MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Most greenhouse growers use plant growth retardants (PGRs) to control the height of their spring and summer crops. However, PGRs have several disadvantages: they can be expensive to apply, can cause stunting if applied at an excessive rate, and their effect may persist in the landscape. There are several environmental height control strategies, such as light and temperature manipulation, that are discussed below. In the next Greenhouse Alert, cultivar selection and cultural strategies for height control will be discussed.

Light quality

Flowering and stem elongation is influenced by the color of light that plants receive, which is known as light quality. Sunlight has similar amounts of red (R) and far red (FR) light. Below a leaf canopy, the amount of FR light is about 20 times higher because leaves absorb R light and transmit FR. The FR light that is transmitted below a plant canopy or an individual leaf will cause shade-avoiding plants such as petunia and poinsettia to elongate rapidly. Therefore, when plants are tightly spaced, most plants respond by rapidly elongating or stretching and the lower leaves often turn yellow and senesce. Plants that naturally grow in shaded environments (such as hosta) do not usually exhibit the shade-avoidance response. Therefore, plants should be spaced as far apart as economically possible so that leaves from one plant do not touch the leaves of an adjacent plant. Also, don’t forget that hanging baskets can also induce the shade-avoidance response on the crop below.

Artificial light sources also influence the amount of red light and far red light that crops receive. Incandescent lamps emit more FR than R light, and thus promote stem elongation. On the other hand, high-pressure sodium (HPS) lamps have about six times more red light than far-red light. The effects of light quality can be seen in the accompanying figures (below), where Coreopsis ‘Early Sunrise’ grown under incandescent lamps (INC) were taller than plants grown under cool-white fluorescent (CWF), HPS, and metal halide (MH) lamps.

Table 1 (below) lists the ratios of red to far-red (R:FR) and other important factors for several lamps that are commonly used in greenhouses and germination chambers. Using HPS lamps to provide photoperiodic lighting can reduce plant stretch, however, the initial cost to purchase HPS cost is higher than for incandescent lamps.

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.

Table 1. Comparisons among some common lamp types used in greenhouses. R = red light and FR = far-red light.