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.
