Objective Peony (Paeonia suffruticosa Andr.) is a renowned woody ornamental plant in China, we investigated the effects of nitrogen (N), phosphorus (P), and potassium (K) application rates and ratios before and after flowering on the growth and ornamental quality of potted peony.

Methods A pot experiment with “3414”incomplete orthogonal regression design was conducted, and five-year-old Paeonia suffruticosa ‘Zi Erqiao’ was used as the test materials. The comprehensive growth and development status of peony were assessed using the integrated membership function values (CMFV). Fertilizer effect models were employed to simulate the mathematical relationship between the CMFVs of peony plants and the fertilizer application rates, and to make recommendation of N, P, and K fertilizer rates for the optimal growth of peony.

Results Application of N, P, and K fertilizer before and after flowering significantly increased peony plant height, stem diameter, crown width, flower diameter, and flower number per plant (P<0.05), and the N, P, and K fertilizer showed significant interaction on the synergistic vegetative and reproductive growth of potted peony. The peony CMFVs under N-P- K three-factor treatments were higher than those under two factor treatments, and the CMFVs under two factor treatments were higher than those under no-fertilizer treatment. The highest CMFV under N₂P₂K₂ (0.73) was 3.17 times of that under N₀P₀K₀. The fertilizer effect on the CMFVs was ranked as N > P > K. Excessive or insufficient application of any fertilizer inhibited peony growth and reduced ornamental effects. The CMFV and fertilizer application rates were well fitted using all the unitary, binary, and ternary quadratic fertilizer effect function equations (P<0.05), and the recommended fertilizer application rates before and after flowering of peony were 4.21 g/plant for N, 2.07 g/plant for P₂O₅, and 2.52 g/plant for K₂O, respectively, with an optimal fertilization ratio of N∶P₂O₅∶K₂O at 1∶0.49∶0.60.

Conclusions Applying appropriate amounts of N, P, and K fertilizers before and after flowering can improve the agronomic traits and promote synergistic vegetative and reproductive growth of peonies. The optimal N, P₂O_5, and K₂O amount before and after flowering of peony are determined to be 4.21, 2.07, and 2.52 g/plant, respectively, with an optimal fertilization ratio at 1∶0.49∶0.60.