Amy, are you sure Shady Grove is "re pentatonic?" I am only familiar with Doc Watson's version and my ears hear it as having la as the tonic. The tune as I think I know it goes
la, la, la,-,re, ti, <so,-, la, la, do, re, mi.-
so, so, mi, do, re, ti, so,-, la, ti. re, ti, la.-
This version has six notes ....Are we thinking of the same song?Murray