It's rated to -15F so it is marginal for me. Harold Greer (Greer Gardens in Oregon)describes it as compact, tight and denselt twiggy. One of the best, most versatile of available dwarfs, ideal for rock gardens.Delightful planted en masse where they will grow together and put on a display when they flower.

And it really does grow to only about a foot tall :)

This one is easy to please and widely grown here in this cooler Z8, tiny leaves, tiny stature.

It becomes a mound of purple for at least three weeks around April or May, when stressed will flower less.

It rarely needs shaping. If it does require pruning to remove dead wood or leafless stems, you need to rethink its growing conditions, it may need more protection from your summer heat, or it may be in too much shade, not be getting the amount of water it requires.
Impeditum is not drought tolerant, keep a constant mulch over the root zone to conserve water.

Yes, it does stay around a foot tall. I have an almost ten year old one. I love them. I love all of the little dwarf Rhodie cultivars.