How are you defining success/failure?

In other words, what causes you to think that your trees should have grown better?

Are these tree bands made of black plastic? If so, were they sheltered from the heat of the sun?

The mix sounds fairly water-retentive. I assume the diatomaceous earth was hard-baked?
Did the mix stay too wet, dry out too soon, compact in the pots?

I would modify the mix thusly: 70 percent bark, 15 percent peat moss, 15 percent perlite.

Lastly, what was your choice of fertilizer specifically?
Perhaps the mix ran out of nutrients prematurely.

Josh

Tap roots are over-rated, and few plants that you bur at a nursery have an actual tap root descending from the original seed radicle because most material is either grown from cuttings or grafted onto understock grown from cuttings.

Tall pots are actually easier to grow in than their shallower counterparts, so I think it's probably safe to stop looking in the direction of container depth, unless the taller pots meant a greater sun load on side walls and higher soil temps, as Josh alluded to in his learned reply.

The areas most likely to have given you problems would be excess water retention due either to media choice or watering habits, high soil temperatures, or nutritional issues. To get to the probable cause, answering Josh's questions would be helpful.

Al

I have these trees growing under artificial light at set temperature so the conditions are well controlled. My criteria for determining success of the project is comparison to trials of growing the same seedlings under the same conditions in the past in standard size 2 gallon nursery pots, where I was getting at least twice the growth during a season, despite having several trees per pot. I don't recall the makeup of the medium. The problem with using these pots is that the trees would soon become rootbound, the pots take up too much space, and the trees would really need to be barerooted to separate from one another - a practice best avoided when dealing with seedling pawpaws.

My impression is that the medium lacked sufficient water retention for the amount of attention (watering frequency) received, and the tall narrow dimension served to emphasize this effect, though I do say that I never really tested the moisture content in the containers and made judgements based on the appearance of the medium lying at the top. These pots, as with the more successful 2 gallon trials, were of necessity kept in bins which collected the runoff water, so they were often standing in an inch or more of water, but I don't believe that the roots of these seedlings extend more than six or seven inches deep - they've barely grown at all.

I used osmocote for fertilizer and there was also some leaf humus. I probably also watered at times with a soluble Miracle Grow fertilizer.