A common cultural practice in greenhouse production is to apply plant growth retardants (PGRs) to produce uniform, compact, and marketable plants. Plant growth retardants can be applied in several ways, including foliar sprays, substrate drenches, liner dips, or bulb, tuber, and rhizome soaks or dips (Barrett, 2001; Blanchard and Runkle, 2007; Currey and Lopez, 2010; Whipker and McCall, 2000). However, the majority of applications are made as a foliar spray or substrate drench (Gent and McAvoy, 2000). Drenches provide more uniform results and increase the duration of effectiveness compared with foliar sprays (Boldt, 2008; Ecke et al., 2004; Gent and McAvoy, 2000). However, the efficacy of PGR drenches can be affected by the amount of a.i., solution volume applied, and substrate components (Barrett, 2001; Barrett et al., 2009).
Root substrates for containerized crops are commonly soilless mixes comprised of various organic and inorganic materials (Nelson, 2003; Raviv and Lieth, 2008). Vermiculite and perlite are common inorganic components, while peat and pine bark are common organic components of soilless substrates used in the production of potted greenhouse crops (Dole and Wilkins, 2004; Fonteno, 1996). Numerous studies have reported that PGR drench efficacy is reduced when organic components are included in substrates, including pine bark (Barrett, 1982; Bonaminio and Larson, 1978; Million et al., 1998a, 1998b; Newman and Tant, 1995; Tschabold et al., 1975) and river waste [a plant product dredged from anaerobic aquatic environments (Benedetto and Molinari, 2007)].
A growing trend among producers is to identify alternative substrate components. Parboiled rice hulls have been identified as an alternative substrate component and are an agricultural byproduct that requires little processing to produce a suitable substrate component (Buck and Evans, 2010; Evans, 2008; Evans and Gachukia, 2004; Holcomb et al., 2008; Sambo et al., 2008). However, there is no information on the effects of parboiled rice hulls on PGR drench efficacy. Our objective in this study was to identify what impact a substrate containing rice hulls as a component would have on PGR applications for controlling plant height or stem length of containerized greenhouse crops.
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