Producers work year-round taking cuttings from herbaceous perennials. This task is difficult because perennials flower at different times of the year. Reproductive tissue on cuttings can inhibit root and vegetative development (Gibson and Cerveny, 2005); therefore, it is highly desirable to keep stock plants in a vegetative developmental state. Herbaceous perennial crops need to produce enough vegetative material to justify the cost of the space they occupy on the greenhouse bench. Furthermore, target uniformity is desired. Perennial producers often prune their plants manually to increase vegetative growth. However, manually pruning perennials to encourage vegetative growth can be labor-intensive (Banko and Stefani, 1996) and can add unwarranted expense (Holland et al., 2007). Producers have sought a more convenient approach to increase vegetative growth. However, PGRs have been less commonly used to enhance vegetative plant growth and remove reproductive tissue (Preece and Read, 1993). However, PGR applications are generally less labor-intensive than manual pruning, although phytotoxicity may be a problem for certain crops (Meijón et al., 2009). Three specific PGRs have been identified for their potential benefits for stock plant production: gibberellic acid, benzyladenine, and ethephon.
The GA4+7 treatment uniformly promotes growth primarily through cell elongation throughout plant tissue (Moore, 1984). Therefore, application of GA4+7 to promote plant growth could result in more propagation material from stock plants. Cytokinins, specifically benzyladenine, have been found to be involved in nearly all aspects of plant growth and development (Leopold and Kriedemann, 1975). Benzyladenine promotes cell enlargement, but not cell elongation as with auxins and gibberellins; therefore, it promotes cell growth in all directions (Preece and Read, 1993). This results in decreased apical dominance if cytokinin levels in the plant are increased (Hartmann et al., 2002). Increasing lateral growth could result in more propagation material from stock plants. Ethephon is an ethylene inducer that enters the plant and breaks down into three molecules: phosphate, chloride, and ethylene. Ethylene has many processes within the plant cell when released into plant systems that affect plant growth and reproductive development (Preece and Read, 1993). Ethephon is widely used to promote auxiliary shoot development without damage to the apical meristem (Hayashi et al., 2001). Increased branching and decreased flower development could result in more vegetative growth for herbaceous perennial stock plants.
Herbaceous perennial responses to PGRs vary across cultural and environmental conditions (Cochran and Fulcher, 2013). Applications of benzyladenine increased branching on ‘Ruby Star’ coneflower (Echinacea hybrid) at rates as low as 300 ppm (Latimer et al., 2011), but ‘Silver Lode’ coral bells (Heuchera hybrid) responded minimally to an application of 600 ppm (Latimer and Freeborn, 2015). ‘Snow Angel’ coral bells and Orange Carpet™ hummingbird trumpet (Epilobium canum ssp. garrettii ‘PWWG01S’) had similar results when treated with GA4+7, but the quality of cuttings had disparities (high quality and low quality, respectively) (Markovic and Klett, 2020). The wide range of possible plant responses indicates the importance of continuing the study of herbaceous perennial responses to PGRs.
Mojave sage (Salvia pachyphylla) and ‘Avalanche’ cape daisy (Osteospermum hybrid) are herbaceous perennials that were used during these experiments. Meetings with greenhouse and nursery operators propagating the two herbaceous perennials resulted in two main production problems with both perennials. These problems were a lack of quality vegetative propagation material from stock plants and low rooting percentage rates during propagation. Based on previous research, it was decided that using PGRs could help resolve these problems.
The main objective of this study was to evaluate vegetative and floral development of mojave sage and ‘Avalanche’ cape daisy after applications of three commercial PGRs with the active ingredients of gibberellic acid, benzyladenine, or ethephon. This study hypothesized that applications of PGRs would result in increased vegetative propagation material with quantity and quality similar to those of mojave sage and ‘Avalanche’ cape daisy stock plants and not affect rooting success.
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