July 2002

Taming the Wild Calibrachoa By Erika Berghauer, Jim Barrett and Rick Schoellhorn

Eye-catching color and vigorous growth have brought nothingbut popularity for Calibrachoa, and it has become one of the industry'shottest vegetatively propagated specialty annuals. Calibrachoa's petite,single, petunia-like flowers, finely textured foliage and trailing growth habitmake it an ideal plant for hanging baskets either alone or in mixed plantings.

As with many vegetatively propagated items, producingCalibrachoa hanging baskets does not come without challenges. Taming vigorousgrowth to produce a more attractive basket, facilitate shipping and keep plantsuntangled in a high-density growing situation are concerns. At the Universityof Florida, we have evaluated the use of growth regulator drench applicationsto slow the growth of Calibrachoa, which should make this high-value crop moreappealing to growers.

Experiments

In the spring of 2000, 'Mini-Famous Pink' and'Mini-Famous Blue' Calibrachoas were grown in 10-inch baskets withfive plugs per pot and were fertilized every watering with 150 ppm of 20-10-20fertilizer. The planting date was February 23. Four weeks after planting, threeBonzi rates were applied as a drench using 15 fluid ounces of solution percontainer. At the time of treatment, plant size (average of plant width andlength of shoots below container rim) was measured at 14.5 inches for Pink and14 inches for Blue, as indicated by the line in Figures 1 and 2, page 10.Plants of this size will make good, salable plants, and a grower would want toslow the growth rate at this point to keep the plants from becoming overgrown.

Mini-Famous Blue appears to be somewhat more vigorous thanMini-Famous Pink and less sensitive to Bonzi, which indicates the variation invigor that occurs with different Calibrachoa varieties. In addition, this trialdemonstrated that both varieties showed favorable responses to the Bonzi drenchtreatments. An application of 1 ppm Bonzi reduced the final plant size by 15percent for Pink and 12 percent for Blue when plants were measured five weeksafter treatment. The high rate of 8 ppm reduced plant size by 28 percent forPink and 27 percent for Blue, as compared to the control plants. Note the linein the figures that indicates the size of the baskets at treatment and that the8-ppm Bonzi drench slowed, but did not stop, growth. After the 8-ppm drench,the Pink Calibrachoa baskets grew 2.5 inches, and the Blue variety grew sixinches (see Figures 1 and 2, page 10).

Taking these varietal differences into account, we carriedout a similar ç trial in spring 2002 with 'Mini-Famous RosePink', 'Mini-Famous Light Blue' and 'Mini-FamousYellow'. For this trial, the 10-inch baskets were planted on February 19and fertilized every watering with 300 ppm 15-5-15 fertilizer. The plants weretreated April 11 (seven weeks after plant date), and the accompanying pictureswere taken on May 18, 2002. With regard to the Rose Pink and Light Bluevarieties, results were very similar to the 2000 trial. As can be seen in thepictures, Mini-Famous Yellow was the most vigorous of the three varieties inthe 2002 evaluation. For the Yellow, 16 ppm Bonzi was required to provide thesame degree of control as the 2 and 4 ppm used on Rose Pink and Light Blue.

Influencing Factors

Bonzi is a helpful tool when trying to control Calibrachoagrowth in production, as illustrated in this study. Differing culturalsituations require adjusted rates according to growing temperature, fertilizerapplications, time of year (light intensities and day length) and productionspacing. Growers in the North, who produce Calibrachoa early in the spring,should be careful not to overapply Bonzi. The lower temperatures and lowerlight levels make plants grow much slower, requiring lower Bonzi rates ascompared to the Southern-grown plants in our study.

Due to the large degree of variation in Calibrachoavarieties, even within a series, growers should assess how vigorous eachvariety is within a series to determine the optimum growth regulator rates.Several breeding companies are actively working to improve their lines ofCalibrachoas, which will result in the rapid introduction of new varieties andchanges to existing varieties. So, growers need to keep up with these changesto know the growth habits of the varieties being used.

Calibrachoas flower faster under long days. Baskets startedearly and without lights will produce more vegetative growth, thus potentiallyrequiring more growth regulator than lit plants. Some liner producers initiateflower buds by lighting the cuttings while they are rooting, which can greatlyreduce the time to flower in the finished container and may reduce the amountof growth regulator needed.

Timing the drench applications depends on how the basketsare produced. In these studies, we applied the drench late, after the basketswere at salable size. This may be a good strategy for growers producing plantsfor a market where larger baskets are needed. A grower that produces baskets onthe bench or one that hangs baskets close together may want to keep the basketsmore compact by applying Bonzi earlier, which prevents the plants from growingtogether and makes shipping easier.

These studies demonstrate the benefit of using a late Bonzidrench at rates of 1-16 ppm to control Calibrachoa baskets. Sumagic could alsobe used for this late drench, but we have not evaluated optimum rates. Anotherstrategy some growers might prefer is to control early growth with growthregulator sprays applied while the plants are still vegetative. Then finish thebaskets with a late drench to hold them at the desired size longer. Dependingon a grower's individual situation and preferences, there are severalchemical options for the spray applications, including B-Nine; Bonzi; Sumagic;a tank mix of B-Nine and Cycocel; and a tank mix of B-Nine and A-Rest.