Grower 101: Lighting With John Erwin

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Are greenhouse temperatures the one input you’re not paying careful attention to? Temperature can affect rooting, germination and flowering. John Erwin of the University of Minnesota shared some temperature advice based on research.

  • The wrong temperature can reduce seed germination and rooting. Be sure to measure media temperature to see where you are. Trays can heat up much more than the air temperature.
  • Plant development depends on average daily temperature. As it increases, leaf unfolding increases to a max rate, then slows. That optimum temperature varies by species.
  • Plants can be grouped into low, medium or high temperature preferring. The grouping depends on species and some are more tolerant of temperature. Different groups should be grown under different temperatures.
  • Photosynthesis varies with plant type and increases as temperature increases to an optimum temperature and then decreases.
  • High temp can decrease flower number, especially short day plants.
  • Many perennials/biannuals can be induced by period of cold temperature for six to 12 weeks.
  • When inducing plants to flower with night interruption lighting with cool night temperatures (less than 60 degrees), light longer (six hours) and/or increase light intensity (more than 10 footcandles).
  • A short-term high temperature exposure can reduce photosynthesis in some plants for days. Some growers heat up houses during the day so they won’t have to be heated at night. More venting and fans need to be used, and that’s why open roof greenhouses do so well.
  • High temps can cause leaves to dry quickly. Chemicals absorbed through leaves can have a harder time getting into the plant and be less effective if the plants are too hot or dry. Apply chemicals to plug trays, allow them to absorb and dry, and then transplant into pots. This method uses less pesticide with better uptake.

For more details on Erwin’s research, eMail him at

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