"Teacherjh" wrote in message ...

In uncontrolled airspace, I believe you still must be on an IFR flight plan to
operate IFR.

Untrue. No plan or clearance is requred.

But you are still IFR (as others have pointed out) and the rules for minimum altitudes
(either in general or on approaches) still apply.

91.173 ATC clearance and flight plan required.
No person may operate an aircraft in controlled airspace under IFR unless that person has -
(a) Filed an IFR flight plan; and
(b) Received an appropriate ATC clearance.