Several plant growth regulators are labeled for use on ornamental crops; however, their effectiveness is often species- or cultivar-dependent (Barrett, 2001; Chamberlayne and Banko, 2003; Keever and Olive, 1994; Kim et al., 1999; Latimer et al., 2001). In addition to the variety of plant growth regulators available, alternative application methods are being developed and added to product labels. Subirrigation occurs when water is applied to plants by ebb-and-flood irrigation or by adding the solution to a saucer under the plant. Plant growth regulators can be added to a tank mix of water and fertilizer and then delivered to the plants using subirrigation. Effectiveness of the plant growth regulator in reducing plant growth might be increased with subirrigation compared with foliar applications, since most plant growth regulators are absorbed by plant roots more readily than by leaves. For example, Wang et al. (1986) showed that root-applied paclobutrazol was translocated throughout apple (Malus sp.) seedlings grown in a continuously aerated nutrient solution, but foliar-applied paclobutrazol did not translocate to the stems or roots. Translocation of paclobutrazol from root uptake has been assumed to occur primarily through the xylem and appears to be less mobile in the phloem (Dalziol and Lawrence, 1984; Sterrett, 1985).
Chemical application concentrations are important in determining monetary costs of a given application. Less active ingredient is required with subirrigation to achieve similar growth suppression compared with applying plant growth regulators to the foliage (Cox, 2003). Million et al. (2002) noted that plant size was reduced more when paclobutrazol was applied continuously through an ebb-and-flood system than when applied as a single application. For begonia (Begonia ×semperflorens-cultorum hybrids ‘Cocktail Gin’), impatiens (Impatiens wallerana ‘Super Elfin White’), chrysanthemum (Dendranthema ×grandiflorum ‘Tara’), and petunia (Petunia ×hybrida ‘Plum Crazy’), 2.1, 4.0, 5.4, and 3.0 times higher application concentrations, respectively, were required with a single application than the cumulative amount applied by continuous application (Million et al., 2002).
The volume of solution absorbed by the plant is important in determining the appropriate application concentration of plant growth regulators. Million et al. (2002) noted that nonuniform uptake of growth regulator solution volume by the substrate can result in the need for greater application rates with a single growth regulator application to achieve the desired growth reduction compared with when growth regulators are applied in continuous applications. A small amount of leaching through the pot may occur when drenching, resulting in loss of growth regulator active ingredient. Uniconazole and paclobutrazol have limited redistribution in plants once deposited from the xylem (Early and Martin, 1988). In the same study, paclobutrazol remained in lower regions of peach (Prunus persica) stems after being absorbed by roots. Flurprimidol chemical structure and mode of action are similar to paclobutrazol and uniconazole, and it is effective as a substrate drench (Rademacher, 2000). However, we found no information on flurprimidol applied through subirrigation.
Bee balm is an herbaceous perennial plant that produces scarlet-red 2- to 3-inch flowers in single or double whorls (Still, 1994). Bee balm is desired by consumers because it is colorful and attracts bees (Apis sp.) and hummingbirds (Trochilidae) to the landscape. Nursery producers have noted that bee balm plants tend to lack uniformity because of variable stem lengths with some being very tall and others very short (L. Marquez and L. Steffy, personal communication). Tall stems are easily broken during shipping and as plants are moved in the market, resulting in less salable plants. Application of plant growth regulators might improve plant uniformity and result in less stem breakage, resulting in a more desirable plant for consumers. Takahiro et al. (2001) treated bee balm with ethephon three times at 1000 mg·L−1. Plant height was reduced by 40% in ethephon-treated plants. No other plant growth regulator research on bee balm was found.
The objectives of this research were to determine 1) the effects of substrate drench application vs. application through subirrigation of paclobutrazol, uniconazole, or flurprimidol on plant growth, and 2) the effect of amount of active ingredient applied on plant growth for each application method and growth regulator.
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