We report an ultralightweight flexible sensory platform using bare die CMOS chips having a light, temperature, and humidity sensor on a flexible polymer substrate. An application-specified integrated circuit designed and fabricated in a 0.35-μm process is used for signal acquisition, conditioning, and further transfer of data to a bare die microcontroller. The whole platform is powered by a bare die solid-state battery. This extremely lightweight (0.44 g), compact (3 x 1.5 cm²), flexible, and mostly transparent platform can be unobtrusively placed on a leaf of a plant to monitor microclimate conditions surrounding a plant for accurate plant growth monitoring. The latter is successfully demonstrated by placing the platform on a plant leaf and monitoring changing environmental conditions (light, temperature, and humidity) for a whole day.