Evening-Out Results with Paclobutrazol
Brian Whipker, North Carolina State University
One of the most popular plant growth regulators (PGR) used in greenhouse floriculture production is paclobutrazol. Bonzi® from Syngenta was the initial paclo registered in the U.S. Fine Americas offers Piccolo® and their newer formulation of Piccolo 10XC®. For growers who desire a greater degree of control, it is the top PGR in their toolbox. Growers have made comments that sometimes the results of a paclo application are inconsistent. Below are some methods to use for improving the consistency of your results.
Where does Paclo Uptake Occur? Paclo is actively taken-up by the plant in the roots, stems and leaves. Uptake is greatest in the roots and stems, and to a lesser extent the leaves.
Why is there a variation in uptake by the plant? It is a function of how paclo is transported within the plant. The xylem is the water-conducting cells of plants. It moves water, nutrients, and chemicals from the roots to the leaves. Paclo applied to the roots and stems is easily is readily transported in the xylem tissue. That is why drench paclo applications are so effective and provide even results.
Why are leaf applications not as effective? Movement out of leaves relies upon the phloem. The phloem tissues are specialized cells which load and move assimilates (food) produced in the leaves to other parts of the plant. Paclo does not readily move into the phloem tissue, therefore a foliar spray application has less effect than a root applied drench.
What is the basis of applying foliar sprays as a known volume over a known area? Recommendation for applying PGRs as foliar sprays has varied over time. Initially the basis was applying enough solution until the leaf glistens. It has also included applying sufficient volume until it just starts to drip from the leaves. Both of these recommendations are very subjective and varied from grower to grower. Hence some growers obtained sufficient control of plant growth, while others had too little or too much.
Over time the recommendation has been modified to a more accurate basis of applying foliar sprays as a known volume over a known area. That is why the current labeled recommendation is to apply 1 gallon of spray solution over 200 square feet of bench area. This basis as helped provide more consistent results across crops.
What are the other effects of this spray volume over a known area? Applying 1 gallon of spray solution over 200 square feet of bench area means that the leaves are wet and there is extra solution that either dribbles down the stem or drips into the substrate. If fact, the recommended rate ranges provided on the label is counting on a small degree of stem and root uptake to control plant growth.
Can spray volume be used as a method of varying the dosage? Yes, this is the basis of a sprench application (applying 1.5 gallon of spray solution over 200 square feet of bench area). The increased volume of spray that is applied means more of the solution comes in contact with the stems and drips down into the substrate for root uptake.
So this allows growers to mix a paclo solution at one concentration and then vary the amount of spray volume applied over the bench area to customize a dose to a plant’s needs. So plants that require less growth control, either because they are slow growing or not a vigorous cultivar, can have less spray volume applied, and this will result in less effect. On the opposite end of the spectrum, a higher volume of water per unit area will provide added control of vigorous cultivars. This is where a grower has to practice the art of PGR applications to customize results.
Why does it appear that my PGR application did not work? There are a number of factors that influence the effectiveness of a PGR application. Optimal rates vary of course by the species of plant, by cultivar vigor, timing (late applications may be less effective), fertilization rates (high P and ammoniacal-nitrogen rates stimulate more growth), and environmental conditions (Figure 1).
In fact, for foliar applications of a PGR, any environmental factor that hastens the drying of the leaf surface after a PGR is applied will have a negative impact on uptake. If the leaf dries too quickly, such as making an application in the middle of the day, then less of the PGR will be absorbed by the plant.
In a experiment conducted at North Carolina State University, we applied a PGR foliar spray and allowed it to dry. The next morning we lightly rewet the leaf surface with clear water to make the leaf glisten. (We avoided applying too much water that would of resulted in runoff.) The end result was an additional 10% of growth control occurred. So the application of any PGR foliar spray should be done when the leaves can remain wet for the longest time to obtain optimal results.
Does my paclo settle in the jug? The answer is yes for all the 0.4% formulations of paclo on the market. The active ingredient (a.i.) in the 0.4% formulations settles out fairly quickly in the jug. If given ample time, the clay particles and xanthan gum used to hold the a.i. in suspension will also settle out to the bottom of the container. (If you place the solution in a clear container, you can observe the clay particle settling after about 2 months, Figure 2.) That is why all the jugs of the 0.4% formulations state that you need to shake the container vigorously for 2 minutes. If the jug is not shaken, then the solution at the top of the jug will contain less paclo and it will be more concentrated at the bottom of the jug. This will have dramatic effects on your results. So remember to shake, not stir your 0.4% paclo jugs before use.
The exception to the above jug shaking rule is Piccolo 10XC®. It is 10% concentrate that is a microemulsion concentrate (MEC) formulation. The advantage of a MEC formulation is the a.i. stays in solution and does not settle.
How do I know if an application actually worked? The simple answer is to leave some untreated controls (check plants) (Figure 3). Check plants allow you to determine how effective a PGR application was and will provide insights on how you may want to modify your rates.
Paclo is a very effective PGR for greenhouse production of floriculture crops. By following the above tips, it will help you get the most effect out of your PGR applications.