Growth Regulators: New Approaches

November 6, 2003 - 13:06

New applications and options for growers top the list of PGR trends for 2004.

Each year when we do this special chemical section in GPN, I'm
a little envious of the insect and disease folks; they always seem to have new
stuff to talk about. New plant growth regulators (PGR) just do not come along
as fast. In fact, for the "What's new in growth regulators" question,
the answer is changes occurring with old chemistry. However, the changes are
bringing new applications and options to growers. Let's review where we are:


I'll start with the Gibberellin (GA) products, since I'm
writing in the middle of poinsettia season, and 25 percent of the crops seem to
be too short, and growers want to know what they can do to increase height.
Valent took over marketing ProGibb 2-3 years ago and made several label changes
that were needed for many years. One of these was the use of ProGibb (active ingredient
is GA3) to stimulate stem elongation and increase plant size. At that same
time, Valent obtained access to an ag product containing GA4+7 and a Cytokinin,
which became Fascination. Fascination is an excellent product, but is only
labeled for Easter lilies to prevent lower leaf yellowing and prolong flower

ProGibb use to increase plant size is not real easy. First,
ProGibb is super concentrated and difficult to measure. Then, there has been
many over application cases similar to the image to the bottom left, on
poinsettias and other crops. Some growers have performed "tests" with
Fascination for this purpose, and it seems to be more user friendly than
ProGibb. Valent is working to expand the Fascination label, and when approved,
we will likely recommend Fascination for this purpose.

Because of the potential negative effects of GA, it is
important to be cautious with its use. Currently, we are suggesting growers try
3-5 ppm and plan to make 1-3 applications at 5- to 7-day intervals, as needed. The
GA effect can be seen in 3-5 days. At least with poinsettias, it is better to
promote growth of short plants early in the crop rather than near the finish.
There are still many unknowns in the use of GA, and growers should do trials
with it to gain experience before treating large numbers of plants.

While 25 percent of the poinsettias are too small, it seems
that about 50 percent must be too tall, and growers are trying to stop them.
This must mean about 25 percent of the poinsettias are just right, which is a
pretty good figure for poinsettias. The use of Bonzi and other products as a
late drench on poinsettias has become an important tool. However, the biggest
question currently is what can be done after the spray cutoff date when the
crop is too tall and there is not adequate bract development to allow for the
late drench treatment. There is a large number of growers who are trying
drenches at this stage using 25-50 percent of the late drench rate. We are
doing a considerable amount of research on this also, so there should be a
better understanding of how this works by the 2004 production season.

New Products

In the anti-GA (growth retardants) group of chemicals, there
are two "new" products to discuss. SePRO should receive label
clearance for Topflor in the next few months. Topflor is not new; it was a
numbered compound back in the early 1980s with Bonzi (Syngenta) and Sumagic
(Valent). However, Topflor was subsequently introduced in Europe and not the
United States. In terms of activity, Topflor fits in the general Á range
of Bonzi, but there are differences. Compared to Bonzi, the optimum rate for
Topflor will be more variable from crop to crop and variety to variety, but
there will be some good fits for Topflor. Watch for its introduction this year,
which will be accompanied by more use recommendations.

As opposed to pesticides, there have been very few generic
growth regulator products, but that situation is changing. Piccolo (Fine
Agrochemicals) was introduced this year, and it contains paclobutrazol, which
is the same active ingredient as in Bonzi. So, we will have to get used to
talking about paclobutrazol products. Our growth regulators are older chemicals
and the patents have expired on all of them. Various companies are making
decisions as to whether or not to introduce other generic products, so we will
likely have additional generics.

Future of PGRs

What this will mean to the grower remains to be seen. Will
prices go down some? We'll see. There certainly will be more competition
between the chemical manufacturers, and the grower could benefit from increased
service as the companies compete to gain and maintain business. Growth
regulators are difficult products for the companies to market. Because of the
numerous variables in their use, the risks associated and the number of crops,
the companies must put a great deal of additional effort into generating
information on how to use the products and communicating that information to
the growers. In short, PGRs are expensive products to produce and sell.

A personal concern of mine as a researcher is whether
development efforts continue. As my next topic below illustrates, there is a
rapid shift underway in how the growth retardants are being used. The
greenhouse industry will benefit in the long term if the companies continue
efforts to develop and disseminate this information. In fact, I predict this
will be one of the ways the companies will differentiate themselves; the
products that come with the most information will be the ones used the most.


As the importance of the more vigorous spring crops has
increased, the need for PGRs has also increased. However, our objectives have
evolved. We now talk about PGR strategies to obtain early control before crops
are too large and then to hold back growth at the end of the crop without
delaying flowering or reducing flower size.

Early control means applications at or near the time of
planting. One of these techniques is called a media spray or media sprench.
This is using spray equipment to apply the chemical to the media which give a
drench type effect. The photos below illustrate this idea with a direct sown
seed crop where hypocotyl elongation is significant. This practice is also
useful in plug production to provide most of the control early and make later
PGR use easier. Media sprays made to the surface of the media prior to planting
liners or plugs can provide very good control of vigorous crops and is
especially useful in smaller containers. Á

Another technique on vigorous liner material that growers
are finding useful for early control is a liner dip or liner drench. Here, the
liners are treated before transplanting by either dipping the root system in a
growth regulator solution or drenching the PGR over the top of the liner tray.
For the dip, the media should be about as dry as when it would be irrigated,
and the liner only needs to be in the solution long enough to become saturated.
Liner dips are again helpful when the vigorous material is grown in smaller pot
sizes, but is also useful for slowing growth of the most vigorous items in
mixed containers.

For control to prevent crops from becoming too large and
overgrown near the end of the crop, we are strongly promoting drench
applications over sprays. Drenches do not affect flower development nearly as
much as sprays do. The drench treatments keep hanging baskets at a marketable
size for a longer period in the greenhouse and at retail.

So, that is where we are with growth regulators -- finding
better ways to use old chemicals.

About The Author

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

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