The present invention relates to a method for determining a growth status of a plant comprising chlorophyll, the method comprising the steps of: illuminating the plant (102) with input light including a light intensity modulation component (205, 206, 207, 208); detecting light emitted from the plant; determining (S702) an offset light intensity (204) surrounding the plant, the offset light intensity being a static component of the input light; determining (S718) a phase and a gain between the input light and the detected light, determining (S720) a growth status of the plant based on a predetermined relationship between input light and detected light, and on the phase and the gain. The invention also relates to a corresponding system and to a computer program product.