Stephanie E. Burnett, Gary J. Keever, J. Raymond Kessler, and Charles H. Gilliam

Coreopsis rosea, or pink coreopsis, is a delicate, erect, finely branched herbaceous perennial, growing 12-18 inches tall, and producing 0.75-inch pink flowers. Characterized by rapid growth, pink coreopsis is often difficult to manage during production, requiring time-consuming pruning for size control. Various plant growth retardants (PGRs), including B-Nine, Cutless, and Sumagic, have been used to control excessive growth in horticultural crops including the closely related species Coreopsis verticillata ‘Moonbeam’ and Coreopsis grandiflora. However, no previous work has been published using PGRs on Coreopsis rosea. The objective of this study was to determine the effectiveness of B-Nine, Cutless, and Sumagic in controlling growth of pink coreopsis.

METHODS
Rooted cuttings of pink coreopsis were transplanted to 4-inch square pots containing Fafard #3 growing medium, and placed pot-to-pot. Before treatments were applied, plants were sheared to 2.5 inches above the pot rim, spaced on 8-inch centers, and provided night-break lighting from 10:00 p.m. to 2:00 a.m. to simulate long days. Growth retardants were applied at 0, 2,500, 5,000, or 7,500 parts per million (ppm) for B-Nine; 25, 50, 75, 100, 125, or 150 ppm for Cutless; and 10, 20, 30, or 40 ppm for Sumagic. Date of the first open flower, shoot height, and growth index [(height + width1 + width2)/3, where width1 is widest point and width2 is perpendicular to width1] were collected at that time. Each plant was rated for market quality on a scale of 1-4 (1 = poor, unmarketable; 2 = fair, acceptable; 3 = highly marketable; 4 = excellent) when one-third of the flowers were open, with a quality of 2.5 or higher considered marketable.

RESULTS
Time to first flower was not significantly increased by any PGR, but was decreased by approximately 3 days for Cutless and Sumagic as compared to untreated controls (see table). At first flower, increasing rates of all PGRs reduced shoot height. Shoot heights at first flower of plants treated with the lowest and highest rates of each PGR were reduced 18 and 25% for B-Nine, 11 and 27% for Cutless, and 20 and 29% for Sumagic, respectively, relative to height of controls. Growth indeces followed trends similar to height.

Time to one-third flowers had opened (full flower) decreased by as much as 4 days with Cutless at rates of 75 ppm and increased slightly with higher rates. Time to full flower decreased as much as 5 days with Sumagic, while B-Nine had no effect on this measurement compared to controls.

Market quality rating of plants treated with B-Nine, Cutless, or Sumagic increased with increasing rate. Highest numerical values for market quality were obtained with B-Nine at 7,500 ppm, Cutless at 125 or 150 ppm, and Sumagic at 40 ppm. Control plants were considered unmarketable based on the ratio of height to pot size, while those treated with all PGR-rate combinations were considered marketable.

The growth retardants, B-Nine at 2,500-7,500 ppm, Cutless at 25-150 ppm, and Sumagic at 10-40 ppm, appear useful in the production of a superior pink coreopsis crop. These PGRs reduced plant size and enhanced plant quality without delaying flowering. In addition to improved quality, the compact size of treated plants should facilitate shipping and handling.