Any suggestions for the best way to join batting pieces so that they are wide enough to fit my quilt. The quilt is a 58" square and my batting is only 48" wide.

You can butt them together and use a wide zigzag or there is a fusible tape you can use made just for this purpose. I have used it with cotton batting, not sure if it will work with poly, I'm sure someone else will clarify

i've used the tape too with cotton, i don't think it works with poly

I've used the tape so far with cotton, poly and Dream Green - works great

i've used the tape too with cotton, i don't think it works with poly

I use a basting needle and whip stitch the pieces together.

I make sure the pieces to join have a straight edge by rotary cutting them. Then I use the mending or zigzag wide stitch to connect the two pieces.

I use leftover pieces of lightweight fuisible interfacing.

I use a wide zig zag too. If the batting has a lot of loft I whip stitch it by hand.

here's my method:

I hand whip scraps together.

I use a slip knot for a knot, go down left hand batting, up immediately next to stitch, needle thru the loop, pull tight.

You are up on the left, sewing about 1/2"-3/8" from the edge. stitch down in between the 2 battings, up on the right batting, down in between, up on the left batting. always up through the batting & down in the seam. When you pull it tight, it butts, no lumping, the stitches inbetween keep it from bunching. stiches are about 3 to an inch. goes really fast.