We describe a sensor for measuring budburst and blossom in plants and trees. A pair of plastic optical fibers is placed behind a bud. One fiber carries modulated light to the bud and the other fiber carries reflected light to detector electronics. When the plant's bud opens, the amount of reflected light changes. The sensor uses lock-in detection to suppress ambient and other light noise. A microcontroller manages the sensor operation and saves data on a secure digital memory card. The sensor incorporates calibration, and the user can set the integrated real-time clock, and the data collection interval through with a simple menu via an RS-232 serial connection. The sensor is power efficient and can operate for an entire growing season without having to replace its batteries. We have conducted initial field testing of the sensor.