Common buckwheat (Fagopyrum esculentum Moench) is an agriculturally and pharmaceutically valuable crop due to its well-balanced essential amino acids and rutin content. However, global mass production of buckwheat is limited because its genetic self-incompatibility results in low seed sets and poor grain yield. Therefore, this study was conducted to classify the modes of pistil-pollen interaction between species belonging to the genus fagopyrum and to determine the optimal combination of outcrosses for the most successful pollinations. Based on the interaction between pistils and pollen, we classified the modes of pollen tube growth during interspecific crosses of Fagopyrum species into five categories: (i) Highly compatible: normal pollen tube elongation and style penetration within $6{sim}24$ hours of pollination, (ii) Slightly compatible: delayed (for $1{sim}6$ hours) pollen tube elongation and normal style penetration, (iii) Incompatible type I: pollen tube inhibition at the stigma, (iv) Incompatible type II: pollen tube inhibition at the style, and (v) Incompatible type III: pollen tube inhibition at the stylodium. Based on the observed pollen tube elongation and the following embryo development, highly compatible pollinations were found to be crosses between F. esculentum x F. cymosum and between F. esculentum (thrum) x F. homotropicum.