Effects of Different Growth Regulators on the Rooting of Catalpa bignonioides Softwood Cuttings

(1) Background: To further improve the rapid reproduction and large-scale application of Catalpa bignonioides. (2) Methods: With young softwood cuttings from a 3-year-old C. bignonioides mother plant used as materials, the effects of indole-3-acetic acid(IAA), indolebutyric acid(IBA) and rhizogenic powder-1(ABT-1) growth regulators at different concentrations on cutting indexes and the dynamic changes in endogenous hormone contents during the rooting of the C. bignonioides cuttings were studied. (3) Results: The rooting of C. bignonioides cuttings could be divided into five stages. There were three types of rooting of adventitious roots. IBA treatment resulted in a high rooting rate and beneficial root morphology. The morphological indexes of the cutting roots after treatment with 1000 mg·L−1 IBA had the best overall quality, which was significantly higher than that of the roots in the control (CK) group (p < 0.05). Although the average longest root length (20.51 cm) under ABT-1 was the longest, its overall average rooting rate was slightly lower than that under IBA. The rooting effect under IAA was generally lower than that under IBA and ABT-1. The endogenous hormone content of the cuttings was found to be closely related to rooting; the IAA and zeatin nucleoside (ZR) content was high, and the ratios of IAA/ABA and IAA/ZR were high. The contents of gibberellin3 (GA3) and abscisic acid (ABA) were low, which had a promoting effect on the rooting of the cuttings. (5) Conclusions: All three kinds of auxin can promote rooting and, of the three treatment groups, the rooting effect of cuttings in the IBA treatment group was the strongest, with 1000 mg·L−1 being the optimum concentration.