Colleen,

You can use floral wire to mount something. The wire only needs to be in there long enough for the plant to attach itself to the background.

You could also use zip ties. Cut the nubby end off and then cut it to the size you need. Cut both ends into points and push them into the foam in a "U" around the plant. They don't stay in very well, but it is better than nothing.

I am thinking if anything maybe a bit of moss would look good, but I like it how it is