Grower 101:Growing Plants Cooler, Part II By John Erwin, Charlie Rohwer and Ryan Warner

In Part I (GPN September 2004) we discussed what happens to plant growth when we grow crops cooler. Here, we will discuss how dropping temperature affects diseases and insects. In addition, to help you decide if there is a benefit for your business we will present real numbers on how much you save on heating costs versus how much you lose through increased crop production time when you drop temperatures. Lastly, we review some quick and inexpensive ways to help reduce your heating costs.

Plant Diseases

One very important consideration when producing plants under cool temperatures is the potential for increased disease pressure. This is the result of three things: 1) Some diseases are more active at cool temperatures; 2) plants, benches and floors stay wet longer after each watering when temperatures are lower, providing a larger window for pathogen spores to germinate (spore germination of many fungi requires moist conditions); and 3) plant health of some species is weakened (optimal plant growth usually occurs at temperatures around 68º F).

Diseases caused by Pythium, Rhizoctonia and Thielaviopsis are promoted under cool temperatures; spore germination and proliferation of water molds require moist conditions. For instance, “damping off” is most severe at 53-68º F. Damping off diseases are not the only diseases promoted by cool temperatures. Thielaviopsis is the pathogen that causes black root rot and, unfortunately, is becoming a more common problem for pre-finished plants, particularly pansies, vinca and petunias. Thielaviopsis can be a problem under cool or warm temperatures, but Thielaviopsis activity is reported to be optimal at 62º F. Another widespread disease promoted by cool temperatures is gray mold caused by Botrytis. Botrytis is often a problem during shipping, when weather conditions are cloudy in spring and/or when crops grow together in a greenhouse, restricting air movement around plants. Previous research identified 71º F as optimal for Botrytis growth and 59º F as optimal for Botrytis spore production.When plants are grown cool, they require less water than when they are grown warmer. In addition, when you grow cooler, media will stay wet longer because standing water will take longer to evaporate off the media surface. Combined, these two factors can increase the root rot disease pressure on plants grown under cool conditions if water management is not altered as well. It is very important to actively scout for diseases and have a plan ready for applying fungicides before the need arises.Here are some suggestions from Steve Nameth, The Ohio State University, for reducing disease severity when growing under cool temperatures:

  • Don’t place seed flats/seedlings on the floor. Floor temperature can be 10-20º F cooler than the air temperature a few feet off the ground; also, having flats on the floor increases the amount of time that free standing water is present, creating germination opportunities for fungal spores. Raising flats off of the floor even a few inches will raise the temperature and reduce direct contact with standing water.

  • John Erwin, Charlie Rohwer and Ryan Warner

    John Erwin is associate professor and Charlie Rohwer and Ryan Warner are graduate research assistants at the Department of Horticultural Science at the University of Minnesota. They can be reached by phone at (612) 624-9703 or E-mail

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