Abstract: Apparatus for treating plants, such as weeds with an herbicide, which plants have grown to at least a predetermined height, including a sensor comprising an adjustable, electrically conductive probe arm adapted to contact such plants coupled to electrical circuitry for generating an electrical signal when such contact is made. The electrical signal opens a solenoid actuated valve which allows the treatment material to flow from a pressurized storage tank through conduits to spray nozzles which dispense the treatment material over the plant. The circuitry includes means by which the solenoid actuated valve is maintained open for a predetermined minimum duration to assume adequate treatment of the plants.