Traditionally, greenhouse and nursery operations formulate container substrates primarily from peatmoss (peat), pine bark (PB), perlite, or vermiculite for the production of most all bedding plants and nursery ornamentals (Nelson, 2012). Of these substrate components, peat and PB are used in the greatest quantities. Peat, PB, and perlite can be expensive and prices can fluctuate due to reduced availability and increasing costs associated with harvesting, processing, and long-distance transport. An emphasis on reducing production costs and an increased interest in using local and regional materials has led to the investigation and development of alternative substrates and substrate components for peat, PB, and perlite.
Many substrate components are agricultural, municipal, or waste by-products. Numerous studies have investigated a variety of substitutes for peat, PB, and perlite such as cotton gin waste (Owings, 1993), wood by-products (Chong and Lumis, 2000; Criley and Watanabe, 1974), municipal leaf and sewage sludge (Bugbee et al., 1991; Rosen et al., 1993), rice hulls (Dueitt et al., 1993; Evans and Gachukia, 2004), ground bovine bones (Evans, 2004a), poultry feather fiber (Evans, 2004b), and shredded rubber (Evans and Harkess, 1997). However, inconsistency and insufficient quantities of by-products are challenging for long-term and sustained use as an alternative substrate or substrate component, especially for large production facilities.
The use of plant growth retardants (PGRs) is a common cultural practice of controlling growth in containerized plants in greenhouse production. Plant growth retardants allow growers to control the rate of growth or flowering, increase water uptake efficiency, hold plants longer in production, and produce uniform, compact, and marketable plants (Whipker, 2015). Methods of applying PGRs include foliar sprays, substrate drenches, liner dips or bulb, tuber, and rhizome soaks or dips (Barrett, 2001; Blanchard and Runkle, 2007; Lopez et al., 2010; Whipker and McCall, 2000). However, the most common application methods are foliar sprays, substrate drenches, or a combination of the two (Gent and McAvoy, 2000). Substrate drenches are preferred because of the precision of application, crop uniformity, duration of effectiveness, minimal environmental impact, and the reduction of potential drift from spray applications. However, the efficacy of PGR drenches can be affected by the amount of active ingredient, volume of solution applied, and substrate components (Barrett, 2001; Barrett et al., 2009).
Paclobutrazol provides size control on many floricultural crops (Barrett and Nell, 1989) and is active when applied to the substrate and taken up through the roots (Barrett and Bartuska, 1982; Davis et al., 1988). In comparison with a peat-based substrate, Quarrels and Newman (1994) reported a significant reduction in paclobutrazol control when drenches were applied to poinsettia (Euphorbia pulcherrima) plants grown in a PB-based substrate. Similar results of paclobutrazol activity inhibition were reported by Barrett (1982) for chrysanthemum (Denranthemum grandiflora) grown in PB-based substrates. Dasoju et al. (1998) found the activity of paclobutrazol drenches to potted sunflowers in coir- and peat-based substrates to be similar at 2 mg/pot. However, a greater activity was reported for coir-based substrates at 4 mg/pot, Lopez et al. (2010) reported that identical PGR drench applications of ancymidol (Abide; Fine Americas, Walnut Creek, CA), palcobutrazol (Bonzi; Syngenta Crop Protection, Greensboro, NC), and uniconazole (Concise; Fine Americas) to ‘Delta Orange Blotch’ pansy (Viola wittrockiana) and ‘Callie Deep Yellow’ calibrachoa (Calibrachoa ×hybrida) grown in a 80% sphagnum peat and 20% perlite- or 20% rice hull-amended (by volume) substrate resulted in similar plant heights and stem length growth patterns.
No information is available regarding the activity of paclobutrazol drenches in peat-based substrates amended with PWC aggregates. Therefore, the objective of this study was to determine if paclobutrazol drench efficacy is affected by PWC used as a substitute for perlite in a peat-based substrate.
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