Are the ridges a problem when you push the root ball out? The sturdiest bottles I find are "Sobe", but they have deep, square-edged grooves that I think would mess up a root ball.
My plan was to put the large bottle cap in the bottom of the bottle, so it would serve as the head of a plunger when I push the root ball out with a pencil or dowel.
Or, the Sobe bottles are so stiff that I could cut a slit down the side with tin snips before adding potting mix. The stiffness would hold it closed.
