Better Calibrachoas with Growth Control

July 10, 2003 - 10:38

PGR trials on the FloraStar Elite Winners and merit awardees demonstrate the effectiveness of early and late PGR applications.

Have you noticed how popular calibrachoas have become? It is
an interesting crop. They can either look very nice or very straggly. The
flowers are small, and unless there are lots of them, the plant does not give
much of a show. In terms of sensitivity to media pH, calibrachoas are one of
our most demanding crops.

This is a young crop from the stand point of breeding and
development; it was introduced only 10-12 years ago. While numerous companies
are doing calibrachoa breeding, each company's series is somewhat variable,
including varieties with different growth habits and sensitivity to
photoperiod. Note: All calibrachoas are long-day plants. None are day neutral;
however, there are big differences in how early in the spring they will flower.

High light and cool temperatures produce much nicer,
more compact plants with a large
number of flowers, which is one of the reasons they always look so good in
those Pack Trial pictures. Calibrachoas, however, can be very aggressive under
the warmer temperatures of Southern production or early summer temperatures of
the North and Northeast. The need for lower media pH means that fertilization
with high ammoniacal nitrogen, such as with a 20-10-20 formulation, is often
used, which also promotes elongation. In addition, varieties that require
longer day lengths for flowering can produce long shoots before they are
saleable. We also cause ourselves more aggravation by putting calibrachoas in
smaller containers.

The past couple of years, we have grown a large number of
varieties as part of our photoperiod research and the FloraStar Elite trials
(See page 22 for results from the Elite trials). The need to produce more
attractive plants led us to doing growth regulator work for our May field days.
For this article, I have chosen the strategy of showing more pictures of the
plants and letting them speak for themselves, as in "a picture is worth a
thousand words."

There are two important concepts for applying growth
regulators to calibrachoas. The first is to achieve early control over
elongation, and several methods are available for accomplishing this. The
second concept is to use a late drench to produce a more attractive finished
plant and to prevent plants from getting out of control at the end of
production and in the garden center. Both concepts are useful for baskets and
for smaller containers.

Early Control

In picture 1, page 70, the plant on the left is a control
(no growth regulator) and the one on the right was sprayed with B-Nine at 5,000
ppm seven days after planting the rooted cutting. The cuttings were pinched at
planting, and the picture was taken five weeks after planting. The control
plant illustrates the open appearance that can develop when laterals are
allowed to elongate too much prior to flowering. For varieties with a trailing
or semi-trailing habit, in baskets, this results in all the flowers being out
over the edge of the container and the basket not having color on top.

Spraying a growth regulator, such as B-Nine, soon after
planting is one method for obtaining early control. Another method is to apply
a growth regulator spray directly to the media at or just after planting. This
is called a media spray and can be done with any of the chemicals that are
active through the media, A-Rest, Bonzi, Piccolo and Sumagic style="mso-spacerun: yes">

An example is Picture 2, above. The plant on the left is a
control pictured five weeks after planting, with the cutting pinched at
planting. The plant on the right received a media spray; the container was
filled with media, the media surface was sprayed with Sumagic at 20 ppm and the
liner was planted. Because of REI requirements, it is usually easier to plant
and spray after the pots have been placed in the greenhouse. We used a spray
volume of 2 quarts per 100 sq.ft., but some growers will use a volume 2-3 times
this with lower chemical concentration. This higher volume is often called a

A third method for achieving early control is shown in
Picture 3, page 71. The control plant is on the left. The other two plants
received a liner dip, where the liner was dipped (root zone only) in a Sumagic
solution and then planted. The concentrations were 1 ppm for the center plant
and 2 ppm for the plant on the right. The root ball should be dipped long
enough for it to soak up the chemical, which is about 10-30 seconds. This
"liner dip" can also be done by drenching the chemical on the tray
from the top. Liners can be treated 1-4 days prior to planting, which makes
this application method generally easy to accomplish. This technique is also
very good for reducing the growth of calibrachoas in mixed containers when they
are combined with less vigorous crops, as it is unwise to apply a growth
regulator to a combination that might contain slow-growing varieties.

Drench applications

Picture 4, page 70, demonstrates the effects of a late
drench on large baskets. The basket on the right, pictured 11 weeks after planting
received a Bonzi drench at 8 ppm seven weeks after planting. There are not more
flowers on the treated basket; it just looks that way because they are pulled
closer together by the reduced shoot length. If the shoots are not elongating
too much, the late drench should be applied just before the plants reach
salable size. This will slow growth and keep plants at a salable size longer
without stopping flowering. The basket on the right is also better for the
consumer since it is not actively growing and does not require watering as

Picture 5, left, shows the effect of drench concentration on
41/2-inch pots. The liners were pinched at planting, the treatment was applied
at two weeks and the picture is at five weeks. The plants from left to right
are control and Sumagic drench at 1/4, 1/2 and 1.0 ppm. For small containers,
the drench usually needs to be applied earlier in the crop than with baskets.
Another illustration is of drench concentration is also pictured on the left in
Picture 6, where the plant on the right received a Sumagic drench at 1 ppm.
Picture 7, left, is an example of a Piccolo drench with the plant on the right
receiving 4 ppm Piccolo.

These are examples of the benefits of obtaining early
control over the elongation of calibrachoas and for using a drench to keep the
plants compact later in the crop. These treatments and rates are examples in
Florida's relatively warm spring. Growers in the Midwest should start their
trials at about one-half of these rates, and in the Northwest and New England
about one-forth of these rates are good starting points. style="mso-spacerun: yes">

About The Author

Jim Barrett is professor of floriculture at the University of Florida and GPN's consulting editor. He can be reached at

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