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The best time to fertilize peony plants is in early spring, just as new growth begins to emerge. This timing allows the plants to absorb nutrients as they start their growth cycle, setting the stage for a healthy and productive blooming season. A second, lighter application can be made in late spring after the plants have finished blooming to support root development and prepare them for the next growing season. Avoid fertilizing in late summer or fall, as this can encourage new growth that may not harden off before winter.