouse with the right growing conditions should prevent many problems before they occur allowing you, as the grower, more time to concentrate on other growing aspects.




“Greenhouse Temperature” by J. Raymond Kessler, Jr
“Managing the Greenhouse Environment to Control Plant Diseases" by Brian Eshenaur and Robert Anderson
“Understanding Humidity Control in Greenhouses”