A rotary tiller device for tilling around trees and other similar obstacles comprises a rotary tilling head with a plurality of tines extending slightly outwardly, downwardly and forwardly from a support which rotates around a vertical axis of the head. A central plate rests upon the ground and defines a tilling depth of the tines. A cover prevents the support from damaging the tree. A mounting system for mounting the unit on the belly of a yard tractor includes a pair of rails mounted in place of the removed belly mower and a hydraulic fluid reservoir tank which slides into position on the rails and defines a support for the tiller head. A linkage between the tank and the tiller head includes a six-bar lever system mounted upon a pivotal plate. The lever system is actuated by a hydraulic cylinder which drives the tiller head in a straight line outwardly and slightly forwardly of a line at right angles to the side of the tractor and in a plane which is generally horizontal and is defined by the plate member at the end of the tank. The straight line actuation of the lever system enables the tiller head to be readily manouevered around trees in a weeding action while the tractor continues in a constant direction.