Getting started with plant growth retardants

Plant growth regulators are intimidating for some growers to use, but by following the proper steps, they can be highly effective tool.

The first step in finding the right plant growth retardant is find the right active ingredient.
Photo: ChiccoDodiFC, Adobe Stock

Plant growth regulators (PGRs) are one of the most ubiquitous tools for producing ornamental plants in the greenhouse. However, PGRs are not as widely accepted as, say, fertilizing plants. Part of this is by necessity. Unlike mineral nutrients, plants don’t require PGRs to live, and they're used to make a good crop a great one. However, part of this is also due to perceived barriers to success with using PGRs. For some, a fear of negative consequences resulting from their use — chiefly stunted plant growth from overregulation — is the main deterrent. Regardless of the reason, I'd like to provide some guidance for the uninitiated about how to start using PGRs.

The first step in learning to use PGRs is to select a forgiving active ingredient, one where an overapplication can minimize the likelihood of unwanted excessive growth control. The number one candidate active ingredient is going to be daminozide (B-Nine; Dazide). On a ppm-to-ppm basis with other active ingredients, it has the lowest activity, and has the shortest duration of growth control. Additionally, it has no root activity, so excess solution applied from sprays isn’t taken up. Alternatively, ancymidol (Abide, A-Rest), flurprimidol (Topflor), paclobutrazol (Bonzi, Piccolo), and, especially, uniconazole (Concise, Sumagic) are active ingredients that should be avoided by new applicators for a few reasons.

First, all these active ingredients can be taken up by root systems, a fact that makes these active ingredients so useful for other PGR applications such as substrate drenches or liner dips. However, it also means excess solution from foliar sprays can be taken up from the growing substrate and may cause excessive growth control. Second, the relative strength of these PGRs are higher than other active ingredients, as reflected in the lower recommended concentrations. Finally, these active ingredients have longer residual activity and greater duration of growth suppression than other PGRs.

It is best for new applicators to avoid using ethephon (Collate, Florel) for height control. Ethephon promotes flower abortion and enhances branching and while these can be beneficial results of intentional ethephon applications for some crops, they may be negative consequences when unintended on other crops.

To avoid over-application, take the recommended concentration, cut it in half and then apply it two weeks apart.
Photo: ChiccoDodiFC, Adobe Stock

Once an active ingredient has been selected, determine an application method. The four methods of applying PGRs include: 1) foliar sprays; 2) substrate drenches; 3) liner (or plug) dips; and 4) bulb (or rhizome or tuber) dips. Each of these application methods has a place in the greenhouse, and there are distinct advantages to each of them. However, the most widely used application method is foliar sprays, and for good reason. All active ingredients can be taken up by the shoot. Additionally, it's common to already be making other foliar spray applications in your greenhouse, likely for some type of pesticide. In order to expect consistent results with foliar sprays, it's important to ultimately apply 2 quarts of PGR solution per 100 square feet of growing area. However, for the beginner PGR applicator using daminozide who hasn't previously learned how to calibrate foliar spray application volumes, you can spray until “runoff” — when foliage glistens and excess solution just starts to run off leaves down to the stem or drips from the leaf tip. Even though the excess daminozide solution won’t be taken up by roots, you don’t need (or want) to spray the crop until the foliage is saturated and excess solution is pouring down leaves.

Next, determine what concentration should be applied to your crop. Granted, a specific recommendation for your exact active ingredient, application method, and crop may not exist. However, there are still many resources that can help you come to the best conclusion. First, start with any sort of technical information about the crop provided by breeders about crop culture- it is common for them to provide PGR recommendations. Look to publications like the Annual and Perennial PGR guides, which provide extensive recommendations summarized from a large number and wide variety of sources. Finally, ask around, whether it is an extension agent or a fellow grower.

One way to mitigate fear of over-application is to take the recommended concentration, cut it in half, and apply it twice two weeks apart. While it is ultimately more desirable to minimize PGR applications to save on labor, this intentional multiple-application approach of low-concentration solutions can go a long way to initiating new PGR users.

But what happens if our worst fear is realized and you get excessive growth control from applying PGRs? Don’t despair, there is hope! First, making the difference between the day and night temperature greater (i.e. a warmer day and cooler night) can promote stem elongation. Secondly, increase the overall average daily temperature can increase leaf unfolding which also may help. But another tool, on that can be applied with precision to affected crops only, exists. A combination of a cytokinin (benzyladenine; BA) and two different forms of gibberellic acid (GA4+7) is available commercially as Fresco or Fascination. When applied as a foliar spray to crops, it can promote stem elongation, and can be used to combat unwanted growth control from PGRs. For example, it is commonly used on poinsettia crops where too much PGR was applied, or on bedding plants where small amounts of PGR was in recycled irrigation water.

Plant growth regulators are some of the most useful tools available to greenhouse crop producers. Their ability to fine-tune a crop from a good one to a great one is something every grower can take advantage of, regardless of the size of their crop or the square-footage of their growing operation. If you aren’t currently using PGRs, I whole-heartedly encourage you to think of those containerized ornamental crops you grow that always could use some growth control and try out some foliar sprays of daminozide. In this case, you can consider it just what the doctor ordered.

Christopher is an associate professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu

February, 2023
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