Plant Growth Regulators

Regulating shoot growth
Most PGRs used in the greenhouse or nursery are used to regulate shoot growth of containerized crops. These PGRs are referred to as "growth retardants." Typical growth retardants are ancymidol, daminozide, chlormequat chloride, flurprimidol, paclobutrazol, and uniconazole. Now that most of the PGR chemistries are off-patent, there are several options available. These PGRs control plant height by inhibiting the production of gibberellins, the primary plant hormones responsible for cell elongation. Therefore, these growth-retardant effects are primarily seen in stem, petiole, and flower stalk tissues. Lesser effects are seen in reductions of leaf expansion, resulting in thicker leaves with a darker green color. 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.
Plant growth retardants also increase the tolerance of plants to the stresses of shipping and handling, as well as retail marketing, thereby improving shelf life and extending plant marketability. Remember, growth retardants do not reduce plant size. They limit the plant's growth rate. You must apply the growth retardant prior to the "stretch." Look for recommendations on the PGR label for time of application.
These recommendations will be given in terms of plant development or plant size, as opposed to production time. For example, uniconazole labels specify that pansies should have attained a minimum height of 4 inches prior to application. Paclobutrazol labels state that bedding plant plugs should be treated at the one to two true-leaf stage and bedding plants (after transplanting) at 2 inches of new growth or when the plants reach marketable size. Generally, growth-retarding PGRs should be applied just prior to rapid shoot growth. This is generally one to two weeks after transplanting a plug, after the roots are established, and as the plant resumes active growth.
On pinched plants, it is after the new shoots are visible and starting to elongate. This is where the art of plant growth regulation is most important. You must learn how your crop grows and when to intervene to obtain the desired results.
Remember to note details of crop development in your records of PGR treatments. For example, due to weather conditions, next year you may need to treat at seven days after transplanting instead of at 10 days after transplanting, which you used this year. You must gauge when rapid elongation will likely occur and treat to counter it.
Many growers use multiple applications of growth retardants to better control plant growth. A single application at a high rate early in the plant production cycle may be excessive if growing conditions are not as good as expected. An early application at a lower rate provides more flexibility, but the tradeoff is the additional labor involved with a second application if it becomes necessary. Some growers improve crop uniformity by using multiple applications of lower rates to affect small corrections in plant growth.
Be aware that excessive rates of many of these PGRs can cause persistent growth control in the flat or even in the landscape. It is always a good idea to evaluate the long-term effects of your treatments by growing some out for yourself and talking with your customers.
Be careful to avoid late applications, especially of paclobutrazol or uniconazole, because they may delay flower opening on bedding plants. However, drench applications of paclobutrazol have provided excellent control of poinsettia height very late in the production cycle without causing the reduction in bract size accompanying late spray applications. Learn the art of using PGRs for plant growth regulation.

Enhancing Lateral Branching
Another group of PGRs used in floricultural crops are those that enhance branching, including ethephon, benzyladenine, dikegulac sodium, and methyl esters. 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, 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. These compounds also can improve flowering and/or bloom size of camellia and baby's breath (Gypsophila), promote earlier flowering and an increased yield of statice (Limonium) and induce flowering of Spathiphyllum. Gibberellic acid is also used to promote growth and increase stem length of other cut flowers like stock (Matthiola), Delphinium, and 'Sweet William' (Dianthus). See product labels for specific uses and recommended rates. Again, timing is critical, because late applications or excessive rates may cause excessive plant stretching, resulting in weak, spindly stems. Chlormequat chloride (a plant growth retardant) used to control stem height of hibiscus and geranium also improves early flowering of these crops.
Removal of flowers. Flower removal is especially desirable for stock plants maintained for cuttings of vegetatively propagated
ornamentals, like Verbena or Lantana. Ethephon 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. Ethylene is the
primary hormone responsible for flower senescence and fruit ripening. It is the "postharvest" hormone.

Sprays
Sprays are generally applied to achieve a relatively short-term response and are appropriate when a small to moderate effect on plant height is desired. When applied as foliar sprays, PGRs must be absorbed and/or transported within the plant. The active ingredient must move through the waxy cuticle layer of the leaf or stem and then into the plant tissue.

Application Rates
There are several factors to consider when applying PGRs in this manner. First, the volume applied per unit area varies depending on the age and size of the crop. The spray volume on the label reflects a typical crop with plants at different stages of development. For young and small plants, an even smaller volume should be used and more required for larger plants, depending on coverage. Identifying an appropriate application time requires insight into and anticipation of how a crop develops.

Factors Influencing the Effectiveness of PGR Sprays
Plants to be treated with PGRs should be healthy, turgid and unstressed–never wilted. Generally, there should be sufficient foliage or stems to absorb the PGR. The triazoles-paclobutrazol, and uniconazole-are absorbed primarily by stem tissue and then translocated upward in the plant. Therefore, consistent and complete coverage of the stems is necessary for uniform effects. In other words, if the stem of one lateral receives an inadequate amount of spray, it will grow faster than the others, resulting in a poorly shaped plant, most noticeable in potted crops like poinsettia or chrysanthemum.

Spray Equipment
Spray equipment must be operating properly, including sprayer pressure and distribution pattern of the nozzles or spray gun. Keep a separate sprayer for applying PGRs. Triple-rinse the sprayer after each application to prevent unnecessary damage to other crops from residues of previous PGR applications.

Soil-Active PGRs
Applicators often spray PGR solutions to "runoff," or until leaves look saturated. When an excess volume of PGR spray and runoff reaches the substrate, this can result in additional height control through substrate activity, depending on the active ingredient used.

Drenches
Drenching is the second most common method of applying plant growth regulators. Drenches are primarily applied to the top of the media of a growing plant, with little or moderate contact with the foliage. The chemical is absorbed by plant roots and translocated to the plants' growing points where it inhibits subsequent elongation. Be sure roots are well-developed before drenching with any PGR. Drenches are often applied to serve one of two purposes: to inhibit stem extension for a long period of time beginning soon after transplant or to "stop" stem extension once a plant reaches its final desired height.

Foliar sprays
Foliar sprays require even application for consistent, uniform results. This is best achieved by applying a known water volume and product rate to a determined area for reproducible results. Fine spray droplets generally achieve better coverage and canopy penetration. Spray run-off should be avoided as it is wasteful and can result in too strong an effect from root uptake. Such run-off can also contribute to some products building up in capillary matting contributing to unwanted growth regulation in subsequent crops.
Special care must be taken when applying paclobutrazol. Low spray volumes may result in poor coverage, insufficient growth control and increased variability within the crop. Conversely applying too high a spray volume can result in spray running into the growing medium where it can have a persistent effect.
Care should be taken to avoid root uptake when treating plugs or recently potted crops with a large exposed surface area of growing medium, by delaying application until the plant's foliage has covered some of the surface. Too high a rate or volume can also effectively stop plant growth, which can render crops unsaleable, particularly young plants, early on in the production phase.
In general, irrigation should not be applied for 24 hours post application, although label recommendations are for shorter time periods for Stablian 750 (12–18 hours) and Cerone (rain-fast within 4–6 hours).

Drench Application
Although time consuming, drenches are easier to apply evenly than foliar sprays providing there is adequate, evenly distributed moisture within the growing medium. Drenches are applied by volume, depending on pot size. Drenches should be applied when roots are actively growing to enable uptake. Wash any excess growth regulator off the leaves to avoid chlorosis.
PGRs with root activity are generally more potent when applied to the growing medium as a drench than as a foliar spray. For drench applications, avoid irrigating crops too soon after application. A drop in temperature post application (spray or drench) can also enhance the effect of the PGR, resulting in a more powerful reduction in growth.

Adjuvants
In the case of Stabilan 750 use of a non-ionic wetter is recommended with to minimise leaf chlorosis. For products containing daminozide and paclobutrazol the label states that they should not be mixed with other chemicals, including adjuvants.
Adjuvants such as Elasto G5 (glycerol/fatty acid polymer) can give improved uptake of products containing chlormequat and daminozide allowing PGR rates used to be reduced without impacting the crop response.