Section 1: Introduction
Quality standards dictate that containerized plants should be proportional to the container in which they are grown. Elongated or excessively tall plants are considered to be of lower quality. Excessively tall plants easily fall over which makes irrigation difficult and they are also difficult to ship and for consumers to handle. Growers must be prepared to prevent excessive stem elongation (stretching) before it occurs because undesirable stretching can be a production reality. Growth must be managed and growers have a number of excellent tools, both non-chemical methods and chemical plant growth retardants (one classification of plant growth regulators) to control excessive stretch. Used together, they can be used to manage plant growth and produce well-proportioned and compact plants.
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, 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 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. However, as with using other stresses to control plant height, applying the correct amount of stress without causing undesirable effects can be difficult. A general recommendation to control plant height is to not exceed 5 ppm phosphorus in the constant liquid fertilization program.
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. A positive DIF occurs when the day temperature is greater than the night temperature. A negative DIF occurs when the night temperature is greater than the day temperature. 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 break) 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.
Section 3: Chemical Growth Control
The most common growth regulators used in greenhouse crop production are the plant growth retardants. Quality standards dictate that most container-grown greenhouse crops be compact, have short internodes, have a height consistent with the container they are grown in and have strong stems. Although short or dwarf cultivars exist for many greenhouse crop species, chemicals that further reduce plant height and increase the compactness and strength of the plant are often required. Growth regulators may also be used to slow growth or "hold" plant material in the greenhouse.
Plant growth is influenced by the class of plant “hormone” called gibberellins, which influence cell elongation. If synthetically produced gibberellins are applied to a plant, it will become tall and spindly. In contrast, if gibberellin production in the plant is reduced, it will be shorter and stronger with thicker stems and smaller leaves. Therefore, most of the commercially available growth retardants function by inhibiting gibberellin synthesis in the plant. There are a number of commercial growth retardants used in greenhouse crop production. Each label has specific recommended dose ranges, recommendations, and precautions.
Daminozide (commercial names: B-Nine®, Compress WSG® and Dazide®). This material is applied only as a foliar spray because it is rapidly broken down when applied to the substrate. It is highly mobile in the plant and will rapidly move from the point of application to all parts of the plant. It is most commonly applied at rates between 1250 to 5000 parts-per-million (ppm). Daminozide is effective on most crops except lilies. It is highly effective in controlling growth of seedlings in plug flats, and it is most effective in cooler climates.
Chlormequat Chloride (commercial names: Chlormequat E-Pro®, Citadel®, and Cycocel®). This material is one of the most widely used plant growth regulators in agriculture because it is also used to prevent lodging in grain crops. In greenhouse crops, it is most commonly used on poinsettias, geraniums, osteospermum, and hibiscus. It is usually applied as a 1000 to 3000 ppm foliar spray. Foliar chlormequat chloride applications often result in a phytotoxic response (chlorosis), but the symptoms are usually covered up with new leaf growth. Substrate drenches are also effective at controlling excessive growth, but because rates are similar to what are used with foliar sprays, the practice is usually not a cost effective option in the U.S. In certain crops (i.e. poinsettia and geraniums), a mixture of daminozide and chlormequat chloride (both at reduced rates) may be used. This usually provides for greater height control and reduces the potential for phytotoxicity.
Ancymidol (commercial names: Abide® and A-Rest®). This chemical is more effective than either daminozide or chlormequat chloride and is used at much lower rates. Concentrations applied are usually in the range of 10 to 200 ppm for foliar sprays and 0.15 to 0.5 mg per 6-inch container for substrate drenches. Ancymidol readily moves through the plant and is usually used on crops where other chemicals are not effective (most notably in bulb crops) or on very high value crops (i.e. plugs). Growers often prefer the use of ancymidol on plugs because of the lack of phytotoxicity and it is a “safer” PGR to apply (because its limited residual allows the plugs to grow out of the growth control effects after being transplanted). Phytotoxicity may occur from applications of high rates of ancymidol (especially under high temperatures) and usually appears as necrotic spots. The primary limitation for ancymidol is that it is a comparatively expensive growth retardant.
Flurprimidol (commercial name: Topflor®). Is a relatively recent introduction into the U.S. market, although it has been available in Europe since the 1990s. Flurprimidol is chemically closely related to ancymidol, but it has a greater degree of activity. Most commercial spray application rates are between 0.5 and 50 ppm. Flurprimidol is also one of the most cost effective growth retardants to use as a drench, with recommended application rates in a similar range as uniconazole on most plants.
Paclobutrazol [commercial names: Bonzi®, Downsize® (labeled for drench applications only), Paczol®, Florazol® and Piccolo®]. Paclobutrazol (as well as uniconazole) is a member of the family of plant growth retardants known as triazoles. These chemicals do not readily move within the plant since they are transported in the xylem. Therefore, triazoles are absorbed by the leaves, but cannot be transported out of the leaves to other parts of the plant. Because of this fact, it is important than when applied as a foliar spray, triazoles should be applied so that the solution contacts the plant stems. The triazole plant growth retardants are the most persistent (long lasting) of the plant growth retardants and because these materials are active at very low rates, the potential for error and crop overdose is greater than with other plant growth retardants.
Paclobutrazol is the most widely used growth retardant for greenhouse-grown floriculture crops in the U.S. It is commonly applied as a foliar spray and the trial rates of 5 to 90 ppm are listed for experimental use, but most commercial spray application rates are between 1 and 50 ppm. It is also effective as a substrate drench. It can be applied as a single high dose drench of 2 to 200 ppm (varies widely among plant species) to provide season long control of excess growth (rates vary among plant species and cultivars). Additionally, the application of low dose drenches of 0.1 to 1 ppm can be used to provide temporary control of plant growth which allows greenhouse managers the ability to apply additional drenches as needed.
Uniconazole (commercial names: Concise® and Sumagic®). Uniconazole is applied as both a foliar spray and as a substrate drench. Experimental use rates of 1 to 50 ppm are listed on the label, but most commercial spray application rates are between 0.5 and 25 ppm. Uniconazole can also be used as a drench, at rates 50% lower than recommended for paclobutrazol. This chemical is commonly used on perennials because it is highly effective on a very broad range of plant species. Uniconazole is also a triazole growth retardant and therefore requires stem contact to be effective.
Section 4: Other Growth Regulators Used In Greenhouse Crops Production
Not all plant growth regulators are used to control plant height. Others are used to cause flower bud abscission, increase branching, and promote flowering and shoot elongation.
Ethephon Phosphonic Acid (commercial name: Florel®). This material is absorbed by the plant tissue, and due to a change in pH once absorbed into the plant cell it releases ethylene. Ethephon is used to promote flower bud abortion and vegetative branching in crops. Although it is used in many situations, it is most commonly used where vegetative cuttings are being produced and in hanging basket production. Ethephon is applied as a foliar spray at concentrations of 250 to 500 ppm. Ethephon may also be used to promote flowering in bromeliads.
Benzyladenine (commercial name: Configure®). Benzyladenine (BA) is used to promote branching and increase flower set. Configure ® has specific label recommendations for Christmas cactus (100 to 200 ppm foliar spray), Enchinacea (300 to 900 ppm foliar spray)and hostas (500 to 1000 ppm foliar spray) and a supplemental use label allowing for experimental applications on any annual, perennial, foliage, or tropical plant grown in a greenhouse. Optimal results occur when the plant is actively growing and is physiologically receptive for growth or flower promotion. Benzyladenine does not readily move within the plant, therefore complete coverage is required.
Gibberellins (commercial names: Florgib®, and ProGibb T&O®). Additional stem elongation may be desirable in some plants such as tree forms of azaleas, poinsettia and geraniums. For the production of tree forms of these species, the general recommendation is to apply a 50 ppm gibberellin (GA) foliar spray after plants are approximately 6 inches tall. Depending on the desired height and size, an additional one or two GA applications may be desired. Gibberellins can also be applied to promote growth and overcome over-application of gibberellin-inhibiting plant growth retardants. For this use, the general recommendation is to apply 1 to 3 ppm GA as a foliar spray and check for growth stimulation after 5 days. The application can be repeated if additional shoot elongation is desired.
Benzyladenine + Gibberellin Combinations (commercial names: Fascination® and Fresco®). These combination products are used on potted lilies as foliar sprays to avoid lower leaf yellowing and leaf drop. A typical recommendation is to apply a 25 to 100 ppm foliar spray. The actual concentration depends on timing (early versus late production applications) and species. Sprays are also beneficial at prolonging flower life of potted lilies.
Section 5: Application of Growth Retardants
To successfully control the height of greenhouse crops with chemical growth retardants, many factors must be considered.
Environmental conditions
The environmental conditions can have a significant impact on the efficacy of a plant growth regulator. Applying plant growth regulators early in the morning when the evaporation rate is lower will allow for greater chemical uptake. Plants should not be water stressed when plant growth regulators are applied as this will increase the risk of phytotoxicity. After application, the plant growth regulator should be allowed to dry and wetting the leaves should be avoided. Plant foliage should be allowed to dry for four hours after Daminozide application before the foliage is rewetted while other plant growth regulators only require one hour.
Under high temperatures most growth retardants become less effective and higher concentrations or additional applications may be required. Also, under high temperatures, the potential for phytotoxicity is increased. Foliar applications of plant growth regulators are more effective under conditions where drying rate is slower (i.e. low light, high humidity, cool temperatures). This is because the active ingredient is not absorbed after drying, so the longer the foliage remains wet from the spray application, the more active ingredient that is absorbed.
Crop
Different crops and even different cultivars of a crop may respond differently to plant growth regulators. Some crops do not respond to certain plant growth regulators or may respond by different degrees. For each plant growth regulator and crop, there is an optimal concentration that should be applied. Also, some crops may be more susceptible to phytotoxicity from certain plant growth regulators than others. The growth retardant label will list all crops on which the material may be applied.
Stage of crop
Crops may be more or less sensitive to growth retardants at different growth stages. Crops should have developed sufficient foliage so that the growth retardant may be applied and so that stunting does not occur. Growth retardants should be applied at the correct stage to prevent undesirable effects on growth. For example, late spray applications of plant growth retardants to poinsettias can result in reduced bract size.
Concentration
For every crop and growth retardant combination there is an optimal concentration required to achieve the desired results. Concentrations too low will give inadequate height control while concentrations too high will result in stunting or phytotoxicity. Even when the correct concentration is applied, excessive volumes (see Volume of Application) of material can still cause stunting or phytotoxicity. This occurs when an excessive volume is applied as a foliar spray and the growth regulator runs off into the substrate. In this case, the plants receive both a foliar spray application and a substrate drench treatment (often referred to as a "sprench").
Method of application
Some growth retardants can only be applied as a foliar spray while others may also be applied as a substrate drench. When applying a foliar spray, the volume applied should be sufficient to wet the foliage (and stems in the case of the triazole growth retardants) but without a significant amount of growth retardant draining into the substrate. When applying a growth retardant as a substrate drench, higher volumes are applied, but with less active ingredient per plant. Typically, substrate drenches are applied in such a way as to provide a specific amount of active ingredient per container (i.e. 0.25 mg per 6-inch container). This requires that not only the correct concentration of solution be prepared but also that a specific amount of solution be applied per container. Therefore, substrate drenches allow for a more exact amount of chemical to be applied per plant (increasing uniformity), but are more labor intensive than foliar sprays. Sometimes a drench application rate recommendation is made using parts-per-million (ppm). With this recommendation, a given solution is prepared at the indicated ppm and sufficient volume of the solution is applied per container to wet the volume of substrate but without leaching. For example, 4 fl. oz is typically applied to a 6-inch container.
Two additional methods of plant growth retardant application are liner soaks and bulb soaks. Liner soaks are used to control excessive growth of vigorous vegetative annuals. For liner soaks, plants in plug trays are irrigated 24 hours prior to treatment to even out the moisture level in the plugs. Then the plug trays are allowed to dry slightly over the next 24 hours to the point of where the plug trays would require a normal irrigation. The plug trays are placed in a plant growth retardant solution for 1 to 2 minutes to allow for uptake of the solution into the root substrate. To allow adequate uptake of the solution, the plug trays should be held for two hours prior to transplanting plugs into final containers. The chemicals used most frequently for liner soaks are flurprimidol, paclobutrazol, and uniconazole. Rates vary for northern locations verses southern locations, by species and cultivar.
Bulb soaks involve mixing a known concentration of a plant growth retardant solution and then soaking the bulbs in the solution for 2 to 10 minutes. The bulbs are then allowed to drain for 2 hours prior to potting. Rates vary by chemical used (flurprimidol, paclobutrazol, and uniconazole), species, and cultivar. This method works very well for hyacinths, tulips and potted lilies.
Volume of application
The general rule of thumb is to spray the foliage evenly with the appropriate concentration of solution just to the point of run-off (some run-off will always occur and is assumed in concentration recommendations). With the triazole growth retardants (i.e. paclobutrazol and uniconazole), the volume must be sufficient to make stem contact. When applied on a large scale, it is assumed that a volume of 2 quarts of the solution will be applied per 100 square feet of bench space. In some cases where a dense canopy occurs, 3 quarts of solution per 100 square feet may be required. In cases where high volumes are required to achieve stem contact and uniform coverage, the volume applied may be increased and the concentration decreased.
Coverage and uniformity
Uniform coverage is essential in order to have uniform growth and crop height. Also, where the triazole growth retardants are concerned, stem contact is required for the growth retardants to be effective.
Modifications with bark-based substrates
The substrate environment impacts the efficacy of substrate growth retardant applications. Composted barks absorb and deactivate growth retardants. Therefore, substrate drench concentrations of growth retardants may need to be increased by approximately 25% when applied to a substrate containing significant amounts of composted bark.
Section 6: Other Benefits With the Application of Plant Growth Regulators
Because of how some plant growth retardants inhibit the production of gibberellin, there are a few other beneficial biochemical processes magnified in the plant. The application of plant growth retardants increase the concentration of chlorophyll in the plant, which results in darker green foliage. Darker green plants have greater consumer appeal.
Along with the more compact growth that occurs with the application of plant growth retardants, there is also a decrease in water use. Thus, in areas where water conservation is a concern, plant growth retardants can be considered a best management practice.
Originally, many of the plant growth retarding chemicals were discovered in fungicide efficacy trials. Therefore, it is not surprising that a few of them (flurprimidol and paclobutrazol) have demonstrated fungicidal activity. This disease protection is an added benefit of using certain plant growth retardants.
Mixing Plant Growth Regulators
When mixing plant growth regulators, great care needs to be given to accurately measuring and applying the chemical. Drench applications vary by pot size and desired dose, so refer to the product label for exact mixing instructions. As always, the label contains the legal mixing information.
Correct Dosage
In order to achieve the desired effect on plant growth, a known dose needs to be applied to each plant. The dose to apply to a crop is based on two factors: (1) the solution concentration and (2) volume of solution applied per area.
Foliar sprays require an even application to obtain consistent results. To accomplish this, a dose is based on measuring out a known amount of chemical and adding it to a known volume of water to achieve the desired concentration. Next, the foliar spray needs to be applied to a known area. Most foliar sprays are applied at the rate of 2 quarts per 100 square feet. By applying this known concentration over a known area provides a known dose per plant.
Drench applications are based on measuring out a known amount of chemical and adding it to a known volume of water. This provides a solution with a known concentration. Next, the solution must be applied at a specific volume of drench solution to each plant or pot. Applying this known concentration with a known amount per pot provides the desired dose per plant. The volume of drench applied increases with the pot size (specifics are listed on each product label). For instance, 2 oz of drench solution should be applied to a 4-inch pot, 3 oz to a 5-inch pot, 4 oz to a 6-inch pot, and 10 oz to an 8-inch pot.
How well do the plant growth regulators work? The only way to confirm the efficacy of a plant growth regulator is to leave a few representative plants untreated. These “check plants” offer a valuable insight into ways to adjust future plant growth regulators applications.
Section 7: Example Growth Regulator Calculations
©M.R. Evans and B.E. Whipker, 2009, 2011, 2014