I've uploaded a patch that should allow you to fix this through the pause menu, under Garden fixes, if you click Restore garden border the surrounding tiles will be returned to their original state.
Right now there is currently no way to make a tile into Dry Grass or Dry Soil, just as you've stated they're kind of just placeholder materials for later versions when I add some Monsters that prefer those biomes. I'm sure I'll add some kind of Decor item which can convert surrounding tiles, or something along those lines.