Section 2: Non-Chemical Growth Control
Although chemical growth retardants are commonly used to control height in greenhouse crops, there are non-chemical methods for reducing plant height, producing more compact plants and "holding" crops until the desired shipping date. In some situations, non-chemical methods might be the only means available for creating shorter and stronger plants or holding crops. Knowing how the growing environment and cultural practices affect plant growth will help greenhouse managers control a crop’s growth, and the ability to manage these non-chemical control options will aid in avoiding excessive growth.
Water stress
Allowing plants to dry between irrigation cycles (and suffering mild water stress without being allowed to dry to the permanent wilting point) reduces plant height and "toughens" the plants (makes shoots and axillary branches stronger). The difficulty in using this method to control plant height is that subjecting the crop to too great of a water stress can also result in negative responses such as flower bud abortion, leaf loss, reduced growth and reduced plant quality. However, as a general rule, subjecting plants to mild water stress (watering just as the plants begin to show a very slight amount of wilting) can be used to "toughen" plants and slow growth when plants need to be held for a later shipping date. Impatiens and tomatoes are two common crops in which water stress is utilized for height control. However, water stress may cause premature bolting if used with crops such as cauliflower and broccoli.
Nutritional stress
Nutritional stress can also be used to control excessive plant height. Shorter plants can be grown by limiting the amount of nitrogen in the constant liquid fertilization program to 50 ppm and relying primarily on nitrate-nitrogen (NO3-) while avoiding ammonical-nitrogen (NH4+) which promotes excessive leaf expansion. Plant height can also be controlled by restricting phosphorus fertilization. By limiting phosphorus, internode elongation can be reduced (A general recommendation to control plant height is to not exceed 5 ppm phosphorus in the constant liquid fertilization program). However, as with using other stresses to control plant height, applying the correct amount of stress without causing undesirable effects can be difficult.
Light
Higher light levels tend to limit plant elongation, thus resulting in shorter plants. Low light levels caused by late spacing of the crop, crowding, too many hanging baskets overhead, or greenhouse glazings with low light transmission (due to dirt, age or unnecessary shading) can lead to excessively tall plants. Therefore, greenhouse managers need to maintain light levels that are optimal for the crops being grown.
Root restriction
Another possible option to control excessive plant height is root restriction by utilizing small containers. This method will limit the amount of plant growth, but irrigation frequency of the crop will likely increase. The primary mechanism by which small containers reduce plant height is that the container holds a smaller volume of substrate. Therefore, the container will hold less water and fertilizer than a larger container and plants will tend to experience nutritional and water stresses more frequently than plants grown in larger containers with larger volumes of substrate.
Pinching
Pinching (also pruning or sheering) can be used to increase branching, improve the shape of the plant and decrease the height of the plant. However, labor costs of pinching and the potential delay in flowering that may occur may make this method of height control an economically unfeasible option.
Thigmotropic responses
Thigmotropic responses are plant responses to physical stresses (i.e. touch). Shaking, rubbing or blowing air across plants has been shown to reduce plant height and reduce internode length. In some cases, rubbing or wind is being used to retard plant height and "toughen" vegetable transplants. The difficulty with these methods has been how to effectively apply the treatment (i.e. rubbing) over a large crop and how to apply it without damaging the crop.
DIF and DROP
A commonly used non-chemical method of height control is temperature manipulation. Growing plants at lower night temperatures reduces growth and elongation. However, temperature may be used in a more precise method to specifically control internode length (and thus stem elongation and plant height). This method of height control using temperature is referred to as DIF.
DIF refers to the difference between the day and night temperatures (i.e. difference). The DIF is determined by subtracting the night temperature from the day temperature (day temperature - night temperature = DIF). A positive DIF occurs when the day temperature is higher than the night temperature (70 ℉ day - 60 ℉ night = +10 DIF). A negative DIF occurs when the night temperature is higher than the day temperature (65 ℉ day - 70 ℉ night = -5 DIF). A zero DIF occurs when the temperatures are the same.
Plants grown under a positive DIF are usually taller than plants grown at a zero DIF, and plants grown under a zero DIF are taller and have longer internodes than plants grown under a negative DIF. As the DIF becomes more negative, plants tend to become shorter. There are some undesirable effects when the DIF is too negative (i.e. chlorosis in lilies). Usually, a -10 DIF has been found to be optimal for controlling plant height of most crops.
A problem with maintaining temperatures higher at night than in the day is that of increased heating costs. However, it has been found that the first two hours in the morning (beginning at day beak) is the period of time that DIF is most effective. Therefore, lowering the temperature below the night temperature (creating a negative DIF) for two hours at sunrise is just as effective at reducing plant height and internode length as is maintaining a negative DIF throughout the night. This practice is commonly referred to as DROP. Many growers create DROP by opening vents and dropping the temperature in teh greenhouse for two hours at sunrise (cooling greenhouse) and then closing vents and returning to normal temperatures.
