Media Containing Bark or Parboiled Rice Hulls Affects Plant Growth Retardant Drenches Differently

The application of plant growth retardants (PGRs) is common in floriculture production to produce uniform, compact, and marketable plants. While peat, perlite, and vermiculite are common substrate components, bark and rice hulls are being used as alternative, and sustainable media components for soilless substrates.  The objective of this study was to evaluate the impact of media components on efficacy of PGRs drenches applied to bedding plants for height control.  ‘Callie Deep Yellow’ calibrachoa and ‘Delta Orange Blotch’ pansy were planted in 11.5-cm pots filled with peat and perlite- (PP), peat and parboiled rice hull- (PRH), or peat and pine bark-based (PB) media.  After planting, 74 mL of deionized water, paclobutrazol, or uniconazole solutions were applied to the media surface of each treatment.  Paclobutrazol (2.0 or 4.0 mg·L^-1) and uniconazole (1.0 or 2.0 mg·L^-1) drenches suppressed calibrachoa stem length for plants grown in each media.  However, at each paclobutrazol concentration and for 1.0 mg·L^-1 uniconazole, control of stem elongation was reduced for plants grown in PB media compared to PP media, while there were no differences between treated plants grown in PP and PRH media.  Similarly, pansy plant height was comparable for plants grown in PRH and PP media and treated with 0.5 or 1.0 mg·L^-1 paclobutrazol or 1.0 mg·L^-1 uniconazole, while plants treated and grown in PB media were taller than  treated plants grown in PP media.  Based on these results, rice hulls can be used as a media component in combination with PGR drench applications without altering rates used for peat and perlite-based media, because they do not reduce PGR drench efficacy.