Compare to P. virginianum was known as P. vulgare for many years and can still be found under that name on many web sites. North America is now regarded as not a home to P. vulgare. Distinguishing these two (or, for that matter, any pair of polypodies) is nonetheless difficult. Differences:
P. virginianum has rhizome scales with a central, darker brown stripe, stipes are scaly, sori are midway between the costa and margin OR closer to the margin (particularly on the basal side of the pinna), sori have paraphyses (branching structures among the sporangia, hand lens required).
P. vulgare has brown rhizome scales with no central stripe, stipes are glabrous, sori are midway between costa and margin OR closer to the costa, sori have no paraphyses.
Compare to Fronds of Polypodium appalachianum and P. virginianum in the photo below: three characters differ: pinnules pointed on P. appalachianum, pinnules rounded on P. virginianum; blade triangular, widest near base with P. appalachianum, blade with more parallel sides, often widest at the middle, for P. virginianum; lowermost pinnule meets rachis creating a distinct notch in P. appalacianum, hard to see here, but click through to the original, somewhat clearer. The apex of lower frond is aberrant.
Parents P. appalachianum and P. sibiricum, both of which differ in minor ways and the latter not cultivated. The former is wider at the base of the blade.
Taste Most of the floras mention an acrid taste; sometimes unstated, this is a means of distinguishing between this and P. glycyrrhiza, the licorice polypody.