Various approaches for generating woodgrain patterns using computer graphics have been proposed so far. However, the generation of various woodgrain patterns with conventional methods is difficult due to the need for the adjustment of numerous parameters to express a real woodgrain pattern. In this paper, a new mathematical approach for generating woodgrain patterns is proposed. Virtual trees are generated by simulating tree growing based on past actual weather information obtained from public organizations, and woodgrain patterns are then acquired by cutting the trunks of the virtual trees. In order to simulate tree growing, growth models of tree are constructed in consideration of dendrological characteristics and environmental conditions. Growth of tree is influenced by various environmental factors, such as sunlight, temperature, carbon dioxide concentration, wind, precipitation, soil nutrient, inclination of ground, survival amongst surrounding trees, etc. With this system, the growth model of trees is constructed based on precipitation, temperature, sunlight, and inclination of ground, which especially have strong effects on tree growth. With this approach, various types of virtual trees can be obtained by changing growth conditions such as period and location of growth without the need to reset complicated parameters of tree species, and then the virtual trees can be cut at arbitrary areas, thus allowing a variety of woodgrain patterns to be easily generated by one parameter setup.