Chapter 18

Plant Growth Regulators for Greenhouse Crops

Factors Affecting Plant Growth Regulator Performance

In order to be effective, PGRs must be applied with adequate coverage, and then be absorbed by the plant and translocated to the site of activity in sufficient concentration to give the desired response. Any factor that affects the rate and quality of plant growth and development can influence a plant’s response to PGRs. Growers must consider all of the factors that affect plant response to PGRs relative to the conditions in their own greenhouses, past experiences with these compounds, and the desired effects.

Plant Factors Influencing Plant Growth

Plant Species and Cultivars

Plant species and cultivars vary greatly in growth habits, and in chemical and environmental sensitivity. PGRs that work on one species may not be effective on another specie. Rates that are effective on one cultivar may be too high or too low for another cultivar of the same species.

Stage of Plant Growth

Plants progress through a cycle of growth stages, which include stem extension, internode elongation, leaf expansion, bud formation, flowering opening, and physiological maturity. In general, the rate of physiological development is more rapid under warm conditions with high light than under cool and low light growing conditions over the same period of time.

Plant Vigor

In general, plants with more vigorous growth habits require a higher rate than those with less vigorous growth habits. Avoid using PGRs or use lower rates on less vigorous plants, as the desired effect may be excessive.

Plant Size

Plant size impacts PGR application. Smaller plants require lower rates or less chemical than larger plants. For example, to achieve the same level of control, a plant in a 6 inch pot requires more drench or spray than the same cultivar in a 4 inch pot produced under similar conditions.

Environmental Factors Influencing Plant Growth

Temperature and Light

Plants grown under low temperature and light may require a lower dosage or fewer PGR applications than those grown under higher light and temperatures. Adjust concentrations accordingly for winter and summer production, especially when using the more active or stronger PGRs.

Growing Media

Growing medium composition directly affects the efficacy of drench applications of certain PGRs. In general, as organic matter content of a growing medium increases, the effectiveness of a PGR drench decreases.

Water Quality and Amount

Water quality affects the activity of some PGRs. A combination of high pH (>7) and high alkalinity (>100 ppm calcium carbonate equivalent) may reduce the effectiveness of many PGRs. The amount of water in which PGRs are applied can also alter performance.

Physical and Chemical Factors Influencing Plant Growth

Residual Chemical Effect

Residual chemical effect refers to the length of time a PGR remains active in the plant after application. The exact length of time a chemical remains active depends on environmental and plant factors previously discussed.

Chemical Uptake and Translocation

Chemical uptake and translocation vary from one PGR to the next. For example, Bonzi, Concise, and Sumagic, (e.g., PGRs with the same active ingredients) are actively taken up by plant roots and are readily transported to shoot tips.


Coverage refers to the volume of solution sprayed per unit area, or for drenching, the volume of solution per pot size or area (if applied to soil beds).


Compared to crop protectants (insecticides and fungicides) PGRs have a relatively narrow acceptable dose range, where overdose can result in negative side effects. Therefore, PGRs are applied in very precise and low concentrations. Extreme care must be taken to mix and apply these chemicals accurately. The crop may be injured if too much chemical is used. For example, Sumagic when applied at too high a rate will result in lilies with a palm tree effect or a section of weak stem, or when applied to poinsettia after the beginning of short days, bracts may be too small.

Spray Droplet Size

The smaller the average droplet size, the greater the coverage and penetration. However, extremely small drops (i.e., fog) result in drift, take a long time (hours) to settle and may require air circulation to achieve good penetration.

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