During perennial plant production, the height many perennials attain during growth makes them top heavy and limits their production. Shipping is also impacted; because of their height, fewer plants can fit on a cart. A prime example of this predicament is with lilies (Liliaceae family). These colorful, summer-flowering, bulbous perennials are commonly used as ornamental landscape plants, cut flowers, and potted plants (Dole and Wilkins, 2005); however, many species are tall, which subsequently reduces their transportability both from the grower to the retailer and from the garden center to the landscape when the shoot has emerged. Tiger lilies can reach a cumbersome height of 120 to 150 cm (Armitage, 2008), limiting their suitability for greenhouse forcing before retail sales (Whipker and Hammer, 1993) because most potted lilies cultivated are typically between 20 and 50 cm (Dole and Wilkins, 2005).
Although plant height can be manipulated by growers by using various cultural methods such as manipulation of light or temperature, inducing water or nutrient stress, restricting root growth, and mechanical means such as pinching or brushing, application of plant growth regulators (PGRs) continues to be commonly used to control height (Whipker and Evans, 2012). Flurprimidol is a PGR that is used to control plant height and can be applied as a foliar spray, substrate drench, or preplant bulb soak (Krug et al., 2005). It functions by inhibiting the catalyst that converts ent-kaurene to ent-kaurenoic acid and thereby interrupts the synthesis of gibberellin, a plant hormone important for cell elongation and promotion of shoot elongation (Rademacher, 2000; Sponsel, 2010).
Recommended application concentrations have been shown to vary within a bulbous species. For example, flurprimidol preplant bulb soak trials of cultivars of hyacinths and tulips exhibited varying degrees of height control (Krug, 2004). Miller (1992) reported that lilies exhibited varied response to PGRs, and flurprimidol has not been evaluated for use on tiger lilies. In addition, PGRs usage on tiger lilies can potentially differ from application on annuals because of their perennial nature. With perennial species, the desired goal is that with the initial PGR treatment growers will be able to reduce plant growth to decrease labor and make transportation easier. Then, once transported to a garden center, purchased by a gardener, and planted in the landscape, the plant will grow to the normal height in subsequent growing seasons. However, limited work has been done on investigating second-season effects of flurprimidol on herbaceous perennials. Sellmer et al. (2001) reported that after 24 weeks of growth in the landscape pampas grass (Cortaderia selloana) treated with ancymidol drenches (0.25 to 4.0 mg/pot) and paclobutrazol drenches (1.0 to 16.0 mg/pot) exhibited no distinguishable differences from untreated controls; however, plants treated with uniconazole drenches at >1 mg/pot were shorter, and at ≤0.5 mg/pot no noticeable differences in height were observed. This evidence suggests that growth after PGR application depends on concentration and type of PGR used (Nørremark and Andersen, 1990) and that trials should be conducted to determine if plants will exhibit a residual response after installation in the landscape. The objectives of these experiments were to determine 1) if flurprimidol preplant bulb soaks could be used to control stem elongation of tiger lilies and make the plants more suitable for retail sales; 2) if a PGR preplant bulb soak treatment affects the second year’s growth of ‘Orange Tiger’; and 3) if the recommended concentration for ‘Orange Tiger’ was applicable to other tiger lily cultivars.
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