Using Topflor, Part I :Bedding and Potted Plants

Plant growth regulators (PGRs) are commonly applied tocontainer-grown plants to control stem elongation and produce high-quality,compact plants. Flurprimidol was a molecule discovered more than 20 years agoby Eli Lily and Company. It was experimentally labeled as EL-500, and initialtrials were conducted on poinsettias, pot chrysanthemums and exacum by JimBarrett, GPN’s consulting editor, at the University of Florida and G. McDanielat the University of Tennessee. Flurprimidol is a “Type 2” PGR, whichis similar in its mode of action to A-Rest, Bonzi and Sumagic. Flurprimidol hasbeen labeled as Cutless for turf use in the United States and commerciallyintroduced as Topflor in Europe for greenhouse crops. Cutless has been trialedextensively by Auburn University researchers on nursery crops such as butterflybush, holly and Mexican sage with growth control comparable to Bonzi andSumagic.

Even though the initial Topflor trials were conducted in theUnited 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 anumber of greenhouse plants such as pot chrysanthemum, dianthus, osteospermumand streptocarpus. Commercial recommendations for applying Topflor foliarsprays have been developed for more than 20 greenhouse crops in Europe, but thelabel does not include many of the plants commonly grown in the United States.

There are a number of factors that can influence theefficacy of all plant growth regulators, including Topflor (See Table 1,below). Foremost is the particular species being grown. In addition, rates willneed to be adjusted according to the cultivar being grown, the concentration ofTopflor being applied and the number of applications made. The optimalconcentration (in ppm) of Topflor appears, in most cases, to be similar toBonzi, although for a few crops’ (e.g., pansies) optimal rates were similar toSumagic.

Topflor is absorbed by the plant through its leaves, rootsand stems. From preliminary experiments conducted at North Carolina StateÁ University, Topflor has a significantly greater absorption through thestem than Bonzi.

In Europe, a 1.5-percent Topflor formulation is used, whilethe new formulation that will be available in the United States is 0.38percent. This new formulation of Topflor has not been tested under U.S. growingconditions or on cultivars available in North America. In addition, Europeangrowers rely upon multiple Topflor applications at low concentrations tocontrol plant growth, while U.S. growers usually prefer a single application.Also, drench applications of PGRs are not commonly done in Europe, and researchis needed to determine optimal drench rates. Research is currently beingconducted at a number of universities: North Carolina State, Purdue, VirginiaTech, University of Florida and Cornell. The goal has been to determine optimalapplication rates for U.S. conditions. These rates, from the Southeast trialsconducted at North Carolina State University, should be viewed as a startingpoint, and growers will need to determine rates for their particular operation.

Trial process

Geraniums. Trialswere conducted with the cultivars ‘Samba’ and ‘Noblesse 99’ from Fischer USA.Cuttings were grown in 6-inch pots, and PGR foliar sprays were initiallyapplied two weeks after transplant. Topflor was applied at 10, 15, 20, 25 or 30ppm, and these rates were compared with Bonzi at 15 ppm and Sumagic at fiveppm. For each of the Topflor rates, another group of plants also received asecond Topflor spray at the same rate two weeks after the initial application.

Under North Carolina growing conditions, both cultivarsrequired rates of 25-30 ppm applied once or 15-20 ppm applied twice, with thesecond application occurring two weeks after the first, to adequately controlgrowth. Splitting the applications will allow growers to customize theirTopflor applications based on the prevalent weather conditions and crop vigor.These findings are comparable to the 15-30 ppm suggested on the French versionof the Topflor label. Rates will need to be adjusted for different locationsand cultivars.

Pot Mums. In ourtrials, we used both a short (‘Yellow Blush’) and tall (‘Duluth’) pot mumcultivar. Plants were given the required long-day treatments and pinched twoweeks 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 ppmand Sumagic at 7.5 ppm. A group of plants also received a second Topflor sprayat the same rate two weeks after the initial application.

Topflor foliar sprays were effective in controlling growthof Duluth, sprayed once or twice. Cultivar differences did occur. A singlespray of 25 ppm provided excellent control of Duluth.

Fall Pansies. Eachyear, Southeastern U.S. growers battle plant stretch with fall pansies. Germantrials indicated that Topflor had a very high efficacy on pansies, and wewanted to determine how well Topflor would do under hot and humid conditions.’Majestic Giants Yellow Blotch’ plants were treated with Topflor foliar spraysat 2.5, 5, 10, 15, 20, 25 or 30 ppm and compared with the other common PGRsused on fall pansies, such as Bonzi at 10 ppm, Sumagic at 2.5 ppm, A-Rest at 10ppm and Florel at 50 ppm. The PGRs were applied two weeks after transplant.

Topflor is very active on fall pansies, with rates higherthan 20 ppm being excessive. Topflor foliar sprays of five ppm provided comparablegrowth control for fall pansies as Bonzi at 10 ppm, A-Rest at 10 ppm or Florelat 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 inother seasons.

Tuberous Dahlias.Growth of pot dahlias can be excessive, and PGRs are required for mostcultivars. The cultivar ‘Ellen Houston’ was grown in 6-inch standard pots, andTopflor was applied as a substrate drench two weeks after potting.

Substrate drenches of Topflor were an effective means ofcontrolling tuberous dahlia growth. Southeastern U.S. growers should begin with1-2 mg active ingredients/pot drenches. Rates may vary by location andcultivar.

In conclusion

Based on the results of our trials, Topflor will be anexcellent addition to a grower’s PGR toolbox. It offers another alternative toBonzi and Sumagic for the management of plant growth. Through additionaltrialing, optimal concentrations will be determined, and growers will be ableto compare each of these PGRs based on its efficacy and price to determinewhich PGR is the most economical option. Look for Topflor research from JoyceLatimer 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.

Brian Whipker is associate professor, Ingram McCall is a research technician, James Gibson is a graduate research assistant and Todd Cavins is a former graduate research assistant (now on faculty at Oklahoma State University), in floriculture at North Carolina State University in Raleigh, N.C.