Plant Growth Regulators for Greenhouse Crops
Using Plant Growth Regulators
The most common plant growth regulators (PGRs) used in greenhouse crop production are the plant growth retardants used for controlling stem elongation, which results in shorter plants. However, PGRs are also used to encourage rooting, promote stem elongation, abort flower buds, and for promoting branching. Many growers have improved the quality and performance of their crops by using plant growth regulators to promote or inhibit plant growth and developmental responses. There are currently five recognized uses of plant growth regulators for controlling plant growth.
Regulating Shoot Growth
Most plant growth regulators (PGRs) used in the greenhouse are used to regulate shoot growth (i.e., stretch) of containerized crops. These PGRs are referred to as “growth retardants.” These PGRs control plant height by inhibiting the production of gibberellins, the primary plant hormones responsible for cell elongation. Other benefits of using these PGRs in plant production include improved plant appearance by maintaining plant size and shape in proportion with the pot and increased shipping capacity with the smaller plants.
Enhancing Lateral Branching
Another group of PGRs used in floricultural crops are those that enhance branching, including ethephon (Florel), benzyladenine (BA; Configure), dikegulac sodium (Atrimmec or Augeo), and methyl esters (Off-Shoot-O). These PGRs are frequently called chemical pinchers because they generally inhibit the growth of the terminal shoots or enhance the growth of lateral buds, thereby increasing the development of lateral branches.
Plant Flowering
Plant growth regulators can be used to enhance flowering. To improve flowering, Florgib 4L, ProGibb T&O, or GA3 4%, which contain the growth promoter gibberellic acid (GA3), can be used to substitute for all or part of the chilling requirement of some woody and herbaceous ornamentals typically forced in the greenhouse, including azalea for florist crops and Aster for cut flowers.
Removal of Flowers
Flower removal is especially desirable for stock plants maintained for cuttings of vegetatively propagated ornamentals, like Verbena or Lantana. Ethephon (Florel) is the primary compound used for flower removal. Once ethephon is absorbed by the plant, it is converted to gaseous ethylene—a natural plant hormone effective in many plant processes.
Other PGR Used in Greenhouse Production
Another specific application of the gibberellin and cytokinin products (Fascination or Fresco) is the reduction of lower leaf yellowing on Easter, Oriental, and LA hybrid lilies. See the label for detailed instructions. These products also may be used to increase bract expansion in poinsettias. Fascination, Fresco, ProGibb T&O, and GA3 4 percent are labeled to promote the growth of plants that have been overregulated by plant growth retardants. These PGRs are very potent growth promoters. Start with low rates, 1 to 3 ppm, and apply at fiveday intervals as necessary.
Active PGR Ingredients
Before selecting a PGR, you need to understand the active ingredient (AI) in each chemical in managing greenhouse crop production. The AI is the compound in the product that can be used to increase or retard plant height, prolong or break dormancy, prolong flower and plant shelf-life, prevent leaf yellowing, abort flowers, or promote rooting, branching and/or flowering (See Table 21.1).
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