Figure 1. Plug heights (cm) of impatiens, marigold, and salvia seedlings grown in plug cells coated on interior surface with paclobutrazol/paint mixes at five different growth regulator concentrations. Horizontal bars represent SE.
The effectiveness of applying a plant-growth regulator as a paint/paclobutrazol mixture coating the interior surface of plug cells was evaluated. Three bedding plant species (Marigold, Tagetes patula cv. 'Bonanza Spray;' Impatiens, Impatiens wallerana cv. 'Blitz White;' and Salvia, Salvia splendens cv. 'Little Tango') were grown in 288 plug trays (24 rows, 12 cells per row) in sections three rows wide with the interior surface of each cell covered by flat latex paint with paclobutrazol concentrations of 0, 12.5, 25, 50, 100, and 200 mg l-1 (0, 0.00125, 0.0025, 0.005, 0.01, and 0.02 mg a.i. per cell respectively).
Significant reductions in impatiens and marigold plug height were observed at paclobutrazol concentrations as low as 12.5 mg l-1 while 50 mg l-1 paclobutrazol was required before a reduction in salvia plug height was observed. Plugs subjected to growth regulator treatments were greener than the control plants. No signs of phytotoxicity were attributed to either paclo-butrazol or the paint. These results suggest that the application of paclobutrazol and paint to the interior of plugs may represent the development of a new practical method of systemic growth regulator application. Chemical name used: þ-((4-chlorophenyl) methyl)-a-(1,1-dimethyl)-1H-1,2,4,-triazole- 1-ethanol (paclobutrazol).
The use of plant growth regulators in plug production is important for crops that stretch soon after plant emergence. This tendency is compounded by plant crowding in a plug tray. Overlapping of plant parts reduces light intensity and changes light quality, resulting in light below plant canopies that is richer in near-infrared and far-red which encourages plant stretching. As a consequence, the application of growth regulators to control plug height is a common practice among plug producers. While drenching is the most effective method of application for some growth regulators (e.g., paclobutrazol), it is impractical for plugs. As a result, spraying is the most common application method for plug production.
The concept of using copper-coated interior containers to control root growth was introduced in the 1970s. Roots containing copper-treated surfaces absorb high amounts of copper (Arnold and Struve, 1989). Pasian and Struve (1996) showed that plant growth regulators, like copper, could be delivered using coated interior containers. They demonstrated that chrysanthemum, Dendranthema grandiflora, plant height could be controlled using containers with the internal surface coated with a paint/paclobutrazol mixture. The objective of this study was to determine the response of three bedding plant species to paclobutrazol/paint mix applied on the interior plug cell surface.
Paclobutrazol was stirred into a white interior flat latex paint (water solution of butyl acrylate/vinyl acetate copolymer, titanium dioxide, aluminum silicate, and amorphous silicate). After mixing the paint and paclobutrazol for 15 minutes, the mix (0.1 ml) was brushed uniformly onto the internal surface of plastic plug tray cells. After painting, the cells were dried for 24 hours before use.
Plug trays (288 cells: 24 rows, 12 cells per row) were cut in sections three rows wide. Of the three rows of a section, two had the cells painted with the paint/paclobutrazol mix, and then were filled with growing medium and seeded. Plug tray sections were divided into seven treatments - control (no paint and no paclobutrazol), paint only (paint with no paclobutrazol), and five paint/paclobutrazol mixes applied at rates 12.5, 25, 50, 100, and 200 mg l-1 (1.25, 2.5, 5, 10, and 20 µg a.i. per cell, respectively). Seeds of marigold (Tagetes patula, cv. 'Bonanza Spray'), impatiens (Impatiens wallerana, cv. 'Blitz White'), and salvia (Salvia splendens, cv. 'Little Tango') were sown by hand in the plug tray sections. The cells were filled with a mix of 82% (by volume) sphagnum peat moss, 18% perlite, and a wetting agent (Greenway 18% Seedling Mix, KLX Growers LTD, Ontario, Canada). Seeds were covered lightly with horticultural grade vermiculite.
After sowing, the seeded plug tray sections were watered with tap water and placed inside an illuminated germination chamber at 21 C. Seedling emergence for marigold and salvia was determined after seven days and after 17 days for impatiens. The plug tray sections were then moved to a greenhouse and the seedlings watered as needed for three days. From then on, tap water was alternated with a 20N-8.7P-16.7K fertilizer solution at a rate of 100 mg l-1 N. After 31 days, plug height (the distance between the rim of the plug cell and the top of the plug) was measured for marigolds and salvias, and after 38 days, for impatiens.
Rate response to paclobutrazol was determined by regression analysis using the GLM procedure of SAS (SAS Inst. Cary, NC). Single-degree-of-freedom contrasts were used to compare the control and the lowest effective concentration of paclobutrazol in paint.
Paint and/or plant growth regulator did not negatively affect seedling emergence of impatiens and salvia (Table 1). More work is required to validate the reduction in marigold seedling emergence. No signs of phytotoxicity were attributed to paint. Plugs subjected to paclobutrazol were significantly shorter and greener than control plants. There was no significant difference in plant height between marigold and salvia plugs for the control and paint-only treatments. These results are consistent with those found by Pasian and Struve (1996) with chrysanthemum plants grown in 485 ml plastic containers. However, impatiens plants in the paint-only treatment were significantly shorter than the control plants (Fig. 1).
Increasing paclobutrazol concentrations produced shorter plugs (Table 2). Concentrations of 200 mg l-1 reduced plug heights 51% for impatiens, 42% for marigold, and 43% for salvia. Pasian and Struve (1996) found that increasing paclobutrazol concentrations in paint from 100 mg l-1 to 150 and 200 mg l-1 did not produce shorter chrysanthemum plants. In the present study, however, higher concentrations of paclobutrazol in paint yielded shorter plugs. One explanation for this finding may be the greater root length:container surface ratio of a plant grown in a plug tray, which allows greater root contact with the walls than a plant in a 485 ml container; hence, the plant absorbs proportionally more growth regulator. More research is required to study the effect of the paint/growth regulator mix on root length density and root distribution inside plugs.
Growth regulator concentrations as low as 12.5 mg l-1 reduced impatiens and marigold plug height (17% and 11% of controls respectively). In salvia, the lowest concentration of paclobutrazol in paint that produced a substantial reduction in plug height was 50 mg l-1 . It is unknown why salvia plugs in the 100 mg l-1 treatment were taller than plugs in the 50 mg l-1 treatment (Fig. 1).
The paint application method is potentially a better method than drench or spray growth regulator treatments. The growth regulator is fixed in paint, and workers are less exposed to it because they do not contact the chemicals coating the plug cells' internal surface. This application technique may be advantageous in light of the increasing stringency on restricted entry intervals of the Worker Protection Standards. As opposed to drenches and sprays, the paint application method may release less growth regulator chemical into the environment. Additional studies are being conducted to determine any environmental impact of this growth regulator application technique. The results obtained with impatiens, marigold, and salvia must also be validated with other plug crops. However, until this method of application is described on the label of commercial growth regulators, it cannot be used by bedding plant growers.
Appreciation is expressed to Uniroyal Chemical for its financial support.
Arnold, A.A. and D.K. Struve. 1989. Cupric carbonate controls green ash root morphology and root growth. HortScience 24(2):262-264.
Pasian, C.C. and D.K. Struve. 1996. A New method of application of paclobutrazol to potted plants. HortScience, 31(4) 655.
| Table 1. Percentage Seedling Emergence for Impatiens, Marigold, and Salvia at Five Different Concentrations of Paclobutrazol. | |||||||
|---|---|---|---|---|---|---|---|
| % Germination | |||||||
| Crop | Control | Paint only | 12.5 mg l-1 | 25 mg l-1 | 50 mg l-1 | 100 mg l-1 | 200 mg l-1 |
| Impatiens | 65 | 83 | 86 | 83 | 83 | 79 | 65 |
| Marigold | 100 | 80 | 96 | 96 | 96 | 86 | 92 |
| Salvia | 75 | 70 | 75 | 83 | 62 | 75 | 75 |
| Table 2. Level of Significance of Two Contrasts (Control vs. 25 mg l-1 and 50 mg l-1) and Two Models (linear and quadratic) for Impatiens, Marigold, and Salvia at Five Different Concentrations of Paclobutrazol. | |||
|---|---|---|---|
| Significance | |||
| Contrast | Impatiens | Marigold | Salvia |
| Control vs. 25 mg l-1 | ** | ** | - - |
| Control vs. 50 mg l-1 | - - | - - | ** |
| Linear model | ** | NS | * |
| Quadratic model | ** | ** | NS |
| * = Significant, ** = Highly significant, NS = Nonsignificant | |||
Figure 1. Plug heights (cm) of impatiens, marigold, and salvia seedlings grown in
plug cells coated on interior surface with paclobutrazol/paint mixes at five
different growth regulator concentrations. Horizontal bars represent SE.