Physocarpus responds well to renewal pruning, but yours are new. And though you've said you don't care about this year's bloom, the pruning gurus recommend letting them bloom out then cutting them back immediately afterwards. It's unlikely you'd harm them by pruning now, but I don't think I'd cut the existing stems back by more than a half or a third this time around. It might take a couple or three seasons to get them to really start to flesh out nicely.

You really do need to let the root system get well established - leaves feed roots as much as roots feed leaves, so it might be better to let them grow for the first year. Taking one or two stems back by a quarter MIGHT not hurt, depending on how they are growing, but is more likely to push more growth at the cut point, putting more stress on the roots. Next spring, taking the longest stems back to ground level, and continuing to take a third of the longest/oldest growth out annually, might not be a bad idea - or you can do it until you have a shape you like, and then only as needed.

If you want fullness at the bottom, you have to take the old growth back to the bottom - you get growth at the point you trim back to - and a little below that - but it may not be growth at the bottom of the plant, but rather half or a quarter of the way up if you cut to a third or a quarter of the way back. Hope that makes sense when you try to visualize it....

Thank you both for your suggestions. You may have saved the lives of some shrubs :). I'll try to be less drastic and more patient with my pruning.

In my experience, physocarpus has a pretty long (one or two growing seasons) "ugly duckling" stage. They have a tendency to shoot out long branches at odd angles. Try to look elsewhere, and don't prune it all during this awkward phase. It will grow lush and start arching in a beautiful, flowing fountain by it's third year in the ground. :)

Can it be pruned to look like a mini tree?