Effects of IBA and NAA treatments on rooting Douglas-fir stem cuttings

The effectiveness of six IBA and four NAA concentrations, four combinations of IBA and NAA concentrations, and control were tested for their ability to enhance rooting frequency (%) of Douglas-fir cuttings. Two IBA and one NAA treatments were also compared to the control for quality of root system. Between 1984 and 1998, six independent studies were conducted in mist or fog environments with the same 11 or 14 clones. Auxin concentrations tested ranged from 0 to 123 mM IBA and 0 to 10 mM NAA. Auxin, clone, and auxin by clone effects were significant in every study, although individual clone analyses showed only two clones to differ significantly for auxin. All auxin treatments but the 10 mM NAA treatment induced significantly greater rooting percentage than the control, but no single auxin, auxin concentration or combination of auxins was clearly superior in every study. The 10 mM NAA concentration was the only concentration tested that reduced rooting percentage to less than the control. Both NAA and IBA appeared to have broad ranges of root-enhancing activity. However, within the effective range of IBA evaluated, 24.6 mM produced the greatest rooting percentage in four of five studies testing IBA. NAA solutions with concentrations between 2.5 and 7.4 mM NAA generally resulted in similar rooting success. Rooting responses to increased IBA and NAA were both nonlinear; rooting decreased with both too little and too much auxin. Combinations of IBA and NAA in the same solution did not increase rooting percentage above what was achieved with one auxin. For root system quality, auxin treated cuttings in one study, had significantly better root systems than control, but there was no difference in the other study in which root quality was estimated.