Flower bud differentiation and development of tree peony were investigated, using 3 cultivars of spring flowering type. Sepal differentiation started in the early July in 'Hanakisoi' and 'Taiyoh'. Petal differentiation started in the late September in 'Hanakisoi', and in the early October in 'Taiyoh' and 'Tamasudare'. Androecium differentiation started in the late October in 'Hanakisoi', and in the early November in 'Taiyoh' and 'Tamasudare'. Gynoecium differentiation started in the early November in 'Taiyoh' and 'Tamasudare', and then in 'Hanakisoi'.
The radial growth of flower bud in 3 cultivars almost completed at the late sepal differentiation stage or early petal differentiation stage, which corresponded with the starting time of chilling treatment for December flowering.