Late-Season Bonzi Drenches: Northern & Southern Perspectives

In 1988, research conducted by Dr. Jim Barrett at the University of Florida demonstrated the potential of Bonzi (paclobutrazol) drenches for poinsettia height control. This technique proved to be particularly effective when used late in the poinsettia season. As Freedom grew in popularity, the use of late-season Bonzi drenches also became more popular, since Freedom has a tendency to elongate after first color, in the weeks immediately before flowering.

By the late 90s, late-season Bonzi drenches became a standard practice for many Southern growers, while many Northern growers were still appropriately cautious. Currently, late-season Bonzi drenches have been sufficiently tested that both Northern and Southern growers can safely use this technique, provided that the proper guidelines are followed.

Why drench?

The main benefit of using Bonzi as a drench, rather than a spray application, is that drenches are highly effective on stem elongation, while less problematic for reducing bract size when applied during the late season (we define ?late season? as the 2-3 weeks prior to anthesis or pollen shed). This does not suggest that Bonzi drenches cannot mess up a crop. Many factors determine whether you achieve the desired results. Climate, timing, application rate, application method and growing media contribute to the success or failure of Bonzi drenches used as a late-season, height control technique on poinsettias.

Climate. After living in a particular climate for many years, we have a tendency to forget how different the climate can be in other parts of the country. Let?s first review how the climate varies from North to South. In August, the Northern and Southern United States have very similar light levels and temperatures. Northern greenhouses can be equally hot or even hotter than Southern greenhouses, since outside temperatures differ little in mid-summer from North to South and many Northern greenhouses lack evaporative pad and fan cooling systems. In September, Northern and Southern climates become different quickly. Typically, Northern locations experience lower light levels and colder temperatures 4-8 weeks ahead of a Southern location. For example, the light levels in Michigan in September are equivalent to those of South Carolina in October.

November is a critical time for bract expansion. At this time, a Southern poinsettia crop is still receiving moderate light levels and average daily temperatures greater than 70° F. Meanwhile, Northern growers may have 30-50 percent less light and relatively cool growing temperatures based on the heating setpoint (average daily temperatures range from 65-68° F). Since plant growth (bract expansion and stem elongation) is more vigorous under warmer temperatures and higher light levels, Southern poinsettia crops can continue to be vigorous throughout November. Therefore, higher rates of growth regulator are required to control poinsettia elongation in warm climates compared to cooler climates, and growth regulators have more Á potential to produce undesirable effects on Northern crops grown under the less-optimal growing conditions. Examples of undesirable effects include small bract size, delayed flowering and excessive height control.

Timing. A key to proper use of Bonzi drenches is to realize that they are late-season plant growth regulator treatments. The most important idea to remember is that the closer a Bonzi drench application is made to full plant maturity (generally the market date), the less likely there are to be problems. The most common problem resulting from a Bonzi drench is applying the drench too early in the crop. Early applications can produce excessively short plants and can reduce bract size. The reason later applications have less potential to excessively reduce bract size is that bracts will have developed to a greater size before the application. Since a growth retardant cannot make a bract grow smaller, later applications have less potential to reduce bract size.

What is considered too early? The answer depends on your growing environment (temperature and light) and the variety. In general, warm temperatures and high light environments produce the largest bracts. On large-bract varieties like Freedom, grown under warm temperatures, some reduction in bract size may actually be desirable. Small-bract varieties, like Success, have less margin for error. In general, Bonzi drenches can be safely applied in the North when the plants are within two weeks of the market date. In warmer climates, applications can usually be stretched to three weeks prior to market, but caution should be exercised.

While we generally speak about application relative to market date, market date is only appropriate if the plant is developing normally. If the plant is developing slower than normal, due to heat delay, for example, then a ?normal? application date may in fact be too early. On varieties like Freedom, application three or more weeks after first bract color should be safe. Application four or more weeks after first color may be more appropriate for late-maturing varieties like Success.

Application rate. The efficacy of the drench is affected by the concentration of the solution and the volume of solution applied to the container. Thus, the final solution concentration and the volume applied determine the actual amount of active ingredient applied. Recommendations may list only the concentration of the drench solution. It is important to realize that these recommendations assume a certain application volume. The standard application volume depends on container size.

The typical recommended Bonzi drench application rates are 0.5-1 ppm in the South and 0.25-0.5 ppm in the North. Vigorous varieties, e.g. ?Monet?, in the Deep South, may require two ppm. The actual rate used will also depend on the prior plant growth regulator applications and the current growth rate. For example, if little plant growth regulator has been applied and growth is vigorous, the higher rate should be used. In contrast, if considerable plant growth regulator applications have been made and the growth rate is moderate, the lower rate should be used.

The media should be evenly moist when the drench is applied, with the plants preferably watered the day before the application. Dry media will allow the growth regulator to run out the bottom of the container, resulting in a less effective application.

Some growers that make a drench application through an emitter irrigation system find applications are more uniform if the volume of solution is doubled and the concentration of Bonzi is cut in half. For a 6-inch pot, a standard 1-ppm solution applied in four ounces would be reduced to 0.5 ppm and applied in eight ounces. The location of the emitter in the container is particularly important when multiple plants are grown in one pot. For example, in large containers with three plants per pot, the emitter should not be near the edge of the container, since this may cause uneven distribution of the Bonzi and uneven growth within the pot.

Application method. Bonzi is labeled for injection into irrigation systems, so it is possible to rapidly apply Bonzi drenches to a large number of containers at one time, but caution must be exercised. First, irrigation systems will not deliver the identical volume of solution to every pot; therefore, some variation is to be expected. If Bonzi is applied through a spaghetti tube system, the grower must place irrigation tubes into empty beakers at the beginning and the end of the irrigation lines. The grower monitors the solution filling the beakers and shuts off the irrigation lines before excess solution is applied. Second, there will always be some active ingredient remaining in the irrigation tubing or on the flood floor following application. Bonzi does not chemically break down very quickly, which means the residual Bonzi will be soluble the next time the irrigation system is used. So you will continue to receive a Bonzi drench effect with subsequent irrigations. The concentration applied will be significantly less with each successive application, but there will be some residual effect. If the poinsettia crop is shipped immediately following a Bonzi drench, there is potential for the following crop to be negatively affected by residual Bonzi. Therefore, it is recommended that irrigation systems be flushed clean and flood floors be properly cleaned before the next crop is grown. Indicator plants, such as cucumbers, can be placed in the greenhouse to test for residual Bonzi.

An alternative application method is to measure the volume of drench solution applied to each and every pot. This works well for smaller growers, but is not convenient for larger growers. Precise dosing equipment manufactured by Dramm, called ?Chemdose,? is available to deliver the proper volume of solution to each container.

Top-watering versus subirrigation can also affect the amount of active ingredient in the root zone. If a Bonzi drench is applied to the top of the container, some of the Bonzi will be tied up in the top of the media; however, few roots are usually in that area. In comparison, subirrigated Bonzi drench applications are more effective since the active ingredient is in the root zone, thus it is more readily available to the plant. Standard recommendations assume a top application of Bonzi, so subirrigation applications should use lower rates. A safe rate as a top application may result in excessive response when applied via subirrigation. So approximately half the top application rate is needed for an effective subirrigation application.

Growing media. Pine bark in the growing media will reduce the effectiveness of Bonzi drenches. Increasing the Bonzi application rate may be necessary to achieve the desired results when applying drenches to media containing pine bark. Growers will have to experiment to identify how much additional Bonzi is required to be effective in a bark-based medium.

A tool in the arsenal

Bonzi drenches are a great tool to have in your arsenal of height control options. They are particularly valuable for varieties like Freedom that have a tendency to grow vigorously right up to the market date. Stem elongation of many varieties begins to slow down in the two weeks prior to market, so Bonzi drenches may not be needed, but can provide a nice insurance policy against late stretch.

Keep in mind that Bonzi drenches can have a tremendous effect on stem elongation. We have observed that most internodes that elongate after a Bonzi drench will achieve no more than one centimeter in final length (0.4 inch). So applications made when the plants are relatively short may prohibit those plants from ever achieving the desired height. The ideal time to apply Bonzi is when your crop is approximately one inch below your desired final height and when the crop is within two weeks of the market date. If the crop has more than one inch of height to achieve before the market date, then a late-season growth regulator may not be needed, or at a minimum should be delayed until a later date.

The standard strategy is to use other plant growth regulators for early- and mid-season height control. Then, have a Bonzi drench as an option for late-season height control. The key is to manage crop height during the early and mid-season so you are not in a position of having to rescue a crop with a Bonzi drench being applied too early (greater than three weeks prior to the market date). Currently, the B-Nine/Cycocel tank mix is a standard for height control prior to flower initiation in the South, and prior to early September in the North. Coloration of bracts can be greatly delayed by high or late B-Nine applications, so excess and late applications should be avoided. If several B-Nine/Cycocel applications are necessary, the pinch date is probably too early and should be delayed in succeeding years.

Many growers are having success controlling early-season stem elongation with Florel, typically when applied the week before and the week after pinch. We currently do not know how late Florel can be applied to a crop without preventing flower initiation and bract development. Future research will provide this answer. After flower initiation and through first bract color, Cycocel is still the standard plant growth regulator. Bonzi drenches provide a late-season technique to bring the crop in safely while reducing the possibility for late-stretch and minimizing bract reduction.

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This article focuses on the use of Bonzi; however, A-rest (ancymidol) is also labeled for use as a drench on poinsettia and is equally effective.