Paclobutrazol And Uniconazole Applications Affect Production Quality And Subsequent Landscape Performance Of Blue Plumbago1

Michael A. Arnold2 and Garry V McDonald3

Abstract - Plumbago auriculata Lam., blue plumbago, was grown in 2.3 L black plastic nursery containers under typical outdoor container nursery conditions during the 1999 and 2000 growing seasons. Paclobutrazol (formulated as Bonzi®) and uniconazole (formulated as Sumagic®) were applied as substrate drenches or foliar sprays. After evaluation of effects of these growth regulators on plant responses during production, the plants were transplanted to landscape beds to evaluate residual or carry-over effects. Paclobutrazol drenches of 40 to 80 mg·L-1 produced a more compact growth form on blue plumbago during production. Unfortunately, these concentrations of paclobutrazol severely retarded plant growth subsequent to transplanting to the landscape. Uniconazole sprays of 60 to 120 mg·L-1 were also effective in producing a more compact plant in the nursery. Landscape trials indicated that uniconazole had some negative residual impacts on growth indices and flower counts on blue plumbago, whereas low level concentrations of paclobutrazol as a substrate drench in the nursery resulted in neutral effects on growth indices and, in some cases, increases in the number of flowers in the landscape. At 5 to 10 mg·L-1, paclobutrazol increased vegetative growth and flowering in the 1999 landscape trials. The 5 mg·L-1 rate also induced a more compact canopy following landscape planting in 2000 without adversely affecting flowering, but the 10 mg·L-1 treatment reduced flowering in the 2000 landscape trials. Residual effects of some treatments were exhibited in a delayed fashion in the landscape after those treatments appeared to have had no effect during production in the nursery, serving as a strong argument for testing residual effects of any production treatments before recommending wide scale implementation.

INTRODUCTION

Paclobutrazol and uniconazole are triazole plant growth regulators that inhibit gibberellin and sterol biosynthesis (Buchenauer1977; Fletcher et al., 2000; Hedden and Graebe, 1985). Paclobutrazol formulations, such as Bonzi®, effectively reduce internode extension on various plant taxa (Arnold, 1998; Arnold and Davis, 1994; Hunter and Proctor, 1992; Sharma and Webster, 1992; Vu and Yelenosky, 1992). This facilitates the production of more compact, denser canopied plants. The popularity of tropical woody and herbaceous perennials for use as summer annuals in hot southern climates is increasing (Arnold, 1999; Riffle, 1998; Sperry, 1991). Selection and evaluation work is ongoing for inclusion of Plumbago auriculata Lam. in the Texas Coordinated Educational and Marketing Assistance Program and SERA-IEG-27 trials (Arnold et al., 1998; 2000; Dunwell et al., 2001). While blue plumbago is an outstanding landscape plant, it tends to develop a rather uneven growth form during conventional nursery production. This often occurs when a few branches elongate more rapidly than the rest resulting in an irregular canopy. Reduced internode extension and/or promotion of lateral branch growth via inhibition of rapidly elongating terminal buds may result in a plant with greater market acceptance. Ruter (1996) and Arnold (1998) have had success with growth regulator applications to improve the appearance of another tropical woody plant, Lantana. Reports of long term residual effects on growth of other species have been documented (Arnold and Davis, 1994; Sharma and Webster, 1992), thus studies with P. auriculata may be useful to identify concentrations of growth regulators that would temporarily inhibit growth in the nursery, but avoid negative long-term effects once plants are transplanted to landscape sites.

The objectives of this study were: 1) to determine dose response curves for P. auriculata to substrate drenches and foliar sprays of paclobutrazol or uniconazole during container production, and 2) to determine if paclobutrazol or uniconazole treated plants exhibit any residual effects of the drenches or sprays applied during production after transplantation to a landscape setting.

MATERIALS AND METHODS

General Conditions.

A promising cultivar of P. auriculata tentatively named ‘Hullabaloo’ was propagated from tip cuttings, 6 to 8 cm in length, obtained from six month old stock plants overwintered in a heated greenhouse. Once rooted, cuttings were acclimated in the greenhouse for 10 days prior to transplanting into a commercial 6 pine bark: 2 peat moss: 1 vermiculite: 1 hadite clay substrate (Horticultural Products, Hope, Ark.) in 2.3 L black plastic nursery containers (Lerio Corp., El Campo, Texas). After being potted, plants were acclimated outdoors for approximately three days under 50 % light exclusion. Plants were then moved to a gravel-surface container production area in full sun. Individual containers were top dressed with 23N-1.7P-6.6K (23-4-8 High N Southern Formula, 8-9 month formulation, Scotts Co., Marysville, Ohio) at the rate of 6.8 kg *m-3 Fertigation was supplied as a constant feed of N at 50 mg·L-1 from a 24N3.5P-13K water soluble fertilizer (The Scotts Co.) via Roberts Spot Spitters (#9 spot spitters, Roberts Irrigation, San Marcos, Calif.), one per container. Sulfuric acid (93.2% H2S04, Harcros Chemicals Inc., Kansas City, Mo.) was injected into the irrigation water to achieve a target pH of 6.5.

For each nursery production study, initial canopy diameter (measured in two perpendicular directions) and height prior to treatment applications were recorded. Similar measures plus the number of blooming flower clusters were recorded when more than half of the control plants were deemed marketable. Plant indices were calculated as height x the first canopy diameter x the second canopy diameter, resulting in an approximation of canopy volume.

Each chemical tested on a given date was treated as an independent concurrent study. The dosage effects of each of the four applications were determined independently. The general linear models procedures in the statistical analysis software from SAS Institute Inc. (1988) were used to perform an analysis of variance on the data. When significant dose responses were detected, first, second and third order set-wise polynomial regression equations were determined for the treatment means. Best fit regression equations, as measured by R2 values, are presented in the appropriate figures.

When testing for residual effects of the production treatments following transplantation to the landscape, a completely randomized planting design was utilized. Canopy height and spread in perpendicular directions and the number of flower clusters were measured at transplanting and subsequently at monthly intervals for the remainder of the growing season. Interactions between time after transplanting and dosage were tested for each chemical and species. If interactions were significant (P≤0.05), polynomial regression equations were determined for each rate of chemical over time. Otherwise, data from the various observation dates were pooled and regression equations were calculated as previously described for the nursery studies.

Preliminary Dosage Trials: Nursery.

 The first set of nursery studies tested paclobutrazol [(2RS,3RS)-1-(4chlorophenyl)-4,4-dimethyl-2-1,2,4-triazol-lyl-pentan-3-ol, formulated as Bonzi®, Uniroyal Chemical Co., Middlebury, Conn.] substrate drenches at 0, 5, 10, 20, and 40 mg·L-1 a.i.; uniconazole [(E)-(+)-(S)-1-(4chlorophenyl) - 4,4 - dimethyl -2- (1,2,4 - triazole -1- yl) - pent - 1- ene - 3 - ol, formulated as Sumagic®, Valent USA Corp., Walnut Creek, Calif.] drenches at 0, 1, 2, 4, and 6 mg·L-1 a.i.; paclobutrazol foliar sprays at 0, 50, 100, 200, and 400 mg·L-1 a.i.; and uniconazole sprays at 0, 10, 20, 40, and 60 mg·L-1 a.i. on P. auriculata. These rates were chosen based primarily on manufacturer recommendations. The chemicals were diluted to the desired concentrations with distilled water, then applied in 30 ml aliquots per container for both spray and drench applications. With spray applications, container surfaces beneath the canopy were covered with paper to avoid drip or errant spray contamination of the substrate surface. Plants were initially potted on May 18, 1999. As new growth emerged on June 7, 1999, five single plant replicates were treated per growth regulator and concentration combination and arranged in a completely randomized design in the nursery. Canopy height, canopy spread in two perpendicular directions, and the number of flower clusters were recorded on June 25, 1999, as a majority of plants reached a marketable state.

Preliminary Dosage Trials: Landscape.

 All plants from the nursery portion of the preliminary dosage trials were transplanted to landscape beds with 0.6 m within row and 0.9 m between row spacings on June 26, 1999. Completely randomized designs were used with five replications per concentration of each chemical. The 3.7 m wide by 12 m long beds were constructed from three to four layers of 10 cm diameter CCA treated landscape timbers. Beds were filled with a fine sandy loam soil. Irrigation was applied as needed from stationary risers with 4 m throws placed on 4 m centers along the sides of the beds to achieve 100 % overlapping coverage. After planting, a milled pine bark mulch was applied to the entire surface in a layer 8 to 10 cm thick. Two weeks after planting and again three more times at 6 week intervals, an 18-6-12 formulation granular fertilizer was broadcast on the plots at the rate of 0.5 kg of N per 100 M2. Height, canopy diameters, and flower number were recorded on June 25, August 10, September 20, and October 27, 1999.

Expanded Dosage Trials: Nursery.

In a second set of production studies, application rates were increased with paclobutrazol substrate drenches at 0, 40, 80, 120, 160 mg·L-1 a.i., uniconazole drenches at 0, 6, 12, 18, and 24 mg·L-1 ad., paclobutrazol foliar sprays at 0, 400, 800, 1200, and 1600 mg·L-1 ad., and uniconazole sprays at 0, 60, 120, 180, and 240 mg·L-1 ad. Cuttings were placed under mist on June 28, 1999, and the subsequently rooted liners were potted to 2.3 L containers on August 16, 1999. Five single plant replicates were treated per growth regulator and concentration combination and arranged in a completely randomized design in the nursery on September 21, 1999. Final nursery measurements as described for experiment 1 were collected on October 18, 1999.

Final Dosage Trials: Nursery.

The final set of nursery studies repeated the most promising test rates for paclobutrazol drench applications. Test rates included paclobutrazol at 0, 5, 10, 20, 40, and 80 mg·L-1 a.i. and utilized a 5 milled pine bark: 1 builders sand (by vol.) substrate. The change of substrate from that used in the preliminary and expanded dosage experiments was necessary because the supplier discontinued the original substrate mix. The substrate was amended with 16N-3.1P-l0.OK controlled release fertilizer (Southern Special, Scotts Corp., Marysville, Ohio) at the rate of 1.2 kg-m3  N, 0.89 kg-m3  of micronutrient mix (Micromax, Scotts Corp.), 3.6 kgm3 of dolomitic lime (Vulcan Materials Co., Tarrant, Ala.), and 1.8 kg-m-3 of gypsum (Standard Gypsum Corp., Fredericksburg, Texas). On May 24, 2000, fifteen single plant replicates were potted in individual containers and arranged in a completely randomized design. Paclobutrazol drenches were applied on June 13, 2000. Final production measures as described in experiment 1 were collected on June 30, 2000.

Final Dosage Trials: Landscape.

All plants grown in the nursery portion of the expanded dosage trials were transplanted to landscape beds on July 3, 2000. Cultural and planting conditions were as previously described. Completely randomized designs were used with fifteen replications per concentration of each chemical. Growth and flowering of individual plants were measured on July 3, August 7, September 12, and October 3, 2000, as previously described.

RESULTS AND DISCUSSION

Preliminary Dosage Trials: Nursery.

 No statistically significant (P<_0.05) responses in canopy growth or flowering were found for either chemical or application method with blue plumbago during the initial phase of production (data not presented). However, there appeared to be some sporadic reductions in internode extension in blue plumbago with the highest concentration drenches, paclobutrazol at 40 mg·L-1 and uniconazole at 6 mg·L-1, so plants from this phase of the experiments were planted in landscape beds to see if the growth regulator responses were simply slow in developing.

Preliminary Dosage Trials: Landscape.

Surprisingly, concentrations of growth regulators that had little or no effect on blue plumbago during the first nursery production experiment had significant impacts on growth and flowering of this species in the landscape (Fig. 1- 4). Significant (P≤0.05) interactions between time after transplanting to the landscape and growth regulator concentrations occurred for paclobutrazol drenches and uniconazole sprays for growth index and flower clusters. No significant interactions between time and growth regulator concentration were found for height with paclobutrazol drenches or uniconazole sprays, nor for height, plant index, or flower clusters for paclobutrazol sprays and uniconazole drenches. The only significant main effect of growth regulator concentration across time for height was for uniconazole sprays. Main effects of growth regulator concentrations over time were significant (P<_0.05) for all four growth regulator and application methods for plant index and flower clusters per plant. The main effects of all measured characteristics were significant (P≤0.05) for time in the field, but that data is not presented as plants would all be expected to grow larger over time in the landscape.

The most intriguing effects were seen with significant (P≤0.05) interactions of paclobutrazol drench concentrations over time (Fig. 1). Paclobutrazol drenches appeared to induce enhanced vegetative growth as evidenced by plant index (Fig. IA) and late season flowering (Fig. I B), particularly at low concentrations. In rare instances, enhanced growth or flowering has been reported for plants during field establishment after nursery applications of growth regulators. Gok and McDaniel (1987) reported accelerated flowering with uniconazole applications to bedding plants. Bruner et al. (2000) reported B -Nine applications advancing the time to first flowering of Canna x geueralis L.H. Bailey ‘Florence Vaughan’, but in the previous year similar applications induced delayed flowering. Paclobutrazol applied as a foliar spray had little effect at concentrations below 200 mg·L-1, but at 400 mg·L-1 reduced the growth index and flowering across sample dates (Fig. 2).Uniconazole drenches elicited minimal reductions in blue plumbago growth and flowering at higher concentrations when averaged across sample dates (Fig. 3A, B). These reductions in plant index were in part due to a reduction in height with uniconazole sprays above 10 mg·L-1 (Fig. 3C), particularly late in the growing season (Fig. 4A). Uniconazole sprays of 20 mg·L-1 induced some late season reduction in flowering, whereas 40 to 60 mg·L-1 nearly eliminated flowering of blue plumbago in the landscape following nursery applications (Fig. 4B). These results illustrate the importance of testing landscape responses of all plants treated in nursery/greenhouse settings with growth regulators to determine possible residual effects on landscape performance.

Expanded Dosage Trials: Nursery.

A second set of production studies was initiated using the highest concentrations tested in the preliminary dosage trials as the lowest non-zero level tested. The increased concentrations utilized during the expanded dosage trials in the nursery induced significant (P≤0.05) reductions in height (Fig. 5A), plant indices (Fig. 513, D), and flowering of blue plumbago (Fig. 5C). Height of blue plumbago was slightly reduced with increasing concentrations of paclobutrazol drenches (Fig. 5A), whereas canopy volumes were more dramatically reduced (Fig. 5B). Drenches of 40 mg·L-1 paclobutrazol resulted in a substantially more compact plant (Fig. 5B), with only a moderate reduction in flower production (Fig. 5C). Concentrations of paclobutrazol above 80 mg·L-1 tended to limit growth excessively (Fig. 513) and cause nearly a 50% reduction in flowering (Fig. 5C). Foliar applications of uniconazole had similar or greater reductions in plant indices, but not at the same concentrations (Fig. 5D). Uniconazole drenches and paclobutrazol sprays had no significant (P≤0.05) effects on height, plant index, or flower number. Uniconazole had no significant effects on plant height or flower number. Data for nonsignificant effects are not presented. Completion of the expanded dosage trails in the nursery on October 18 did not permit adequate time to test the residual responses of plants in this portion of the nursery trials in the landscape prior to the onset of frost.

Final Dosage Trials: Nursery.

Interestingly, plants appeared to be more responsive to paclobutrazol drenches during the third series of production trials which were completed in the second year of the study (Fig. 6). Causes for the differential results are uncertain, but may have been related to tissue receptivity, temperature, or perhaps differences in substrate composition. Temperatures were much warmer during the 2000 test period than in 1999 and a different substrate was employed, although both were primarily pine bark based substrates. Million et al. (1998a, 1998b) found differential responses within two herbaceous species with the same growth regulators but using different substrates.

Although the main effect of paclobutrazol on height was not significant at P≤0.05, the main effects of paclobutrazol on plant indices and flower clusters were significant. Reductions in plant indices were apparent with as little as 5 mg·L-1 of paclobutrazol drench (Fig. 6A) and substantial reductions were present with 10 mg·L-1 or greater concentrations. While most plants in all of the treatments were considered marketable, the plants treated with 20 to 80 mg·L-1 did appear more uniform than the controls or plants treated with lower rates. A reduction in flowering was seen at intermediate drench concentrations (Fig. 6B), but the total differences in flower clusters were only between two and three clusters from the best to worst treatments and may not represent a response of commercial significance during production. Because flowering tended to be somewhat cyclic in nature for this species, these minor differences might be attributable to variations in flowering cycles. The inconsistent responses to lower rates might pose a problem in production settings.

Final Dosage Trials: Landscape.

No time by concentration interactions were significant for any measures in the 2000 field trials, but there were significant (P≤0.05) main effects of paclobutrazol drenches on height, plant index, and flower clusters when averaged over time. The increases in flowering and growth responses to low concentrations of paclobutrazol drenches observed for the 1999 landscape trials were not repeated in the 2000 landscape trials (Fig. 7). In the 2000 landscape trials, height was slightly reduced and the plant indices were substantially reduced by paclobutrazol concentrations above 10 mg·L-1 (Fig. 7A, 7B). Flowering was reduced by concentrations above 5 mg·L-1, but the effects were most pronounced at greater concentrations (Fig. 7C). The reductions in plant indices at a low concentration, 5 mg·L-1, were beneficial to the appearance of the plants as the canopy appeared more compact without an adverse effect on flowering. In contrast to the positive growth and flowering responses in the 1999 plantings (Fig. 1), plants in the 2000 landscape trials were stunted by paclobutrazol drench concentrations of 10 mg·L-1 and greater (Fig. 7A, B).

General Conclusions.

Paclobutrazol drenches of 40 to 80 mg·L-1 appear to produce a more compact growth form on blue plumbago during production. Unfortunately, this level of paclobutrazol severely stunted plants subsequently transplanted to the landscape. Uniconazole sprays of 60 to 120 mg·L-1 were also effective in producing a more compact plant in the nursery. Landscape trials indicated that uniconazole has some negative residual impacts on growth and flowering of blue plumbago, whereas low level applications of paclobutrazol as a substrate drench in the nursery resulted in either neutral (final dosage trials) or positive effects (preliminary dosage trials) on growth and flowering in the landscape. The paclobutrazol flowering enhancements were somewhat unusual, and did not occur in the second year’s trials, thus it would be imprudent to recommend paclobutrazol for this effect. However, 5 to 10 mg·L-1 paclobutrazol drench did enhance landscape growth in the 1999 trials and tended to induce a more compact canopy following landscape planting in 2000. If this response remains consistent, it might be desirable from a landscape maintenance perspective.

In general, paclobutrazol and uniconazole applications were too inconsistent in their effects to make recommends on their use during production of P. auriculata. The substantial residual effects of these growth regulator applications on the growth and flowering of P. auriculata in the landscape as a result of production applications underscores the often neglected need to test the long-term effects of nursery or greenhouse growth regulator applications on field performance.

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1Received for publication 9 April, 2001and in revised form 14 November, 2001.           Use of trade names in this publication does not imply endorsement by the authors, the Texas Agricultural Experiment Station, or Texas A&M University of the products named, nor criticism of similar ones not mentioned. This study was funded in part by a grant from the Uniroyal Chemical Company, Middlebury, Conn. and the Texas Ornamental Enhancement Endowment.

2 Associate Professor of Landscape Horticulture, Dept. of Horticultural Sciences, Texas A & M University, College Station, TX 77843-2133.

3Graduate Research Associate, Dept. of Horticultural Sciences, Texas A&M University, College Station, TX 77843-2133