I've done the following technique successfully with Mugo on a healthy plant, in its original pot (OR SLIP potted up one size) without any prior root pruning. Bear in mind that there is always a chance of the tree being weakened or dying. The Technique: From fall until bud-swell in spring, I removed all of the current buds and even pruned back the current years growth on some of the most vigorous shoots (making sure to leave adequate needles ~ 10 pairs). The following spring as sap rose from the roots, hidden buds swelled further down the branches and some backbudding did occur. With a Mugo this far grown out, there still may NOT be enough backbudding in the interior but you can only tell by giving it a try. This technique could be duplicated in other subsequent growing seasons as long as the plant puts out enough vigorous growth after the initial use of the technique. If there is not such a vigorous response after the 1st use of the technique, give it a year or two rest and try it again. Some folks may disagree with this technique on Mugo but like I said, I've used this technique on several of my Mugos acquired as nursery stock and it had decent results.
