Plant Growth Regulator Primer

By Pete Mihalek|November 23, 2009

For some growers, the acronym PGR can mean one of two things – plant growth regulator or plant growth retardant, both of which tend to get used interchangeably. “This is okay, but can be misleading,” says Jason Fausey, Valent research and development specialist. “I like the term regulator, simply because it can be applied to those that inhibit and speed up growth.”

Todd Bunnell, ornamental research manager at SePRO, adds, “Plant growth retardants are growth regulators that stop plant growth, specifically mitotic development.”

Most products are actually retardants, according to Peter Konjoian, Konjoian Floriculture Education Services, especially if one considers they are used to inhibit (retard) internode elongation.

Regulator or retardant, what follows is a breakdown of popular PGR types by active ingredient (AI) used in the greenhouse industry:

Ancymidol
Activity	Applied as either a foliar spray or substrate drench, this AI acts as a gibberellin inhibitor, which reduces internode elongation for a more desirable, compact plant.
Primary Crops:	Container-grown ornamentals, foliage plants, bedding plants/plugs
Products:	Abide, A-Rest
Additional Info:	Kevin Forney, Fine-Americas technical services manager, says, “Sequential applications of Abide at lower application rates can provide more uniform growth control and improved crop safety compared to a single application at a higher use rate.”

Benzyladenine (BA)
Activity:	This AI is a synthetic cytokinin that promotes cell division, cell differentiation from undifferentiated tissues and inhibits dominant growth of the apical meristem, which promotes axillary shoots (branching). BA also delays the aging process of leaves.
Primary Crops:	Poinsettias, variety of lilies
Products:	Configure, Fresco, Fascination
Additional Info:	In Fresco and Fascination, BA is packaged with two types of gibberellic acid, which stimulates and lengthens cells. Forney says Fresco can be used to reduce lower-leaf yellowing and necrosis on certain lilies. “Fresco can also be used on poinsettias to increase plant height and bract size. This can be especially useful to help overcome stunted growth following an excessive application of a plant growth retardant,” he says.

Chlormequat Chloride
Activity:	This AI is a gibberellin inhibitor, which reduces internode elongation for a more desirable, compact plant. It is used to reduce stem elongation. Like daminozide, chlormequat chloride is a “forgiving” PGR that will require multiple applications.
Primary Crops:	Geraniums, poinsettias
Products:	Citadel, Cycocel
Additional Info:	Chlormequat chloride and daminozide are two AIs that are commonly tank-mixed for more active results. This combination has shown to provide effective growth reduction while minimizing the potential for phytotoxicity, says Fine-Americas’ Kevin Forney. “They have an excellent synergy,” says Dave Barcel of OHP. “It’s not like one plus one equals two, but more like one plus one equals three in activity.”

Daminozide
Activity:	Used exclusively as a foliar spray, this AI is well absorbed into leaves and also a gibberellin inhibitor, which reduces internode elongation for a more desirable, compact plant.
Primary Crops:	Chrysanthemums, bedding plants/plugs, hydrangeas and azaleas
Products:	Dazide, B-Nine
Additional Info:	B-Nine is a more forgiving PGR, says Dave Barcel, OHP technical specialist. “It’s used at fairly high rates (typically 2,500 to 5,000 ppm),” he says. “If you happen to make a mistake or your measurement isn’t accurate, you won’t overstunt the plant.” Fine-Americas’ Forney adds that Dazide should not be applied within seven days of applying products containing copper, for risk of phytotoxicity (leaf burn).

Ethephon
Activity:	Once this foliar spray is absorbed by the plant, three main effects will take place: inhibition of flower initiation and development, stimulation of lateral branching, and inhibition of internode elongation.
Primary Crops:	Bedding plants, ornamental crops
Products:	Florel
Additional Info:	“If the plant to be treated with Florel is under any type of stress, the application of ethylene will exaggerate the normal signs of stress,” Konjoian says. Konjoian notes Dr. John Erwin at the University of Minnesota is currently conducting research on Florel – looking at conditions (humidity, temperature and drying time) as they affect Florel applications.

Flurprimidol
Activity:	Acts as an inhibitor of enzymes catalyzing the steps in the gibberellic acid (GA) biosynthetic pathway that involve oxidation of ent-kaurene to ent-kaurenoic acid, a GA precursor.
Primary Crops:	Bedding plants/plugs, bulbs, poinsettias, herbaceous perennials, other containerized crops
Products:	Topflor
Additional Info:	When used as a drench application, this active ingredient seems to be two to four times (ppm) as active as paclobutrazol formulations, says Todd Bunnell, Turf & Ornamental Research Manager at SePRO.

Gibberillic Acid (GA3)
Activity:	An application of this strong growth regulator will elongate plant cells. Primarily a spray, this AI is relatively active on what it comes in contact with, and shaping and stretching relies heavily on timing.
Primary Crops:	Used on ornamental crops looking for additional growth (stretch).
Products:	Florgib, ProGibb
Additional Info:	“Florgib works best when using water of neutral or slightly acidic pH,” Forney says. “ProGibb is excellent for tree growth – think poinsettia trees,” says Valent’s Fausey.

Paclobutrazol
Activity:	This growth inhibitor can be used as a spray or a drench and is absorbed primarily by the stems and roots of the plant, moving upward through the xylem to slow the production of gibberellic acid.
Primary Crops:	Azaleas, bedding plants, garden mums, poinsettias, herbaceous perennials, other containerized crops
Products:	Bonzi, Paczol, Piccolo
Additional Info:	Nancy Rechcigl, Syngenta field technical manager, says Bonzi can reduce vertical growth by as much as 75 percent for up to four weeks. Piccolo was the first paclobutrazol-based PGR labeled for use as a liner dip treatment, says Fine-Americas’ Forney. With typical application rates between 5 to 15 ppm, OHP’s Barcel warns against careless rate applications with this AI. “If you get sloppy, you’re going to be in trouble,” he says. Typical overdose rates for PGRs and Paclobutrazol in particular will end up with downward-cupped, extremely dark green, leathery leaves.

Uniconazole
Activity:	This AI inhibits the gibberellin biosynthesis. It is rapidly absorbed through the root, stem and leaf, and then transported to the meristem from the xylem.
Primary Crops:	Bedding plants, perennials, lilies
Products:	Concise, Sumagic
Additional Info:	While absorbed quickly into leaves, the AI does not readily move out of the tissue, so spray applications must be directed toward plant stems, Forney says. “Concise has been shown to increase the quality of plants (including darker colors, thicker leaves, stronger stems and increased water retention) even in the absence of noticeable growth control,” he adds. Valent’s Fausey notes that Sumagic has a recent label addition for use on greenhouse tomatoes and fruiting vegetables.