Disclosed is an apparatus (1) for applying substances to an area of interest (13). The apparatus (1) includes one or more sensors (8, 10, 12) for sensing one or more sub-areas (14) of the area of interest (13). Each sensor (8, 10, 12) generates one or more detection signals each associated with a respective sub-area (14) upon detection of a sub-area (14) considered to have a chlorophyll intensity above a threshold level. The apparatus (1) also includes a control circuit (2) which produces a control signal associated with each detection signal and one or more outlets (7, 9, 11) responsive to control signals from the control circuit. The control circuit is operable in a first mode in which a control signal is applied to an outlet (7, 9, 11) to maintain it open except when a detection signal is present.
