Fig. 1. The relative strength of different triazole and triazole-like plant growth retardants varies. Note that while ‘Divine White’ New Guinea impatiens treated with flurprimidol and paclobutrazol were similar with a concentration, growth of plants treated with uniconazole was suppressed more at lower concentrations compared to plants treated with flurprimidol and paclobutrazol.
Graphic: Christopher J. Currey

Plant growth retardants (PGRs) are an extremely useful tool for controlling plant growth and producing plants that are aesthetically balanced in size with their containers. When it comes time to decide which PGR to use, there are numerous options available to growers.

Among the different active ingredients used for growth control, there is a group that stands out and includes ancymidol (Abide, A-Rest), flurprimidol (Topflor), paclobutrazol (Bonzi, Paczol, Piccolo, Piccolo 10XC) and uniconazole (Concise, Sumagic). Sometimes they are referred to collectively as the “triazoles” and, though only paclobutrazol and unizonazole are true triazoles, all four active ingredients are very closely related. Some growers are intimidated by these compounds, as they are generally the strongest active ingredients available. However, these effective chemicals can be used by anybody if you keep a few key points in mind.

While nearly all growth-retarding compounds affect plants similarly, by inhibiting gibberellic acid (GA) synthesis and suppressing cell elongation, active ingredients can inhibit GA synthesis differently. Ancymidol, flurprimidol, paclobutrazol and uniconazole all inhibit GA synthesis the same — by the step where ent-kaurene is converted to ent-kaurenoic acid. It is important to keep in mind how these four active ingredients all have the same site of action, while being different than other retardants. Tank mixes are an approach to enhance growth control from PGR sprays. If you are interested in using ancymidol, flurprimidol, paclobutrazol and uniconazole in a tank mix, be sure to use them with a PGR that inhibits GA differently (chlormequat chloride or daminozide) or has a completely different mode of action (ethephon).

Like all PGRs, ancymidol, flurprimidol, paclobutrazol and uniconazole are all absorbed by shoots. Since all four compounds are absorbed by shoots, foliar sprays are a viable application technique. However, these four active ingredients are also absorbed by roots, which is why they are the primary active ingredients used for substrate drenches and liner dips. The fact that these compounds are absorbed by both shoots and roots can make the application of these chemicals more complex. It can be challenging for spray applications, because excess solution running off can be absorbed by the substrate, resulting in additional control. However, this can be used to your advantage and these chemicals can be applied as a “sprench,” which is a very high-volume spray that purposefully results in solution applied to both the plant and growing substrate.

There is not enough room in any one article (or a series of articles!) to provide recommended concentrations for ancymidol, flurprimidol, paclobutrazol and uniconazole for the variety of crops we grow in greenhouses; in fact, it can be hard to find recommended concentrations for some active ingredients. However, growers can use published recommended paclobutrazol concentrations as a starting point to determine appropriate concentrations of ancymidol, flurprimidol and paclobutrazol that may be used. For foliar sprays, ancymidol, flurprimidol and uniconazole concentrations should be twice, the same or half the concentration recommended for paclobutrazol, respectively. As an example, if all you could find was a recommended 40 ppm paclobutrazol for a crop, you may also consider spraying 80 to 160 ppm ancymidol, 40 ppm flurprimidol or 20 ppm uniconazole. While the relationship between chemicals is similar for drenches, flurprimidol has greater activity when applied as a drench. Therefore, for drenches, ancymidol would again be applied at twice to four times the concentration of paclobutrazol, whereas flurprimidol or uniconazole would be applied at half the concentration.

If you currently use PGRs, and are looking to expand into new active ingredients, ancymidol, flurprimidol, paclobutrazol and uniconazole are worth considering. Compared to other anti-GA retardants, these chemicals are stronger and give longer-lasting control and are suited to several application techniques. As always, conduct in-house trials to determine which active ingredients work best for you and your facility.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University.