Hi, very nice starter! Pieris make good bonsai too. Don't cut any more from underneath though, because it's the lower branches (often called sacrifices for this reason) that help fatten the trunk, and once you've grown out and trained your tree over time (years often) to a more or less final design, you can always cut them (or even just one you've left) off then if they don't fit in. I suggest that you put it straight into the ground for the season now, and then in November move it into a large but somewhat shallower pot til the spring. When you do move it (I say Nov. because hopefully it should be dormant by then, but maybe Dec.'s better where you are) try to break up the bottom third of the rootball some to spread it out, and put a tile or slate under it so it will no longer grow downward, but out. Next (early) spring, cut off old hardened and probably circling roots, spread the remaining (half?) of the ball some more and then let it continue to grow in the same pot (replace lost soil with a gritty, non-peaty mix to which you could add some bark mulch). If you want to take off individual branches to open it out a bit, get an idea where you want to go with design, do it late next winter, shortly before root pruning.

I also think that by the third year you can move it into a bonsai pot if a nice pad of young feeder roots has develop under the trunk, and of course along the way you can also (late spring is good) trim twiggy foliage to help get some shape into it.

Lucy, I recieved a suggestion similar to yours in another forum. He suggested that I slip-pot it into a larger pot and let it grow wild till winter, then aggressively root prune it and plant it in the ground or a training pot with a tile to create some nebari, and then move it to a bonsai pot the next winter, or at least begin styling.

It's looking like a pretty open and shut case so far! Any other ideas or suggestions?

Yes - relax and let it grow at whatever pace it chooses. Get some more trees to fuss with, but keep it to a dull roar as you can fuss them too much. Been there, done that and payed for it.

I know this is years too late but maybe it will help some other people with pieris questions. I had bought one last summer (mountain fire, I believe) that was most awesome. I did everything perfectly. But, slowly I began to kill it, (despite having researched a ton before I even started.) Over the course of the next few months it began to lose leaves but I still could not figure out what was wrong. Finally, I found a little three word sentence that would have saved its life, had I found it sooner. "Doesn't tolerate lime." I have well water, and tho I don't have a softener (so no salt), it is loaded with lime deposits. Since then I have purchased three more and only used rain or bottled water, and all are thriving. Also, dig down around the base of the plant a bit, you may discover a treasure of a nabari. I have yet to see a pieris that didn't have some awesome nabari features. (I have worked on around 8, all had great nabari)