Acquisiton of crop growth stage information can not only help to analyze the relation between the crop growth process and environmental condition, but also to guide the field operation effectively. Therefore, different growth stages of rape crops are monitored with the visual system constructed in this paper, and the first critical growth stage of rape is detected automatically, which is seeding emergence stage. The rape should be first extracted from the image. Considering the the impacts of the complicated environment and climatic changes, HI color segmentation method is adopted to segment the crops from the background. Then, two limited conditions, cotyledon area and density, are applied to judge whether it is at seeding emergence stage. Eventually, the experimental results are compared to the ones from other mature methodologies and manual observation, and it shows that the proposed methodology is effective and feasible, and it can provide support for precision agriculture.