The string (this was the usual way of mounting some time ago) or tape will be enough to hold them upright especially behind glass. Are you planning on matting them as well??

When I frame pressed flowers/leaves I put one dot of Elmer's glue with a toothpick at top, bottom of leaves, or under the center of a flower........just a dot.

I framed some ferns several years ago and actually used decopage glue... spread under the leaf placed on the watercolor paper then dried it. I then coated the upper side while atached to the paper. I have done this with pressed flowers too. So far so good. As it begins to dry I place wax or blotter paper on tob and a heavy book.