A tiller for installation at one side of a tractor, having a retractable tiller frame movable by a power mechanism which is normally actuated by a horizontal sensor arm through a linkage system. Here, there is manual override enabling the tractor driver to actuate the power mechanism regardless of the position of the sensor arm. There is also a manual disengagement member to place operation of the actuator entirely under manual control. Also, the tiller frame carries a rotary head having a series of depending tines, and a sharp cutting blade depends from the tiller frame closely adjacent to the head for cutting off weeds that tend to wind around the tines. The power mechanism preferably comprises hydraulic structure for holding the tines down in the soil at a predetermined vertical position, while a manually controlled valve when actuated, enables a predetermined obstacle force to push the head upwardly.