We investigated the effect of habitat environment and colony characteristics on floral bud development in Rhododendron brachycarpum native to Ulleung Island of Korea. Two habitats were considered. Habitat B had a remarkably higher altitude and steeper slope than habitat A. Therefore, fewer types of trees existed on the upper layers, and the density was low. Hence, the degree of light transmission within the community was higher in habitat B. Additionally, flowering ratio of shoots was higher in habitat B, and the amount of light was also relatively higher. The lower the amount of light, lesser the floral development, the higher the leaf area and the specific leaf area, lower the dry weight and chlorophyll content. The total carbohydrate (T-C) and nitrogen (T-N) in shoots are more important than the relative T-C and T-N ratios per leaf. Finally, this study revealed that the floral bud development and flowering of native rhododendrons are strongly associated with the degree of assimilate accumulation in shoots, and that sufficient assimilate accumulation in shoots is essential for stable flowering.