An adhesive and visualized hydrogel‐based detector is designed to monitor the solar radiation dose required for plant growth by the discoloration of methylene blue (MB) under sunshine. The detector is based on hybrid hydrogels prepared from photoinitiated polymerization. The combination of polypropylene (PP) fabrics acting as a substrate and the photoinitiated polymerization on the graphitic carbon nitride (g‐C3N4) surface enables a smaller thickness and a more homogenous distribution of the g‐C3N4 nanosheets. Thus, the hybrid interpenetrating network (IPN) hydrogel on the PP fabrics with a thickness of 1.5 mm presents an improved photodegradation capability. In addition, better mechanical properties and lower weight of the hydrogel‐based detector are achieved by the presence of the PP fabrics. In combination with the stickiness from a polyurethane resin acting as glue, the obtained detector can be easily pasted on the leaf of a plant as demonstrated for the example of Epipremnum aureum. The radiation dose exposed to the leaves can be accurately traced from the change in color of the detector to ensure a sufficient radiation for plant growth and avoid possible burn to the leaves under sunshine.