Bedding plant growers have the opportunity to utilize urban waste comp
ost in their growing media, but variation in substrates that constitut
e the compost could influence the efficacy of growth regulator drenche
s, This experiment investigated the effect of 0%, 30%, 60%, and 100% c
ompost made from biosolids and yard trimmings on the efficacy of a pac
lobutrazol drench on Impatiens wallerana Hook.f. 'Accent Red'. Paclobu
trazol was applied at a,i. drench rates of 0, 0.016, 0.032, 0.060, or
0.125 mg/pot. Growth of control plants (0 mg) increased linearly as th
e percentage of compost in the medium increased from 0% to 100%. Growt
h of plants in all media decreased as paclobutrazol concentration incr
eased to approximate to 0.016 mg with no further decreases at higher c
oncentrations. The efficacy of paclobutrazol varied with the medium. F
or example, paclobutrazol at 0.016 mg reduced shoot: dry mass by 31% r
elative to control plants in the presence of 100% compost but only 17%
in the absence of compost. The percentage of growth reduction, relati
ve to control plants, increased as the percentage of compost in the me
dium increased. Chemical name used: pha(1,1-dimethylethyl)-1H-1,2,4-tr
iazole-1-ethanol (paclobutrazol).