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Plant growth regulators (PGR?EUR??,,????'???s) are chemicals used on a wide range of floricultural crops. Products currently on the market are labeled for control of plant height (chemical growth retardants or CGR?EUR??,,????'???s such as A-Rest, B-Nine, Bonzi, Cycocel, Florel, and Sumagic), for stimulation of lateral branching (Florel), for substituting for a cold storage requirement (GibGro 4LS), or for promoting flower initiation or earlier flowering (B-Nine, Cycocel, Florel, and GibGro 4LS).
By far, the majority of plant growth regulators employed in floriculture are chemical growth retardants used for control of plant height. However, the application concepts and techniques are given below for all PGR?EUR??,,????'???s labeled for use on floricultural crops. While digesting the following text, keep in mind that chemical growth retardants are a class of plant growth regulators; not all PGR?EUR??,,????'???s are CGR?EUR??,,????'???s.
Before applying a PGR, a grower should consider the reason for using the PGR. Recall that PGR?EUR??,,????'???s are applied to plants to regulate plant development and to stimulate a desired growth response. For example, chemical growth retardants are used to retard growth, resulting in shorter plants. A closer look at how CGR?EUR??,,????'???s work may help emphasize the importance of correct choice and use of PGR?EUR??,,????'???s.
Most of the available growth retardants are anti-gibberellins; i.e., they inhibit the synthesis of gibberellins such as gibberellic acid (GA3) within the plant.
Gibberellins stimulate cellular elongation, so without them, cells do not elongate as much, and plants do not grow as tall. Ethephon is not an anti-gibberellin; ethephon releases ethylene, which reduces elongation in some crops.
Since CGR?EUR??,,????'???s, as do all PGR?EUR??,,????'???s, affect a specific process in the plant, it is essential they be applied in a manner that assures the most efficient response. Both monetary and environmental costs are too great to apply PGR?EUR??,,????'???s carelessly. This conscientious effort to use the minimum amount of chemical as effectively as possible is the basis for Best Management Practices. Best Management Practices for using PGR?EUR??,,????'???s can be divided into different categories: timing, target tissue, dosage, application technique and environmental conditions.
Timing of PGR applications must be matched to the proper plant stage of development to achieve the desired goal. Usually the labels on the products will give good descriptive stages of plant development to assist with correct application timing.
For example, the GibGro label also defines when to make applications. If GibGro 4LS is being used to partially substitute for cold storage of azaleas, plants should be at Stage 5 of floral development (i.e., style elongated and open when treatment is initiated. Applying prior to this stage could result in abnormal flower development.
For most plants, apply CGR?EUR??,,????'???s such as A-Rest, B-Nine, Bonzi, Cycocel, Florel, and Sumagic after the plant has developed sufficient foliage (photosynthetic area, leaf area) to prevent excessive stunting of the plant?EUR??,,????'???s development. For example, a Bonzi spray on pansy plugs should not be made until three true leaves are present on the plants. Earlier applications may stop plant growth completely rather than merely reduce elongation.
Apply CGR?EUR??,,????'???s just prior to rapid shoot elongation; e.g. after pinching and newly developing shoots are visible, but before shoot elongation has occurred.
Remember that CGR?EUR??,,????'???s are chemical growth retardants not chemical plant reducers—they cannot shrink plant growth already present. Make your final application before the stage when floral size will be reduced. If growth retardants are applied too late, the size of the flowers can be reduced and floral development can be slowed. A good example would be bract size reduction and delay of bract color in poinsettia due to late application of A-Rest, B-Nine + Cycocel, Bonzi, Cycocel, or Sumagic.
Timing of a PGR application should be based on a physiological stage of plant development such as the number of leaves present, the length of shoots, or plant diameter, not chronological age such as three weeks after pinching. Recommendations given in terms of chronological age are merely guidelines that have been correlated to physiological stages and should only be used as rough estimates as to when to apply plant growth regulators. Also remember to read the label when deciding on timing for a PGR application. Many labels suggest the proper plant stage of development for chemical application.
The target tissue or plant part to receive the chemical depends on the chemical and the plant species.
Foliage and Stems: Chemicals can be sprayed on, or shoots can be dipped into a chemical solution at time of transplanting. If dipping shoots into a solution, use a consistent soaking time, such as 10 seconds. For sprays, remember that areas are sprayed not individual plants. Imagine yourself painting a wall with a roller. You want uniform coverage over the entire area?EUR??,,????'??+-not heavy clumps (every time you pass over a plant). This is the only way to assure even coverage and consistent results.
With many PGR?EUR??,,????'???s such as A-Rest, Bonzi, and Sumagic, spraying different volumes of the same concentration spray over a given area gives different levels of control. If growers “spray to runoff” or “spray to glistening”, every grower will apply a different amount, and there is no way of predicting the results.
Proper coverage is essential for consistent results (see Application Technique section). For example, Bonzi and Sumagic are not effective if only applied to plant leaves. Transport of Bonzi and Sumagic to the growing point in the plant is most effective if it is applied to the stem or if it is absorbed through the roots. If an insufficient application volume is used and stems are not wetted, poor height control will be the result. On the other hand, since root uptake is effective, an over application of Bonzi or Sumagic using too large a volume will result in the excess spray dripping onto the substrate leading to root uptake of the chemical and causing too great a reduction in growth.
PGR?EUR??,,????'???s can be applied to the substrate (growth media or soil), if the root system is the target tissue. This is the same as a soil drench; drenches use larger volumes of solution per plant or pot, but usually at lower concentrations than a spray or dip. Drenches can take more time to apply than sprays, and require exact metering of volume delivered per pot for consistent control of elongation.
PGR?EUR??,,????'???s can be applied directly to the roots or the underground portion of the plant such as bulbs, corms, or tubers, prior to planting. An example is a Bonzi soak, which is labeled as a preplant treatment for freesias.
Read the label; do not guess on dosage. Keep in mind that a dosage is the product of concentration of solution applied and volume of solution applied per area. If either are incorrect, results could be unpredictable and nonrepeatable. Take care to always apply the correct dosage to all plants treated.
As with the target tissue, the method of delivering a PGR depends on both the chemical used and the plant species to be treated.
With some plants it is possible to dip the plant shoot, or underground portion (bulb, corm, tuber) into a growth retardant solution prior to potting. This method is labeled and is effective for applying B-Nine to the shoots of rooted chrysanthemums just prior to potting. This method is fairly accurate, if each plant remains in the solution for the same amount of time, and if each plant has approximately the same size shoot or bulb. Unfortunately, this method is not feasible with many crops, and dipping plants in a common solution could result in the spread of disease organisms.
Applying a growth retardant in a drench form is fairly easy. Measure out a known amount of chemical, add it to a known volume of water, and apply a known volume of the drench to each pot or plant. As pot size increases, usually the volume of drench recommended also increases. If using a drench, make sure the volume and concentration of the solution are correct for the size pot to be treated.
There are drenching machines available on the market that emit a set volume of drench then turn off for a short period of time prior to the next dose to allow applicators to place the nozzle into the next pot to receive the drench. Do not apply any plant growth regulator through an irrigation system, unless the label lists chemigation as a legal application technique. Currently, it is legal to apply A-Rest through drip, overhead boom, sprinkler and flood (subirrigation) systems. Consult the label for specific application recommendations.
A spray application can be more difficult to apply evenly than a drench, but with attention to detail, is the method of choice. Some chemical labels recommend to spray to runoff; that is, spray each plant until spray visibly just begins to drip off of the foliage. Depending on the size of the plant, the sprayer?EUR??,,????'???s objectivity and other factors, varying amounts of chemical will be applied to each plant. It is much safer and more accurate to base spray application on areas, not plants.
Apply a known volume of spray evenly to a known area (square footage), regardless of how many plants are in that known area. The general recommendation for sprays is to apply half gallon per 100 square feet of cropping area. This volume is sufficient to comfortably walk 25 feet while spraying a four foot-wide bench, thus the basis for the recommendation. If the area is sprayed evenly, it assures that each pot will receive the same amount of spray, regardless of how many containers are in the area.
To help with uniform distribution, the sprayer should be equipped with a pressure gauge and pressure regulator to assure uniform output. Unless the sprayer is emitting a constant volume, spray will not be applied evenly over plants.
Spray droplet size can affect spray distribution on the plants. In general, the smaller the droplet size, the more even the coverage and the greater the effectiveness (efficacy) of the chemical. Therefore it is possible that two growers applying the same treatment but with different spray nozzles could achieve different degrees of effectiveness. For consistency, try to use the same nozzle type for all PGR applications.
Spray adjuvants, compounds added to assist the action of the active ingredient, such as a wetting agent can affect PGR efficacy. Some PGR products such as A-Rest, Bonzi, and Sumagic advise against adding a wetting agent. B-Nine already contains a wetting agent and may not require the use of an additional wetting agent. If a wetting agent is needed to allow a spray material to more thoroughly cover plant surfaces, such as with a Cycocel spray on poinsettias, use as little as possible to avoid phytotoxic side effects. Start off using a low concentration and by experimenting with water on test plants, slowly increase the wetting agent concentration. Add only enough wetting agent to prevent droplets from beading up on the leaves; a higher concentration is unnecessary and could cause leaf burning.
The efficacy of a PGR can be affected by: 1) the environment at time of application; 2) the status of the plant at application; and 3) post-application treatment of the plant.
A good example of an environmental factor affecting PGR efficacy is the effect substrate components can have on CGR?EUR??,,????'???s. A bark-containing substrate will reduce the efficacy of A-Rest and triazole growth retardants like Bonzi and Sumagic, when applied as a drench. Therefore, drenching may not be the application method of choice for these PGR?EUR??,,????'???s if using a bark-based substrate; or growers should account for the presence of bark when deciding on the concentration of PGR to use.
Another environmental factor of concern is the time of day selected for applying PGR?EUR??,,????'???s. Research with foliar sprays of nutrients has shown that the time of day chosen for an application can affect a plant?EUR??,,????'???s ability to absorb a chemical. Morning applications, when evaporation rates are low, are more desirable than later in the day. The longer the solution wets the tissue, the greater the chance for chemical uptake. If possible, make spray applications on a cloudy day to allow the chemical solution even more time to be absorbed prior to evaporation of the water from the surface of leaves and stems.
Plant water status can affect chemical efficacy. Plants should not be wilted or stressed at application. A turgid plant is more able to absorb and translocate a PGR than a wilted plant.
With respect to post-application handling of treated plants, B-Nine has been shown to require a long period of time (up to four hours) for complete absorption after a spray application. If the plants are watered (wetting the foliage) too soon after a B-Nine application, the unabsorbed chemical will be washed off and the efficacy of the B-Nine will be reduced. This is not true for rapidly absorbed PGR?EUR??,,????'???s such as A-Rest, Bonzi and Sumagic, and plants either drenched or sprayed with these chemicals can be watered as soon as one hour after application without a reduction in chemical efficacy. The effects of post-application watering on Florel and GibGro efficacy are not known.
Best Management Practices for plant growth regulators do not end with the guidelines given above. For a total program, growers should always monitor the effectiveness of applied treatments to assure that the treatments are working and to help fine-tune the amount of chemical needed. Monitoring treatment effectiveness may indicate that less chemical is needed. Always leave a few untreated controls mixed in with the treated population. This allows comparisons for effectiveness of the PGR treatment. For growth retardants, monitor plant growth for the treated plants regularly to know when plants begin to “grow out” of the treatment and to help in deciding if and when another application is needed.
When used properly, chemical plant growth regulators are effective tools to help produce a high quality crop. When misused, they can reduce crop quality and increase production costs. Uniformity and consistency in application are crucial to attain predictable and desirable results.
Research on floricultural crops has shown many other potential uses for PGR?EUR??,,????'???s, such as gibberellic acid (GA3) substituting for cold storage of hydrangea and hastening flowering of cyclamen; however, only those uses listed on a product label can be implemented legally. Plant growth regulators are regarded as pesticides, and it is a violation of Federal and State Law to use these products in a manner inconsistent with their labeling. Hopefully, expansion of current labeling will be possible in the future to allow growers to take advantage of research results showing more efficient cropping and higher quality plants through best management use of plant growth regulators.
Raleigh, North Carolina
Francisco Uviña, University of New Mexico
Hardscape Oasis in Litchfield Park
Ash Nochian, Ph.D. Landscape Architect
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