Abstract

Abstract In two experiments conducted under nursery conditions in 1998 and 1999, the herbaceous perennials, Achillea x ‘Coronation Gold’ and Gaura lindheimeri ‘Corrie's Gold’ were treated with one of four plant growth retardants (PGRs): B-Nine at 2,500, 5,000, or 7,500 ppm; Bonzi at 33, 66, or 99 ppm; Cutless at 50, 100, or 150 ppm; Pistill at 500 or 1,000 ppm; or left untreated. Only Pistill delayed flowering of achillea while Bonzi and Cutless accelerated flowering of achillea. In 1998, B-Nine provided little growth suppression of achillea; however in 1999, when plants were pruned just prior to treatment and thus less reproductively developed, height at the most effective concentration of 7,500 ppm was reduced 33% compared to untreated plants over the duration of the experiment. At the most effective concentration of 99 ppm, Bonzi retarded growth 15% and 31%, over the duration of the experiment, while Cutless at 150 ppm suppressed growth 21% and 46% in 1998 and 1999, respectively, compared to an untreated control. Plants treated with the most effective concentration of Pistill were 13% and 21% shorter than non-treated plants over the duration of the experiment in 1998 and 1999, respectively. However, in 1998 quality ratings of Pistill-treated plants were lower than for control plants, and the duration of control was shorter than with B-Nine, Bonzi, or Cutless in 1999. Only Cutless provided consistent growth suppression of gaura; plants treated with the most effective concentration of 150 ppm were an average of 19% smaller at 60 days after treatment (DAT) in 1998 and 13% smaller than control plants from 28 to 70 DAT in 1999. B-Nine and Pistill provided more transitory control. In 1999, the most effective concentration of B-Nine suppressed growth 9% from 28 to 56 DAT. Plants treated with the most effective concentration of Pistill of 500 ppm averaged 23% smaller at 14 and 28 DAT than control plants. Application of Pistill delayed flowering 11 to 13 days in 1998.

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