Mugo pine is an evergreen shrub that bears needles in groups of two and 1- to 2-inch cones. There are mugo pines that are dwarf at under 3 feet, and some that are 20 feet tall and almost as wide. Mugo pine is often cultivated as a bonsai and is hardy down to U.S. Department of Agriculture zone 2. The natural form is low and very wide. Pruning should be done to encourage the natural shape and strengthen the shrub. Mugo is a slow-growing plant that will likely not need annual pruning.
Hone and clean pruning tools before you start a project. This will ensure clean cuts and healthy plant tissue. Hold the blade of your tool at a slight angle against the sharpening stone and stroke it across two or three times. Wipe the blade and moving parts with oil.
Trim mugo pines in spring when the candles or new growth have formed. Pines are the only evergreen that are pruned when actively growing. When candles are full sized but before the needles have developed, cut the candle in half at a slight angle. Pruning in this manner encourages bushiness and compact form.
Remove any damaged or dead limbs and twigs. Dead plant material will not have needles or the needles will be brownish red. Now is a good time to check for sawfly and pine needle scale.
Make any cuts back to the healthy growth when removing large pieces of wood. Cut 1/4 inch outside the branch collar, which is a slightly swollen area on the secondary wood just off of the primary wood. Cuts on branch ends are made at a slight 45-degree angle and just past a growth node.
Re-pot and prune mugo bonsai at the same time. Pinch off the candles by 2/3 in two stages. First pinch the largest candles back and one week later pinch the smaller ones back. In fall, rub off the buds on each branch to just two and thin the needles from the top down, with the heaviest removal on top so sunlight can penetrate.