I save the plastic 6 packs too. I also take used styrofoam coffee cups and poke a dranage hole in the bottom with a pencil. Then I can plant seeds, cuttings, or seedlings in them. When it's time to remove them for re-planting, if they don't dump out easy, just tare the cups off the plant. With some seedlings, like veggies, I leave the top inch or so and remove the bottom part. That makes a good cut worm barrier and will hold extra water till the roots take hold in the soil.
Another thing. If you plant something that sends down a long tap root, I've cut the flat bottom off and set the cup on another larger cup or pot full of soil. I always use that method when planting Sable palm seeds.
