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PGRs for Growth Regulation of Buddleia spp.

Thu, Apr 4th, 2019, created by Joyce Latimer

Butterfly bushes (Buddleia spp.) are great perennial flowering plants to meet thecurrent interest in pollinator gardens. These very vigorous plants aretypically grown as herbaceous perennials although they become quite woody inthe landscape. We’ve conducted several PGR trials with different cultivarsusing uniconazole (Concise, Sumagic) and/or paclobutrazol (Piccolo, Piccolo 10XC, Bonzi, Pac O) and this is a good place to summarize some of those results.

As with most of our herbaceous perennials, Buddleia cultivars vary in theirresponse to PGRs (Table 1). As a very vigorous crop Buddleia requires early PGR applications. In our studies, the PGRswere applied as liner soaks prior to potting (‘Honeycomb’) or as foliar spraysor substrate drenches (10 fl. oz. per trade gallon pot) applied 10 to 14 daysafter potting. As you can see with the cultivars listed below, the growthresponse was maximized at 3 or 4 weeks after the PGR application (WAT). Much ofthis growth control, relative to the untreated control, was lost by 6 WAT.

Table 1. Maximum growth control(percent shorter than untreated control plants) of Buddleia species/cultivarsusing uniconazole (uni, Concise) or paclobutrazol (paclo, Piccolo) applied asfoliar sprays, liner soaks (2 min) or drench (10 fl. oz. per trade gallon pot)as measured at 3 or 4 (depending on experiment) and 6 weeks after treatmentapplication (WAT).


Buddleia cultivar


PGR/application method/rate

Maximum height control

3 or 4 WAT


Pink Delight

Uni spray 60 ppm








Uni liner soak 2 ppm




Paclo liner soak 4 ppm







Royal Red

Uni spray 60 ppm




Uni drench 0.2 ppm




Paclo spray 160 ppm




Paclo drench 2 ppm








Uni spray 60 ppm




Uni drench 1.5 ppm



The rapid resumption of growth suggests a potentialbenefit to planning to make multiple applications during production. Theincrease in the length of the internodes as the PGR effect is lost, is avisible indicator of the need to make a second application (see photo 1). Thischange in elongation is clear in the Buddleia‘Lochinch’ plant treated with 0.5 ppm substrate drench (second from left, Photo1). Also notice the significant growth control with the 60 ppm foliar spray ofuniconazole at 4 WAT which persisted through a 6 WAT measurement.

 In summary, Buddleiais generally responsive to either uniconazole or paclobutrazol. However, theresponse varies with cultivar. Therefore, you will need to run your own trialsor start with lower application rates and make additional applications asnecessary. Once you narrow down the rate, consider planning on making a secondapplication later in the crop cycle to attain season long control.

About the Author:

Joyce Latimer

Professor, Horticulture, Virginia Tech

Joyce Latimer is a Professor and Greenhouse Extension Specialist at Virginia Tech in Blacksburg VA. Her specialty is in perennials production, problem solving, and plant growth regulation of herbaceous perennials under greenhouse and nursery conditions. Joyce also works with greenhouse energy efficiency education and implementation programs.

Joyce has been recognized for her Extension service to the industry with the Alumni Award for Outreach Excellence (Virginia Tech, 2010), Horticultural Initiative Award (Southeast Color Connection, 2010), and the Outreach Excellence Award (Virginia Tech College of Agriculture and Life Sciences, 2007).

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