Natural or Introduced?

While botanizing in a St. Joseph County floodplain woods a week ago, I came across a population of Matteuccia struthiopteris, Ostrich Fern, a rare plant in Indiana. Ostrich Fern is often planted as an ornamental fern. It is known (as a naturally occurring plant) from St. Joseph County and adjacent LaPorte County, both from single collections by our own Keith Board. Plants of the Chicago Region (1994) states that it is found in "moist, often calcareous woodlands, either on slopes or floodplains." The woods in which I was botanizing was a floodplain woods with some degradation, but there were no signs a former home site or of this population being an escape from a nearby residence. In fact, one of its associates at this location was Asimina triloba, with which it also apparently grows at a known location in Berrien County, Michigan.

My question is this. With landscaping using native plants as well as well-intentioned "restoration" becoming more widespread, how do we track naturally occurring populations of rare plants versus those that have been introduced, or does it even matter? Specifically, how do I know whether this population was naturally occurring and if I should collect and report it?