Although the natural florescence of the tree peony is short, it can be lengthened by forcing culture. In this study, both defoliation or gibberellic acid (GA₃) treatment individually induced tree peony (Paeonia suffruticosa 'Luo Yang Hong') flowering under forcing culture, and their combination (D + G) accelerated flowering with a GA₃-overdose-like phenomenon, indicating that synergism between defoliation and GA₃ treatment may occur. Both defoliation and GA₃ treatment induced a GA response, including (i) increased GA₃ production, (ii) increased PsCPS and PsGA3ox expression, and (iii) decreased PsGA2ox, PsGID1c, and PsGID2 expression; both treatments also positively influenced non-structural carbohydrate (NSC) accumulation. According to the expression of five PsSWEETs, PsSWEET2 and PsSWEET17 may redundantly exercise the crosstalk of defoliation and GA₃ treatment by NSC distribution, whereas PsSWEET12 may act by GA modulation; no synergism resulting from the D + G treatment was detected. Tissue-specific analysis indicated that, in sepals, PsSWEET2 and PsSWET7 are both induced by defoliation and GA₃ treatment, whereas PsSWEET2 expression showed synergism with the D + G treatment. In summary, defoliation and GA₃ treatment synergistically induce tree peony flowering under forcing culture, and NSCs are suggested as key intermedia. Moreover, sepals may play key roles in their synergism, although more direct evidence is still needed.