It should not matter if the wheels are in the ditch, . The mower and all 4 wheels should be riding the slope of the ditch,
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I'm not to concerned with mowing down inside a ditch, just with keeping the top of the weeds/grass growing down inside of the ditch about the same level as the lawn grass.
This is why I'm toying with the idea of an adjustable telescoping arm that extends out over the ditch.
The mowing device would be attached to the underside of this arm.
One of my ditches has sloped sides but the other 2 ditches have sides that are straight up and down.
One of the ditches can be trimmed from either or both sides, but the other 2 can only be approached from one side.