Basics of Using Plant Growth Retardants in the Nursery


While many of our blog readers may be seasoned plant growth retardant (PGR) users, many of our nursery producers are relatively new at using them. At the Cultivate ’15 show in Columbus last month, Dr. Amy Fulcher (University of Tennessee) and I spoke to a room of nurserymen. We found that many of them had little experience with using PGRs. With this in mind, I decided to repeat a bit of the introductory information that we shared with them as a refresher for all of our blog readers.

First of all, you need to recognize that PGRs are production tools, just like your fertilizer or water applications. They need to be used in conjunction with your other cultural practices. So, you must PLAN to use PGRs as part of your production protocol to manage plant growth. Recognize that PGR effects on flowering in herbaceous perennials can be less predictable than with many of our annual bedding plants. However, the principles of “safety” are the same. Preferably, treat before flower initiation. In general, early applications minimize the effects on plant flowering while maximizing growth regulation.

Which PGR Should You Use for Growth Regulation?

There are many products available for plant growth regulation which are typically applied as foliar sprays. With respect to obtaining consistent application results, I like to categorize the PGRs according to their ease of application.

  1. Not Soil Active – Dazide (daminozide) – only the PGR material applied to the plant is available to the plant. The overspray in the substrate is not absorbed by the roots of the plant.
  2. Limited Soil Activity – Citadel (chlormequat chloride) – PGRs that may be absorbed from the substrate but usually not in quantities great enough to complicate their application. In contrast to greenhouse floricultural crops, we seldom see Citadel used alone in treating herbaceous perennials. However, it is frequently used in a tank mix with Dazide.
  3. Soil ActiveAbide (ancymidol), Piccolo and Piccolo 10 XC (paclobutrazols) and Concise (uniconazole) – are readily absorbed by the root system from the substrate solution. These PGRs are absorbed by both the shoot (leaves and/or stems and petioles) as well as by the root tissues. These PGRs are typically more potent than those that are absorbed by the foliage alone.

How Does the PGR Category Affect the Dosage?

The soil activity category affects the ease of the PGR application. For PGRs with no or limited soil activity, the dosage is simply the concentration of the PGR in the solution (the ppm or mg/liter). Application volume must be sufficient to allow good coverage of the foliage but will not affect the efficacy of the application.

With soil active PGRs, the dosage depends on both the concentration of the PGR and the volume of the solution applied to the crop. Since the PGR in the solution that runs off the plant into the media or that hits the media directly is also available to the plant, we have to consider both the concentration and the volume applied to determine the dosage.

For example, applying 50 ppm paclobutrazol at the label-recommended volume of one gallon per 200 sq.ft. gave excellent growth control of hollyhock. Alcea volume However, applying 50 ppm paclobutrazol at twice the recommended volume (HV, 2 gallons per 200 sq.ft.) resulted in an overdose response. Increased application volume of soil active PGRs increases the plant response to the PGR application due to the additional PGR that is applied to and retained by the medium. The treated hollyhock plugs had good coverage of the foliage with both application volumes. So the excessive growth reduction was due to the additional PGR in the medium of the high volume spray. Furthermore, uniformity of response to PGR sprays depends on the uniformity of the applicationHydrangea uniformity. Practice spray application techniques to apply a consistent volume evenly over the production area to attain a uniform plant response.


If you are new to using PGRs, choose PGRs that are not soil active, for the ease of consistent application. The daminozide and chlormequat chloride tank mix can be very potent and is effective on a wide range of herbaceous perennials. Learn to use PGRs or train specific people to use them so that you can consistently apply any of the PGRs that meet your production needs. Adjust your testing rates based on your part of the country (lower rates in the northern areas) and run your own trials before treating large numbers of plants. Include new crops in your trials, especially crops that are problem crops in your nursery – herbaceous or woody. And, be sure to keep records on your applications – and your results. Effective use of PGRs is still very much an art – but it is based on science.

For more information, see Dr. Brian Whipker’s video blog, “Increasing PGR Foliar Spray Efficacy”,