Late-Season Bonzi Drenches: Northern & Southern Perspectives By Jim Faust and Royal D. Heins

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

By the late 90s, late-season Bonzi drenches became astandard practice for many Southern growers, while many Northern growers werestill appropriately cautious. Currently, late-season Bonzi drenches have beensufficiently tested that both Northern and Southern growers can safely use thistechnique, provided that the proper guidelines are followed.

Why drench?

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

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

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

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

What is considered too early? The answer depends on yourgrowing environment (temperature and light) and the variety. In general, warmtemperatures and high light environments produce the largest bracts. Onlarge-bract varieties like Freedom, grown under warm temperatures, somereduction in bract size may actually be desirable. Small-bract varieties, likeSuccess, have less margin for error. In general, Bonzi drenches can be safelyapplied 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 priorto market, but caution should be exercised.

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

Application rate. Theefficacy of the drench is affected by the concentration of the solution and thevolume of solution applied to the container. Thus, the final solutionconcentration and the volume applied determine the actual amount of activeingredient applied. Recommendations may list only the concentration of thedrench solution. It is important to realize that these recommendations assume acertain application volume. The standard application volume depends oncontainer size.

The typical recommended Bonzi drench application rates are0.5-1 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 rateused will also depend on the prior plant growth regulator applications and thecurrent growth rate. For example, if little plant growth regulator has beenapplied and growth is vigorous, the higher rate should be used. In contrast, ifconsiderable plant growth regulator applications have been made and the growthrate 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 mediawill 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 anemitter irrigation system find applications are more uniform if the volume of solutionis doubled and the concentration of Bonzi is cut in half. For a 6-inch pot, astandard 1-ppm solution applied in four ounces would be reduced to 0.5 ppm andapplied in eight ounces. The location of the emitter in the container isparticularly important when multiple plants are grown in one pot. For example,in large containers with three plants per pot, the emitter should not be nearthe edge of the container, since this may cause uneven distribution of theBonzi and uneven growth within the pot.

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

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

Top-watering versus subirrigation can also affect the amountof active ingredient in the root zone. If a Bonzi drench is applied to the topof 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 Bonzidrench applications are more effective since the active ingredient is in theroot zone, thus it is more readily available to the plant. Standardrecommendations assume a top application of Bonzi, so subirrigationapplications should use lower rates. A safe rate as a top application mayresult in excessive response when applied via subirrigation. So approximatelyhalf the top application rate is needed for an effective subirrigationapplication.

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

A tool in the arsenal

Bonzi drenches are a great tool to have in your arsenal ofheight control options. They are particularly valuable for varieties likeFreedom 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 tomarket, so Bonzi drenches may not be needed, but can provide a nice insurancepolicy against late stretch.

Keep in mind that Bonzi drenches can have a tremendouseffect on stem elongation. We have observed that most internodes that elongateafter 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 mayprohibit those plants from ever achieving the desired height. The ideal time toapply Bonzi is when your crop is approximately one inch below your desiredfinal height and when the crop is within two weeks of the market date. If thecrop has more than one inch of height to achieve before the market date, then alate-season growth regulator may not be needed, or at a minimum should bedelayed until a later date.

The standard strategy is to use other plant growthregulators for early- and mid-season height control. Then, have a Bonzi drenchas an option for late-season height control. The key is to manage crop heightduring the early and mid-season so you are not in a position of having torescue a crop with a Bonzi drench being applied too early (greater than threeweeks prior to the market date). Currently, the B-Nine/Cycocel tank mix is astandard for height control prior to flower initiation in the South, and priorto early September in the North. Coloration of bracts can be greatly delayed byhigh or late B-Nine applications, so excess and late applications should beavoided. If several B-Nine/Cycocel applications are necessary, the pinch dateis probably too early and should be delayed in succeeding years.

Many growers are having success controlling early-seasonstem elongation with Florel, typically when applied the week before and theweek after pinch. We currently do not know how late Florel can be applied to acrop without preventing flower initiation and bract development. Futureresearch will provide this answer. After flower initiation and through firstbract color, Cycocel is still the standard plant growth regulator. Bonzidrenches provide a late-season technique to bring the crop in safely whilereducing 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.

Jim Faust and Royal D. Heins

Jim Faust is an assistant professor at Clemson University and a former graduate student of Royal D. Heins, a professor at Michigan State University. They may be reached via phone at (864) 656-4966 or E-mail at

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