I started a bunch of pawpaw from seed. The way an air pruning system works is that you start with a small air pruning container. This prunes the tap root which causes upstream branching. Studies show that most of the branching occurs in the first 4" before the prune. These secondary roots then prune when they hit the air as well. You use a very well drained mix like Promix with these containers which has lots of small voids in it. The roots begin filling these voids. When you first plant in these and water them, the water almost immediately drains out the bottom. As the roots fill the container, they become hard to water. Water just seems to sit on the container and it takes a long time adding a little water at a time to saturate it and get water to come out the bottom. This indicate the container is full and it is time to transplant to the next container.
When you transplant you want a container that allows for no more than 4" in all directions since that is where most of the branching will occur when those secondary and tertiary roots eventually get pruned.
I have the most success starting trees from seed under lights in the winter and then transplanting them from 18s to 1 gal Rootbuilder II containers at 12 to 16 weeks. In late spring (after acclimating the trees to my deck), I transplant a second time to 3 gal Rootubilder II containers. I either plant the seedlings from 3 gals to the field in fall or I over winter them in a cold room and plant them in early spring.
This system works great for most of the trees I’ve planted. This includes chestnuts, Allegheny chinquipins, pear, apple, jujube (started from root cuttings), Filberts, Dwarf Chinquapin Oaks, and more. The jury is still out on pawpaw. I have heavy clay as well.
Because pawpaw are photosensitive when young, I kept my pawpaws on my lower deck that is shaded for 2 growing seasons. They did really well growing in the containers. I planted them in full sun in the field last spring. Most leafed out well. Some went dormant in the summer. They had no protection from deer but I don’t see much evidence of deer browsing them. Some held their leaves until this fall and went dormant when expected.
Next spring will tell how many survived. My trees are in a wildlife setting and get zero care in the field. They all slow down when they are surviving on the nutrition in my poor native soils, but they do much better than bare root trees.