In plant production, growth regulators are commonly used as rooting enhancers. This study compared the effect of the indole 3-butyric acid (IBA) and two commercial brassinosteroids: Brassinolide (BL) and 24-epibrassinolide (24epi BL) on rooting of stem cuttings in two cultivars of ninebark ‘Dart’s Gold’ and ‘Red Baron’. Changes in the contents of endogenous auxins, several organic compounds, and the activity of catalase, an antioxidative stress enzyme, were monitored to gauge the effects of treatments on cuttings’ condition. Stem cuttings were placed in a rooting medium and sprayed with water solutions of the researched compounds. In both cultivars, brassinosteroids positively affected the degree of rooting and root length, acting similarly to, or better than, the auxin. In the plant growth regulators (PGR)-treated cuttings, the contents of chlorophyll and of total soluble sugars as well as catalase activity were higher relative to the water control while the level of hydrogen peroxide was lower.