Plant growth regulators (PGRs) are commonly applied to
container-grown plants to control stem elongation and produce high-quality,
compact plants. Flurprimidol was a molecule discovered more than 20 years ago
by Eli Lily and Company. It was experimentally labeled as EL-500, and initial
trials were conducted on poinsettias, pot chrysanthemums and exacum by Jim
Barrett, GPN's consulting editor, at the University of Florida and G. McDaniel
at the University of Tennessee. Flurprimidol is a "Type 2" PGR, which
is similar in its mode of action to A-Rest, Bonzi and Sumagic. Flurprimidol has
been labeled as Cutless for turf use in the United States and commercially
introduced as Topflor in Europe for greenhouse crops. Cutless has been trialed
extensively by Auburn University researchers on nursery crops such as butterfly
bush, holly and Mexican sage with growth control comparable to Bonzi and
Even though the initial Topflor trials were conducted in the
United States, the chemical was not introduced to the U.S. market at that time.
In Europe, Topflor has been extensively trialed since the early 1990s on a
number of greenhouse plants such as pot chrysanthemum, dianthus, osteospermum
and streptocarpus. Commercial recommendations for applying Topflor foliar
sprays have been developed for more than 20 greenhouse crops in Europe, but the
label does not include many of the plants commonly grown in the United States.
There are a number of factors that can influence the
efficacy of all plant growth regulators, including Topflor (See Table 1,
below). Foremost is the particular species being grown. In addition, rates will
need to be adjusted according to the cultivar being grown, the concentration of
Topflor being applied and the number of applications made. The optimal
concentration (in ppm) of Topflor appears, in most cases, to be similar to
Bonzi, although for a few crops' (e.g., pansies) optimal rates were similar to
Topflor is absorbed by the plant through its leaves, roots
and stems. From preliminary experiments conducted at North Carolina State
Á University, Topflor has a significantly greater absorption through the
stem than Bonzi.
In Europe, a 1.5-percent Topflor formulation is used, while
the new formulation that will be available in the United States is 0.38
percent. This new formulation of Topflor has not been tested under U.S. growing
conditions or on cultivars available in North America. In addition, European
growers rely upon multiple Topflor applications at low concentrations to
control plant growth, while U.S. growers usually prefer a single application.
Also, drench applications of PGRs are not commonly done in Europe, and research
is needed to determine optimal drench rates. Research is currently being
conducted at a number of universities: North Carolina State, Purdue, Virginia
Tech, University of Florida and Cornell. The goal has been to determine optimal
application rates for U.S. conditions. These rates, from the Southeast trials
conducted at North Carolina State University, should be viewed as a starting
point, and growers will need to determine rates for their particular operation.
were conducted with the cultivars 'Samba' and 'Noblesse 99' from Fischer USA.
Cuttings were grown in 6-inch pots, and PGR foliar sprays were initially
applied two weeks after transplant. Topflor was applied at 10, 15, 20, 25 or 30
ppm, and these rates were compared with Bonzi at 15 ppm and Sumagic at five
ppm. For each of the Topflor rates, another group of plants also received a
second Topflor spray at the same rate two weeks after the initial application.
Under North Carolina growing conditions, both cultivars
required rates of 25-30 ppm applied once or 15-20 ppm applied twice, with the
second application occurring two weeks after the first, to adequately control
growth. Splitting the applications will allow growers to customize their
Topflor applications based on the prevalent weather conditions and crop vigor.
These findings are comparable to the 15-30 ppm suggested on the French version
of the Topflor label. Rates will need to be adjusted for different locations
Pot Mums. In our
trials, we used both a short ('Yellow Blush') and tall ('Duluth') pot mum
cultivar. Plants were given the required long-day treatments and pinched two
weeks after potting. Four weeks after potting, Topflor was applied at 10, 15,
20, 25 or 30 ppm, and these rates were compared with Bonzi at 50 Á ppm
and Sumagic at 7.5 ppm. A group of plants also received a second Topflor spray
at the same rate two weeks after the initial application.
Topflor foliar sprays were effective in controlling growth
of Duluth, sprayed once or twice. Cultivar differences did occur. A single
spray of 25 ppm provided excellent control of Duluth.
Fall Pansies. Each
year, Southeastern U.S. growers battle plant stretch with fall pansies. German
trials indicated that Topflor had a very high efficacy on pansies, and we
wanted to determine how well Topflor would do under hot and humid conditions.
'Majestic Giants Yellow Blotch' plants were treated with Topflor foliar sprays
at 2.5, 5, 10, 15, 20, 25 or 30 ppm and compared with the other common PGRs
used on fall pansies, such as Bonzi at 10 ppm, Sumagic at 2.5 ppm, A-Rest at 10
ppm and Florel at 50 ppm. The PGRs were applied two weeks after transplant.
Topflor is very active on fall pansies, with rates higher
than 20 ppm being excessive. Topflor foliar sprays of five ppm provided comparable
growth control for fall pansies as Bonzi at 10 ppm, A-Rest at 10 ppm or Florel
at 50 ppm. Sumagic at 2.5 ppm provided a greater degree of growth control.
Rates will have to be adjusted for cultivar vigor and for plants being grown in
Growth of pot dahlias can be excessive, and PGRs are required for most
cultivars. The cultivar 'Ellen Houston' was grown in 6-inch standard pots, and
Topflor was applied as a substrate drench two weeks after potting.
Substrate drenches of Topflor were an effective means of
controlling tuberous dahlia growth. Southeastern U.S. growers should begin with
1-2 mg active ingredients/pot drenches. Rates may vary by location and
Based on the results of our trials, Topflor will be an
excellent addition to a grower's PGR toolbox. It offers another alternative to
Bonzi and Sumagic for the management of plant growth. Through additional
trialing, optimal concentrations will be determined, and growers will be able
to compare each of these PGRs based on its efficacy and price to determine
which PGR is the most economical option. Look for Topflor research from Joyce
Latimer on perennials in next month's GPN.
Use of trade names in this publication does not imply endorsement by NC State University of products named nor criticism of similar ones not mentioned. The authors would like to thank Southern Gem Greenhouse, Fischer USA, Flower Fields, Wagner's Greenhouse and the Paul Ecke Ranch for supplying plant material; Fafard for the root substrate; Scotts for the fertilizer; Dillen Plastics for the pots; and the North Carolina Commercial Flower Growers' Association and SePRO Chemical Co. for grant support.
With Topflor on its way to market, you'll soon have another PGR in your tool box. This first of a 2-part series summarizes Topflor's efficacy on bedding plants.