Excessively large bedding plants are less desirable than compact plants as they are difficult to handle and prone to breakage (van Iersel and Nemali, 2004). Damaged plants are often unmarketable, so growers implement management practices that limit plant height and promote overall compactness. Growers typically manage excessive stem elongation with chemical growth regulators such as PGRs (Whipker, 2017). A concern associated with PGRs is that they are not labeled for all crops. PGRs also have the potential to cause phytotoxicity in sensitive species, leading to unmarketable plants. Although PGRs may still adequately control height in these sensitive plants, damage may occur to the flowers or foliage. This is the case for vinca (Catharanthus roseus), which develops symptoms of black spotting in response to the PGR paclobutrazol (Barrett and Nell, 1987). New guinea impatiens will develop chlorosis due to applications of chlormequat chloride and may exhibit flower bud abortion and excessive stunting from applications of paclobutrazol, flurprimidol, or uniconazole (Currey et al., 2016; Justice and Faust, 2015). Phytotoxicity often varies by cultivar and the quantity of active ingredient that is applied to the plant.
Nutrient restriction is an alternative way to control growth in greenhouse crops (Gibson et al., 2007; Henry et al., 2017; Justice and Faust, 2015). Phosphorus restriction has a direct effect on limiting internode elongation, resulting in more compact plants (Nelson et al., 2012). Past research determined that growers can use lower P concentrations during bedding plant seedling production to grow compact plants with a greater root to shoot ratio than those grown with nonlimiting P concentrations (Huang and Nelson, 1994). Phosphorus restriction may be successfully implemented for seedling production, which takes only a few weeks, whereas growing to flowering may take several months (McMahon, 2011). Over time, P concentrations in the substrate may be depleted, leading to reallocation of tissue P and deficiency symptom development. This is especially true for bedding plants produced in soilless substrates which have limited P-holding capacity (Marconi and Nelson, 1984). Present research suggests using 5–15 ppm P to maximize growth of bedding plants grown in soilless substrates (Henry et al., 2017), although few studies have been published regarding low P as an alternative to conventional PGRs.
Studies by Hansen and Nielsen (2000, 2001) investigated low P fertility as an alternative to PGRs using 0.3–31 ppm P with several floriculture species, including argyranthemum (Argyranthemum frutescens), aster (Aster novi-belgii), pentas (Pentas lanceolate), and hybrid rose (Rosa ×hybrid). For most of these species, significant height control was only observed when limiting P fertilization to ≤1.5 ppm (Hansen and Nielsen, 2000, 2001). A study by Justice and Faust (2015) indicated that limiting P fertilization in a liquid fertilization program could provide height control for hybrid impatiens (Impatiens ×hybrida). This study demonstrated that moderate height control was achieved when limiting P fertilization from 12 to 6 ppm P, but greater height control was observed between 6 and 3 ppm P (Justice and Faust, 2015). Other findings suggest that 5 ppm P or less can provide significant growth control for bedding plant production (Henry et al., 2017). However, no study has compared the effect of P fertility directly with that of a PGR. The objective of this study was to determine if restricting P fertilization could result in comparable growth control for bedding plants grown with higher P concentrations in combination with a standard PGR application.
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