We propose a biologically motivated method for creating animations of opening flowers. We simulate the development of petals based on the observation that flower opening is mainly caused by cell expansion. We use an elastic triangular mesh to represent a petal and emulate its growth by developing each triangular region. Our simulation process consists of two steps. The system first grows each triangle independently according to user-specified parameters and derives target rest edge lengths and dihedral angles. The system then updates the global shape to satisi' the rest lengths and dihedral angles as much as possible by means of energy minimization. We repeat these two processes to obtain keyframes of the flower opening animation. Our system can generate an animation in about 11.5 minutes. Applications include the creation of graphics animations, designing 3D plant models, and simulation for aiding biological study. In contrast to existing systems that simulate the development of flattened 2D petals, our system simulates the growth of petals as 3D surfaces. We show the feasibility of our method by c:reating animations of Asiatic lily and Eustoma grandiflorum.