In getting prepared for this year’s order of Dolgo seedling rootstock from Blue Hill, I came across my notes from last year. I don’t know if I did it the best way. It worked though.
I ordered 1/4“ last year. I planned to put them into three trade gallon pots, trimming off only any defective roots. In round numbers, 80% fit just right, 10% were too big, and 10% would have fit into smaller containers.
My air-pruning three trade gallon pots had straight sides (weren’t tapered) and were 9” in diameter. I poured the potting mix in 7” deep. One cubic yard of bulk potting mix was enough to fill just over 70 pots. (I compressed it a bit, especially when making sure that the grafted stem was sticking straight up).
When I started to run short on potting mix, I tossed 1” of rotting wood chips from a flower bed into the bottom of the pot first. Since I normally set my air-pruning pots on concrete for their summer, I didn’t think it would matter if the lowest 1” had some air gaps. The Dolgo seedling roots grew through the chips just fine as far as I could tell.
For the 10% that were too big, I first tried some five trade gallon pots. They were a little bigger in diameter than the three trade gallon ones, but the issue with fitting the roots into them was that the roots were not longer in all directions, just in one or two directions. I put 1” or so of rotting woodchips in each bag first. The Walmart bags formed a roughly elliptical cross-section with 9” and 13” axes. That allowed for an extra 4” of root length, and when I placed the tree in off-center, I gained even a bit more room for long roots. I then put 6” of potting mix over the top. I think one cubic yard of the potting mix would have been about right for 55 bags.