6/28/2024 - By Dr. Aaron Palmateer

Plant growth regulators (PGRs) are used on a wide variety of ornamental crops to improve rooting, increase shoot formation, shorten internodes, and induce flowering. Whether it is annual plants in greenhouse production or a hedgerow of ficus in the landscape, PGRs can save time and money.

Benefits of PGRs

Additional benefits that can result in substantial cost savings when using PGRs include enhancing overall plant quality by maintaining proportionally sized plants, even if it is nursery crops or foliage plants where a larger plant size is desirable. Applicators should consider growth promotion, often targeting low-dose applications, for holding plants that are notorious for stretching.

Dr. Jim Barrett at the University of Florida did a great service by promoting low-dose substrate drenches of select PGRs (i.e., flurprimidol, paclobutrazol, uniconazole) to control the growth of poinsettia. He demonstrated that doses of 0.1 ppm paclobutrazol were effective for slowing growth, yet time-limited, so the poinsettia crop could be at the ideal size for sale.

The Role of PGRs

PGRs play a significant role when it comes to manipulating leaf and bract size, increasing branching, increasing bud set, breaking dormancy in both seeds and established plants, and improving plant shelf life in the store. Regulating plant growth is key for an industry that relies on plant quality for sales. PGRs provide the ability to hold plants in production longer while using fewer resources, such as reducing irrigation frequency (due to smaller plant size) and producing a plant product that meets the customers' demand for size, shape, and overall plant quality.

PGRs are not classified as insecticides or bactericides but a quick dive into the scientific literature reveals that PGRs can and have been used for controlling pests and reducing bacterial leaf spots. Way back in the late 1980s and early 90’s Drs. Lance Osborne and Ann Chase tested the effects of chlormequat chloride (Cycocel) on two-spotted spider mites as well as three bacterial leaf spot diseases on hibiscus. The pathology research results indicated regardless of hibiscus cultivar (10 were screened) or bacterial pathogen (Pseudomonas cichorii, P. syringae, or Xanthomonas campestris pv. malvacearum) the Cycocel-treated hibiscus plants were resistant to the bacteria.

Further results indicated that hibiscus plants treated with Cycocel were less susceptible to two-spotted spider mites (Tetranychus urticae) and it was not just due to surface chemical residue. Note that Cycocel is now available as Altercel from OHP.

When to Apply PGRs

PGRs are most effective when applied at the appropriate times to regulate plant growth or development. PGRs will not shrink an overgrown plant, so they must be applied preventatively. Growers should establish and maintain records of application rates based on small-scale treatments under actual use conditions, for individual species and varieties.

Growers in northern, cooler climates will use rates at lower ranges. Growers in warmer, southern climates will use rates at the upper end of the ranges. Most PGRs can be applied as a spray, sprench, dip, or drench, and the more product that is taken up by the roots will last longer compared to foliar applications. Early applications of PGRs are used to slow down growth using lower rates, while later applications are used to hold finished size and flowering and typically require higher rates.

Contact Your Harrell’s Rep for More Information

PGRs are a powerful addition to horticultural programs and have the ability to save you resources, labor, and money overall. To learn more about PGRs and how you can utilize them in your operation, reach out to your local Harrell's Rep.

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