Growth Regulators: New Approaches By Jim Barrett

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


I’ll start with the Gibberellin (GA) products, since I’mwriting in the middle of poinsettia season, and 25 percent of the crops seem tobe 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 changesthat were needed for many years. One of these was the use of ProGibb (active ingredientis GA3) to stimulate stem elongation and increase plant size. At that sametime, Valent obtained access to an ag product containing GA4+7 and a Cytokinin,which became Fascination. Fascination is an excellent product, but is onlylabeled for Easter lilies to prevent lower leaf yellowing and prolong flowerlife.

ProGibb use to increase plant size is not real easy. First,ProGibb is super concentrated and difficult to measure. Then, there has beenmany over application cases similar to the image to the bottom left, onpoinsettias and other crops. Some growers have performed “tests” withFascination for this purpose, and it seems to be more user friendly thanProGibb. 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 isimportant to be cautious with its use. Currently, we are suggesting growers try3-5 ppm and plan to make 1-3 applications at 5- to 7-day intervals, as needed. TheGA effect can be seen in 3-5 days. At least with poinsettias, it is better topromote 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 trialswith it to gain experience before treating large numbers of plants.

While 25 percent of the poinsettias are too small, it seemsthat 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 apretty good figure for poinsettias. The use of Bonzi and other products as alate drench on poinsettias has become an important tool. However, the biggestquestion currently is what can be done after the spray cutoff date when thecrop is too tall and there is not adequate bract development to allow for thelate drench treatment. There is a large number of growers who are tryingdrenches at this stage using 25-50 percent of the late drench rate. We aredoing a considerable amount of research on this also, so there should be abetter understanding of how this works by the 2004 production season.

New Products

In the anti-GA (growth retardants) group of chemicals, thereare two “new” products to discuss. SePRO should receive labelclearance for Topflor in the next few months. Topflor is not new; it was anumbered compound back in the early 1980s with Bonzi (Syngenta) and Sumagic(Valent). However, Topflor was subsequently introduced in Europe and not theUnited States. In terms of activity, Topflor fits in the general ç rangeof Bonzi, but there are differences. Compared to Bonzi, the optimum rate forTopflor will be more variable from crop to crop and variety to variety, butthere 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 genericgrowth regulator products, but that situation is changing. Piccolo (FineAgrochemicals) was introduced this year, and it contains paclobutrazol, whichis the same active ingredient as in Bonzi. So, we will have to get used totalking about paclobutrazol products. Our growth regulators are older chemicalsand the patents have expired on all of them. Various companies are makingdecisions as to whether or not to introduce other generic products, so we willlikely have additional generics.

Future of PGRs

What this will mean to the grower remains to be seen. Willprices go down some? We’ll see. There certainly will be more competitionbetween the chemical manufacturers, and the grower could benefit from increasedservice as the companies compete to gain and maintain business. Growthregulators are difficult products for the companies to market. Because of thenumerous variables in their use, the risks associated and the number of crops,the companies must put a great deal of additional effort into generatinginformation on how to use the products and communicating that information tothe growers. In short, PGRs are expensive products to produce and sell.

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


As the importance of the more vigorous spring crops hasincreased, the need for PGRs has also increased. However, our objectives haveevolved. We now talk about PGR strategies to obtain early control before cropsare too large and then to hold back growth at the end of the crop withoutdelaying flowering or reducing flower size.

Early control means applications at or near the time ofplanting. 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 adrench type effect. The photos below illustrate this idea with a direct sownseed crop where hypocotyl elongation is significant. This practice is alsouseful in plug production to provide most of the control early and make laterPGR use easier. Media sprays made to the surface of the media prior to plantingliners or plugs can provide very good control of vigorous crops and isespecially useful in smaller containers. ç

Another technique on vigorous liner material that growersare finding useful for early control is a liner dip or liner drench. Here, theliners are treated before transplanting by either dipping the root system in agrowth 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 potsizes, but is also useful for slowing growth of the most vigorous items inmixed containers.

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

So, that is where we are with growth regulators — findingbetter ways to use old chemicals.

Jim Barrett

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

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