I was just trying to see if anyone uses packing peanuts in their soil? I just got two new trees and they were packed with nuts. I was thinking maybe if these were tore up they could work well in soil.
I have used chunks of styrofoam along the sides and bottom, to take up space in place of soil when working with a tree that has a small root ball, but not packing peanuts as "part of the mix".
I used them a few years back and all worked ok really. When I went to inorganic though the Pumice is light so I use that with Traction grit for some weight and stability. I can tell you though if you use them like perlite I never had a problem.
I mixed some packing peanuts (not the corn starch ones) with soil to pot up a collected taxus in a large nursery can. I didn't have enough soil for the job. It worked okay for that.
I use them to partially fill very deep cascade type pots. I put them in the bottom as needed to leave room for the root ball and planting medium above them.