generally the options for grandmother's flower garden quilts are:
use 1/2 hex's to fill in the edges- making it square/straight- then bind as normal
use bias binding & carefully stitch (clip at the inside points, miter the outside) time consuming- but nice finish
or
make a facing to finish the edges- stitching on right sides together- clip inside points, fold to back & press
or
since the edges are already turned under all the way around you can simply applique the top to a background (either leaving whole or trimming away all the fabric under the flower garden- just leaving a border around the top-which can then be squared, sandwiched, quilted & bound normally.