In this paper we propose a method to segment individual leaves of crop plants from UAV imagery for the purposes of deriving phenotypic properties of the plant. The crop plant used in our study is sorghum Sorghum bicolor (L.) Moench. Phenotyping is a set of methodologies for analyzing and obtaining characteristic traits of a plant. In a phenotypic study, leaves are often used to estimate traits such as individual leaf area and Leaf Area Index (LAI). Our approach is to segment the leaves in polar coordinates using the plant center as the origin. The shape of each leaf is estimated by a shape model. Experimental results indicate that this approach can provide good estimates of leaf phenotypic properties.