During spring conditions in Michigan, typical rates of paclobutrazol used as a liner dip on aggressive vegetative annuals range from 4 to 8 ppm, including crops such as petunia, scaevola, argyranthemum and calibrachoa, and 8 to 12 ppm for verbena.  Lower rates are generally used for products containing uniconazole, such as 1 to 2 ppm for petunia, 2 to 4 ppm for calibrachoa, and 3 to 4 ppm for argyranthemum, scaevola and verbena.  These rates generally provide a long-lasting response (six to eight weeks), and lower rates should be used if a shorter response period is desired.  In locations with warmer, sunnier weather, higher rates may be needed to achieve the desired response.

Finally, the media moisture content can have a significant impact on the uptake of the liner dip solution, and thus the plant response.  If the media is moist, not much PGR solution will be absorbed by the media, and thus a smaller response will be attained.  Conversely, if the media is very dry, much more PGR solution will be absorbed, producing a stronger response.  We suggest that growers ensure that the media of liners and plugs is slightly moist, and not wet or dry, at the time of application.  This will ensure a more uniform response within the plug tray and from one application date to another.

For much more information, you may wish to review two articles that appeared in GPN magazine:

PGR Liner Dips On Bedding Plants, by Matthew Blanchard and Erik Runkle, Michigan State University.

Reducing PGR Liner Dip Variables, by Rebecca Schnelle and Jim Barrett, University of Florida.

As with all plant growth retardants, we strongly encourage growers to conduct their own trials on a small scale to determine the most appropriate rates for their crops and growing conditions.