PGR Liner Dips On Bedding Plants
Controlling plant height of bedding plants can be one of the biggest challenges for greenhouse growers, especially when trying to produce containers or hanging baskets that contain more than one plant species. One new strategy is to dip or soak the media of a transplant (seedling plug or rooted cutting) in a solution of plant growth regulator (PGR) before transplanting into the final container. This PGR application method is termed "liner dip" and has proven to be an effective height control method on both bedding plants and herbaceous perennials. However, more research-based information is needed to determine how application rates influence plant response over time, especially in Northern climates such as the Midwest and Northeastern United States.
In the spring of 2005, we performed an experiment to determine the effects of a pre-transplant liner dip application of Piccolo (paclobutrazol, Fine Americas, Inc.) and Sumagic (uniconazole, Valent U.S.A. Corporation) to five species of bedding plants. These chemicals were selected because they are readily absorbed by plant roots and have relatively long-lasting activity. Although Piccolo is not currently labeled for liner dip applications, in many respects a liner dip is similar to drench and bulb soak applications.
The objective of this study was to compare the responses of different application rates of these two chemicals so recommendations can be made for greenhouse growers in Northern climates. This article summarizes the results of our study and provides suggestions for rates and information on how to use liner dips on bedding plants.
Experimental Protocol
On March 3, 2005, Raker's Acres, Litchfield, Mich., delivered argyranthemum, calibrachoa, petunia, scaevola and verbena plants to the Michigan State University (MSU) floriculture research greenhouses, East Lansing, Mich. These plants were chosen because of their commonality and vigorous plant growth characteristics. Plants were received in 50-cell plug trays and subsequently grown at 68¡ F under a 9-hour photoperiod. All plants were given a soft pinch and then grown until they filled the plug cell and were ready for transplant.
Plants were irrigated on the day preceding the PGR dip to make sure the media was moderately dry at the time of application, and chemical uptake was uniform among treatments. On March 14 each plug tray was cut into sections containing 10 plants. Each 10-cell section was then placed into a tray containing one of several PGR concentrations of Piccolo or Sumagic. These application rates were based on previous PGR research studies at MSU and trials with PGR liner dips performed at the University of Florida. Plants were allowed to absorb the PGR solution for 30 seconds. The PGR dips were performed in the greenhouse under high light conditions (3,500 foot-candles). See Figure 1, below, for a summary of the experimental protocol.
Three hours following application, plants were transplanted into 41?2-inch-round pots filled with a commercial soilless medium (Suremix, Michigan Grower Products, Inc.). Plants were grown in a glass-glazed greenhouse with a constant temperature setpoint of 68¡ F under a 16-hour photoperiod.
Results
Argyranthemum. Stem elongation was significantly reduced in plants that received a PGR liner dip of Sumagic at all rates or Piccolo at 8 or 16 ppm (see Figure 2, page 44). For example, at four weeks after application, average plant height was 11?4 inches (24 percent shorter than nontreated plants) in all treatments except Piccolo at 4 ppm. At seven weeks after treatment, the highest application rates continued to have an effect on inhibiting stem elongation.
Plants that were treated with Sumagic at all rates or Piccolo at 8 or 16 ppm flowered an average of eight days later than untreated plants. On the day that each plant flowered, the total number of flower buds was counted, and the average was calculated for each treatment. The total number of flower buds ranged from 34 to 40 and was not statistically different among treatments.
Calibrachoa. The calibrachoa we used, 'Callie Dark Blue', has a prostrate, trailing growth habit, making it a great plant for hanging baskets or mixed containers. To determine the effects of PGR dips on inhibiting stem elongation, the length of the longest stem was measured (referred to as plant height). All rates of Piccolo or Sumagic significantly reduced plant height by 21?4-61?4 inches (20-59 percent shorter than nontreated plants) (see Figure 3, below). As application rates of Piccolo or Sumagic increased, the amount of stem elongation decreased.
There were no significant differences in time to flower among any of the treatments; however, the number of flower buds at first flower decreased when higher rates of Piccolo or Sumagic were used. For example, plants dipped in Piccolo at 8 or 16 ppm had on average 18 fewer flower buds than untreated plants. Petunia. Controlling stem elongation in vigorous petunias can be especially challenging. In our study, Piccolo and Sumagic at all rates were effective at reducing stem elongation on 'Vivid Red'. At four weeks, the plant height of treated plants was 11?4-21?4 inches (22-42 percent) shorter than untreated plants (see Figure 4, page 49). Higher application rates of Piccolo or Sumagic resulted in the strongest response. The average time to flower was similar among treatments and ranged from 21 to 26 days. Plants treated with Piccolo or Sumagic had on average 5-10 fewer flower buds than untreated plants.
Scaevola. All rates of Piccolo or Sumagic were effective in reducing stem elongation in scaevola 'Jacobs White' (see Figure 5, page 49). Plants treated with Piccolo or Sumagic at the highest rates (8 and 16 ppm, respectively) resulted in excessive inhibition of growth. Plants dipped in Piccolo at 16 ppm were 41?4 inches shorter (54 percent) than nontreated plants. At seven weeks after application, the effects of the PGR dip still persisted, and these plants were very compact. Piccolo and Sumagic at all rates delayed flowering by an average of nine days and reduced the number of flower buds by an average of 13, compared to untreated plants. Verbena. Trailing verbenas are aggressive plants that can be challenging to control with PGRs. We observed that lower rates of Piccolo or Sumagic were slightly effective at inhibiting stem elongation on the semi-trailing 'Rapunzel Red' (see Figure 6, below). Sumagic at 8 ppm was highly effective and reduced plant height by 31?4 inches (39 percent) after four weeks. In general, all rates of Piccolo were slightly effective in controlling stem elongation, and plants were 13?4 inches (22 percent) shorter compared to untreated plants. The average time to flower ranged from 20 to 23 days and was similar among treatments. Plants treated with Sumagic at 8 ppm produced 12 fewer flower buds at first flowering compared to untreated plants.
Utilizing The Results
Our results indicate that pre-transplant PGR liner dips can be an effective strategy to control stem elongation in aggressive bedding plants. We suggest Piccolo liner dip rates of 4-6 ppm for petunia and scaevola, 6-8 ppm for argyranthemum and calibrachoa, and 8-12 ppm for verbena. For Sumagic, we suggest liner dip rates of 1-2 ppm for petunia; 2-4 ppm for calibrachoa; and 3-4 ppm for argyranthemum, scaevola and verbena when used in mid-spring in Northern climates. Liner dips at these rates could inhibit extensive growth when transplanted into larger containers. This might be particularly appealing when vigorous plants are grown in the same container as less aggressive plants. The vigorous ones could receive a liner dip before transplant, allowing the less aggressive plants time to develop and get a "head start."
The effectiveness of both chemicals at reducing stem elongation generally increased as application rate increased. This dose-response is similar to other PGR application methods such as spray or drench. When the study was terminated at seven weeks after treatment, Piccolo and Sumagic at the highest rates tested continued to reduce stem elongation in argyranthemum, calibrachoa, scaevola and verbena. The effect of Piccolo and Sumagic on time to flower and the total number of flowers per plant varied among species. Flowering was delayed by an average of 1-2 weeks in argyranthemum and scaevola treated with Piccolo or Sumagic at all rates. In calibrachoa, petunia and verbena time to flower was not reduced by any PGR treatment. The number of flowers in calibrachoa, petunia and scaevola decreased at higher rates of Piccolo or Sumagic.
The efficacy of PGR liner dips in controlling plant height is affected by both environmental conditions and cultural practices (see sidebar above). Two of these factors include the duration of the dip and moisture content of the media. In our study, plants were dipped in the PGR solution for 30 seconds. Allowing plants to absorb a PGR solution for a longer duration will likely increase the magnitude of the response. In addition, the growing medium in this study was moderately dry at the time of treatment, thus increasing absorption of the PGR solution. The volume of PGR solution absorbed will be different under other moisture conditions. Growers should develop their own repeatable application protocols for PGR liner dips before using this height-control strategy.
Our PGR liner dip recommendations are based on growing conditions in Michigan during mid spring, which is when we performed the study. Rates may need to be adjusted for other locations or during different production times. We encourage growers to perform their own trials on a small scale to determine desirable rates for their growing conditions and for each bedding plant species and cultivar.
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