A plant wavelength sensor device for estimating a growth parameter (Eg), comprising: a light emission unit including at least three light emission members (261-266), each light emission member being configured for emitting measurement light (P) in a different wavelength range λn, where n is an index representing a natural number from 1 to at least 3, to irradiate a target plant (Cr); a light receiving unit including for each wavelength range λn a corresponding light receiving member (311-316) for respectively detecting the measurement light (P) as reflected light (Pr) from the target plant (Cr); a memory configured for storing a growth parameter (Eg), the wavelength ranges and the number thereof k required for estimating said growth parameter (Eg); and a control unit for controlling the light emission unit and the light receiving unit , wherein when the growth parameter (Eg) is selected the control unit is configured to extract the combination of the wavelength ranges λ1 to λn and the corresponding number k thereof, required for estimating the selected growth parameter, from the data stored in the memory; to control the light emission unit to simultaneously emit measurement light (P) of all wavelength ranges λn on a periodic basis; to control the light receiving unit to detect the reflected light (Pr) from the target plant (Cr) only in the wavelength ranges corresponding to the predetermined wavelength ranges λ1 ... λn; and to calculate either the intensities of the light reflected (Pr) from the target plant (Cr) as reflected light intensities with respect to the detected wavelength ranges corresponding to the growth parameter (Eg), or the reflectivities of the target plant from the reflected and detected measurement light (P) with respect to the wavelength ranges corresponding to the growth parameter (Eg). a light emission unit including at least three light emission members (261-266), each light emission member being configured for emitting measurement light (P) in a different wavelength range λn, where n is an index representing a natural number from 1 to at least 3, to irradiate a target plant (Cr); a light receiving unit including for each wavelength range λn a corresponding light receiving member (311-316) for respectively detecting the measurement light (P) as reflected light (Pr) from the target plant (Cr); a memory configured for storing a growth parameter (Eg), the wavelength ranges and the number thereof k required for estimating said growth parameter (Eg); and a control unit for controlling the light emission unit and the light receiving unit , wherein when the growth parameter (Eg) is selected the control unit is configured to extract the combination of the wavelength ranges λ1 to λn and the corresponding number k thereof, required for estimating the selected growth parameter, from the data stored in the memory; to control the light emission unit to simultaneously emit measurement light (P) of all wavelength ranges λn on a periodic basis; to control the light receiving unit to detect the reflected light (Pr) from the target plant (Cr) only in the wavelength ranges corresponding to the predetermined wavelength ranges λ1 ... λn; and to calculate either the intensities of the light reflected (Pr) from the target plant (Cr) as reflected light intensities with respect to the detected wavelength ranges corresponding to the growth parameter (Eg), or the reflectivities of the target plant from the reflected and detected measurement light (P) with respect to the wavelength ranges corresponding to the growth parameter (Eg). <EOS>