Hydrangeas (Hydrangea sp.) are popular ornamental plants due to their enormous, round flower heads blooming in white, pink, red, purple, or blue in spring and summer. It was listed as the third most used plant in a 2011 survey of 4000 landscape professionals (GIE Media, 2011). There are at least 70 hydrangea species native to southern and eastern Asia and the Americas (Wikipedia, 2014). In the United States, only five hydrangea species are widely cultivated (U.S. National Arboretum, 2005), and an estimated $73 million hydrangea plants were sold annually (U.S. Department of Agriculture, 2009). Bigleaf hydrangea is one of the most widely grown hydrangea species in the United States with over 1000 cultivars (Dirr, 2012). Some cultivars are best pruned or pinched on an annual basis to prevent them from being “leggy” and to promote lateral branching.
Pruning or pinching is a common technique in hydrangea production to reduce plant size, stimulate lateral branching, improve overall appearance, and enhance flower development. But hand pruning or pinching is labor intensive and costly. If done improperly, it can also reduce flowering in hydrangea plants (U.S. National Arboretum, 2005). Alternatively, a wide range of plant growth regulators (PGRs) with diverse modes of action, sites of absorption and activity are available to control height and promote lateral branching. Their use varies with plant species, chemical concentration, time of application, and environmental conditions. For example, ethephon has been reported to effectively reduce the height of ‘Sonora Jingle’ poinsettia (Euphorbia pulcherrima) by 33% and ‘Sonora White’ poinsettia by 31% when 700 mg·L−1 ethephon was sprayed three times on 29 Aug., 20 Sept., and 13 Oct. 2005, respectively (Sun et al., 2011). Ethephon acts via release of ethylene, which is absorbed by the plant and causes cells to limit elongation and increase in width (Taiz and Zeiger, 2010). Unlike ethephon, most PGRs inhibit the biosynthesis of gibberellins within plants and therefore function as antigibberellins to suppress stem elongation (Bailey and Whipker, 1998). For example, seashore mallow (Kosteletzkya virginica) plants treated with chlormequat chloride at 750, 1000, or 1500 mg·L−1 or paclobutrazol at 60 mg·L−1 were effective for height control and lateral branching (Hilgers et al., 2005).
Symptoms of phytotoxicity are a known issue with most plant growth retardants. The severity of phytotoxicity depends on plant species or cultivar and the chemical concentration. Foliar application of ethephon to ‘Sonora Jingle’ and ‘Sonora White’ poinsettia exhibited phytotoxicity as indicated by reduced biomass accumulation (Sun et al., 2011). Leaf chlorosis appeared on seashore mallow plants immediately after application of paclobutrazol, chlormequat chloride, and chlormequat chloride/daminozide, and disappeared 8 weeks after treatments (Hilgers et al., 2005). This phytotoxicity would reduce plant marketability and should be avoided by choosing an optimized concentration of PGR for producing compact plants.
Dikegulac sodium [18.5% dikegulac sodium (Augeo®; OHP, Mainland, PA)] is a PGR labeled to reduce or break apical dominance in plant shoots and to enhance lateral branching by disrupting cell wall integrity. Dikegulac sodium could be used to produce more compact plants with an increased number of flowers. It can also be used to enhance the overall structure and appearance of ornamentals including bedding plants, herbaceous plants, perennials, woody ornamentals and trees. Dikegulac sodium sprayed between 4000 and 6000 mg·L−1 to mock orange (Murraya paniculata) hedge decreased apical dominance among lateral shoots and enhanced uniform regrowth without causing visible damage (Kawabata and Criley, 1996). Norcini et al. (1993) recommended that a foliar spray of dikegulac sodium at 1600 mg·L−1 could potentially reduce the amount of pruning required during ‘Barbara Karst’ bougainvillea [Bougainvillea ×buttiana (B. glabra × B. peruviana)] production. However, many factors including species, ingredient dosage, cultural and environmental conditions affect the efficacy of dikegulac sodium on plant growth and development. For example, foliar application of dikegulac sodium at 500, 1000, or 1500 mg·L−1 did not control the height and increase the number of branches of seashore mallow (Hilgers et al., 2005). However, Grossman et al. (2013) reported that the application of dikegulac sodium to herbaceous perennial liners [‘Autumn Joy’ butterfly stonecrop (Sedum spectabile), ‘Bright Eyes’ garden phlox (Phlox paniculata), ‘Walker’s Low’ dwarf catmint (Nepeta racemosa)] before transplanting increased branching, with higher rates of dikegulac sodium being more effective in liners. Carter et al. (1996) observed that dikegulac sodium at 250, 500, or 750 mg·L−1 increased shoot number without decreasing frond length of boston fern (Nephrolepsis exaltata). Al-Juboory and Williams (1991) observed that dikegulac sodium not only stimulated branching, but also reduced branch length in golden pothos (Scindapsus aureus). Dikegulac sodium has also been reported to reduce the stem length of ‘Enchantment’ and ‘Joan Evans’ midcentury hybrid lily (Lilium ×hybrida), but prevent the development of flower buds and plant senescence (Hanks and Menhenett, 1980). Norcini et al. (1993) observed that dikegulac sodium stimulated profuse flowering of both ‘Barbara Karst’ and ‘Rainbow Gold’ bougainvillea during middle spring to early summer. Symptoms of phytotoxicity such as yellow and narrow leaves were also present in finished plants of ‘Autumn Joy’ butterfly stonecrop, ‘Bright Eyes’ garden phlox, ‘Walker’s Low’ dwarf catmint, ‘Moonshine’ yarrow (Achillea filipendulina) that were treated with 1600 mg·L−1 dikegulac sodium (Grossman et al., 2013).
Dikegulac sodium has been used to successfully control height of Little Lime™ hardy hydrangea [H. paniculata ‘Jane’ (Cochran and Fulcher, 2013)] and ‘Limelight’ hardy hydrangea (Cochran et al., 2013). Both hydrangea cultivars set flower buds on current season wood. However, ‘Merritt’s Supreme’ bigleaf hydrangea produces blooms on old wood. Therefore, the timing of foliar application and dosage of dikegulac sodium might be different for hardy hydrangea and bigleaf hydrangea. The objectives of this project were to investigate the efficacy of dikegulac sodium on lateral branching applied as a foliar spray and possible phytotoxicity on ‘Merritt’s Supreme’ bigleaf hydrangea in two greenhouse experiments at two locations, El Paso, TX, and Kosciusko, MS.
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