Late-Season Bonzi Drenches: Northern & Southern Perspectives

August 20, 2002 - 10:55

Learn the key components for successfully using late-season Bonzi drenches on poinsettia, regardless of your location.

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

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

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.

About The Author

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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