Using Paclobutrazol to Control Height of Poinsettia ‘Freedom’

Late-season height control of poinsettia (Euphorbia pulcherrima) is difficult since most chemical growth retardants adversely reduce bract size when applied after first bract color. Paclobutrazol (Bonzi) controls stem elongation late in poinsettia crop development but can excessively reduce bract size if improperly applied. Two experiments were conducted to quantify how paclobutrazol application influenced height and bract area of ‘Freedom’ poinsettia. In the first experiment, paclobutrazol was applied at 1 mg·L–1 (ppm) in 118-mL (4.0-fl oz) volumes per pot [(a.i.) 0.12 mg/pot (28,350 mg = 1.0 oz)] as a drench to a new group of plants weekly from the initiation of short days until 1 week before anthesis. Maximum reduction in height and bract area was obtained when paclobutrazol was applied immediately after short days, and the response to paclobutrazol decreased as application time was increasingly delayed toward anthesis. In the second experiment, paclobutrazol was applied weekly after first bract color as either a drench or subapplication at various concentrations. Plant height and bract area were reduced by 23% when 2 mg·L–1 [(a.i.) 0.24 mg/pot) paclobutrazol was applied through subapplication at first color. The effects of paclobutrazol on height and bract area reduction decreased as application time was progressively delayed. Concentrations lower than 1