Friday, November 7, 2008

Greenhouse - Controlling Plant Height with DIF and Early Morning DROP

There are a number of ways to manage plant height in the greenhouse that do not involve the use of growth regulator chemical sprays or drenches. Modifying day and night temperatures can be very effective. The following is a short article on the subject.

Modifying plant height with day and night temperature difference (DIF)

The term DIF refers to the difference between the average day and night temperature and is calculated as: DIF = day temperature – night temperature

The difference between the day and night temperature can influence internode elongation and thus stem extension. Stem elongation is promoted when the day temperature is warmer than night temperature (positive DIF). During the opposite environmental conditions, when day temperature is cooler than the night temperature (negative DIF), stem elongation is inhibited. A zero DIF refers to conditions where day and night temperatures are equal.

Knowledge of how DIF affects stem elongation can be used to regulate plant height. For example, plants grown at a day temperature of 68°F and a night temperature of 50°F (+18°F DIF) will have a taller finished height than plants grown at a day temperature of 68°F and a night temperature setpoint of 77°F (−9°F DIF) ( Figure 2). An intermediate height response would occur if these plants were grown at a day and night temperature of 68°F (0 DIF).

Although using a negative DIF may sound like a great environmental tool to control plant height, this technique may not be cost-effective with the current high cost of energy for greenhouse heating. However, by understanding the DIF concept you can avoid environmental conditions that promote unwanted plant stretch. For example, in response to increasing energy costs, many growers are lowering the night temperature setpoint to save on heat. When using this strategy to reduce your energy bill, remember that you are creating a positive DIF environment and stem extension will be promoted. In addition, you may be delivering a lower average daily temperature, and thus crop timing could be delayed.

Early morning DROP

Another environmental option for controlling plant height is to lower the greenhouse temperature for several hours at the beginning of the day. This strategy is termed an early morning DROP or DIP and has a similar response on stem extension as a negative DIF. A recommend strategy for using an early morning temperature DROP is to turn your heating off 30 minutes before sunrise and allow the greenhouse temperature to decrease 7 to 10°F below the day temperature setpoint. Maintain the cool temperature DROP for two to three hours after sunrise and then increase temperature back to the day setpoint for the remainder of the day. This is much more economical than full DIF.

Reprinted from "Nonchemical height control strategies for greenhouse crops" by Roberto Lopez and Matthew Blanchard in the April 14, 2006 edition of Greenhouse Alert Newsletter from Michigan State University.

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