To produce uniform plants, growers of herbaceous perennials control plant growth by using both cultural controls, including plant nutrition, light, and temperature, and chemical controls, i.e., PGRs (Albrecht and Tayama, 1992; Whipker et al., 2006). PGRs can increase branching and improve plant architecture in plants by releasing apical dominance and allowing dormant lateral buds to expand, which results in fuller, well-branched plants (Cline, 1991). However, using PGRs can be challenging, because plants respond in different ways to PGRs depending on species, cultivar, growing conditions, and the PGR used (Gent and McAvoy, 2000).
BA, a synthetic cytokinin, is a PGR that increases branching when sprayed on ornamental plants (Latimer and Freeborn, 2008; Martin and Singletary, 1999). The number of branches doubled in Echinacea (L.) Moench ‘White Swan’ and ‘Double Decker’ treated with foliar sprays of 300 or 600 mg·L−1 BA at ≈2 weeks after transplanting to quart-sized (1.1-L) pots (Latimer and Freeborn, 2008). Of 18 juvenile perennials treated with BA as a foliar spray at rates of 1000, 2000, or 4000 mg·L−1, 89% had an increase in the number of lateral branches, although no additional branching was seen in the higher two rates as compared with 1000 mg·L−1 (Martin and Singletary, 1999). However, in this study, plants treated with higher rates were more compact and had decreased leaf size.
DS has been used as a chemical pinch to prevent elongation in woody plants as well as to increase branching in herbaceous plants. Banko and Stefani (1995) assessed the use of DS on container-grown woody ornamentals. Of the nine species they studied, DS effectively controlled growth in five, producing plants that were more compact and appeared to have denser shoot growth. When 3-year-old field-grown Lavandula ×intermedia Emeric ex Loisel. ‘Twickle Purple’ plants were treated with 0.5%, 1.0%, or 2.0% Atrinal (PBI Gordon, Kansas City, MO; containing 20% DS), the numbers of vegetative shoots and inflorescences and oil yield increased significantly compared with controls (Porter and Shaw, 1983). Branching of Gaillardia aristata Pursh. ‘Gallo Yellow’ was increased by foliar sprays of 400 or 800 mg·L−1 DS (Latimer and Freeborn, 2010). However, not all species are responsive to treatment with DS; as referenced previously, Banko and Stefani (1995) found five species responded to DS out of nine studied.
PGRs are often applied after transplant, when plants are in active growth and have started to expand. Treating liners (rooted cuttings) before transplant can have significant benefits for growers including reduced time of application and reduced chemical cost. Grossman et al. (2012) reported increases in branching in four of five herbaceous perennials treated with BA before transplant; of these, only two had persistent increases in branching after plants grew to finished size. The question of when to apply growth regulators to achieve increased branching in finished plants is an important one for growers who want to grow out liners to finished, marketable plants. The objective of this study is to examine the effects of application time of DS, BA, or a combination of the two on the growth and branching of herbaceous perennial plants during liner production and grow out of finished plants.
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