Chapter 18

Plant Growth Regulators for Greenhouse Crops

Cultural Factors Used to Control Plant Growth

Knowing how the cultural practices can affect plant growth will help in managing a crop’s growth. Interest in using cultural practices has grown because of the tighter controls placed on the use of agricultural chemicals, the public's negative perception of chemicals in general, and the cost in applying PGRs. What follows is an outline of cultural methods for controlling plant growth, which may be effective alone or in combination with low levels of PGRs.

Container Size

Root restriction can be used to control plant growth by utilizing a small container or by increasing the number of plants per pot. This method works especially well when other production parameters such as ample light, i.e. wide spacing, and proper nutrition are provided.

Plant Density

The distance between plants is another important factor to consider. Spacing affects the amount of light, water, and nutrients that are available to individual plants. In general, plants grown at closer spacing grow taller and have smaller stem diameters than those grown further apart.

Scheduling Greenhouse Crops

Scheduling is an important part of greenhouse crop production. Accurate schedules are required to grow plants to marketable size at the right time of year. Poor scheduling may cause growers to experience having small or non-flowering plants, having overgrown plants at the height of the season or empty benches with several weeks of selling season still ahead.

Irrigation Practices

Mild to moderate water stress can also be used to control plant growth. Mild to moderate water stress can be implemented by continuously allowing the root zone to dry and then irrigating right before the plants wilt or when early symptoms of wilt are observed.


Reducing or withholding fertilizer tends to slow overall plant growth. The nutrients which have the most effect on the size of greenhouse plants are nitrogen and phosphorus. The biggest effect of withholding a water-soluble fertilizer is nitrogen deficiency. Unfortunately if nitrogen deficiency conditions go on too long the plants will be too small. A phosphorous deficiency is somewhat more difficult to create than a nitrogen deficiency. However, if carefully managed a mild to moderate phosphorus deficiency will result in a desirable reduction in growth and no foliar symptoms.

Mechanical Conditioning

Mechanical conditioning is an excellent means of regulating the growth of vegetable transplants and some ornamental bedding plants. It improves the stature, appearance, handling characteristics, and overall quality of treated plants.


Brushing transplant shoots is very popular method with vegetable transplants and ornamental bedding plants. Generally, brushing reduces plant height, and commonly leaf area and dry weight, but increases stem and petiole strength. The goal with plant brushing is to make the plants bend without breaking leaves or stems or causing any other damage. Methods of application have include brushing plant shoots with a small broom, a suspended aluminum bar or PVC pipe, a steel bar suspended in a cloth sling, a wooden dowel rod, single or multiple layers of burlap, and a sheet of polystyrene foam.


Pinching is often used to increase branching, shape plants and reduce plant height.  Pinching removes the apical dominance of the shoot which prevents branching.  Apical dominance results from the production of auxin, a natural plant hormone by the terminal growing point and young leaves. Removing the terminal growing point and young leaves (pinching), removes the source of auxin and allows dormant buds below the pinch to grow. When and how a plant is pinched is important in relation to branching and time of flowering.

Light Quantity, Quality, and Photoperiod

Plant growth and developmental processes are regulated by light quantity, quality, and photoperiod. Together these three parameters strongly influence the growth of plants in the greenhouse, including plant stature, growth habits, the transition to flowering, and end point plant productivity.

Light Quantity

Light quantity refers to the intensity or concentration of sunlight and varies with the season of the year. One of the easiest ways to produce shorter plants in the greenhouse and reduce the need for PGR treatment is to maximize the amount of light plants receives to reduce stem elongation or “stretch.”

Light Quality

Light quality refers to the color or wavelength reaching the plant surface. The ratio of red to far-red light affects stretching and branching of many plant species.


Photoperiod or light duration refers to the amount of time that a plant is exposed to sunlight. Natural photoperiods can be manipulated to create artificially long days or short days. Under natural long days, short days can be created by blocking out all light with black plastic or cloth.


Temperature manipulation can be used very effectively to control plant growth. Lower temperatures reduce plant growth and as a result lower rates of PGRs are required for plants grown under lower temperatures. Conversely, higher temperatures enhance plant growth and higher rates of PGRs are necessary for adequate growth regulation. Some growers use differential day/night temperatures (DIF) to control growth. DIF is defined as the difference between day temperature (DT) and night temperature (NT).

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