Abstract: An automatic spot weed sprayer which is self-adjusting in the vertical to compensate for changes in crop height as the device moves along a row of crop. In a preferred embodiment, the invention is secured to the tool bar of a tractor which also carries a series of cultivators for removing weeds between crop rows. A photoelectric sensor passes down each row, just above the crop, to detect weeds which typically grow faster and therefore higher than the crop itself. When the beam is broken by a weed, a signal is sent to a Programmable Logic Computer (PLC) which then triggers a spray nozzle to dispense herbicide by a time delay based on the speed of the tractor. Concurrently, two sets of vertically spaced photoelectric sensors continually monitor crop height and signal the PLC to raise and lower the spray bar to compensate for crop height changes as the sprayer travels about the field.