We aimed to explore the effects of different concentrations, in particular, high concentrations, of exogenously applied ethephon and methyl jasmonate on gum duct formation in three broad-leaved tree species, Cerasus × yedoensis, Prunus mume and Liquidambar styraciflua. Intact shoots were treated with ethephon and methyl jasmonate in lanolin paste at concentrations of 0.1%, 1%, 2%, 5%, and 10% (w/w). The ethephon treatments induced gum duct formation in the xylem adjacent to the cambium in all three species, whereas the methyl jasmonate treatments did not. The highest induction of gum duct formation was observed after 1–2% ethephon treatments in C. × yedoensis and P. mume, and after 5–10% ethephon treatments in L. styraciflua. Meanwhile, the treatments with higher ethephon concentrations resulted in a lower induction of gum duct formation in C. × yedoensis and P. mume. In addition, we examined gum duct formation at sites distant from the treatment sites in C. × yedoensis and P. mume shoots treated with 10% ethephon. Gum duct formation was found to be the highest at sites 2 cm away from the treatment site (in the acropetal direction). We show that at least in C. × yedoensis and P. mume, trees have an optimal concentration of ethephon to induce gum duct formation, and that concentrations higher than the optimum suppress the induction.