Mugo pines (Pinus mugo, USDA zones 2-8) need minimal but targeted pruning to keep them strong, healthy and neat. The hardy, slow-growing needled evergreen plant — sometimes called a Swiss mountain pine or dwarf mountain pine — can be allowed to grow as a broad, bushy shrub or cultivated as a bonsai. The frequency and method of pruning largely depend on the shape and appearance you prefer.
When to Prune Mugo Pine
Mugo pines are slow-growing, so they may not need annual pruning if they are functioning as general decorative shrubs or being used for garden screening. How often you prune your mugo pine depends entirely on how large or small — or, perhaps, bushy or tall — you'd like it to be. Otherwise, the main consideration is to keep an eye on growth to ensure your mugo pine isn't getting too messy or encroaching on other plants.
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When needed, prune in late winter or early spring. This is when the tender new buds of growth, known as candles, will have formed but haven't yet developed needles. Avoid pruning in fall. Any cut branches will stop producing buds completely if snipped outside of growing season.
Once every two years or so should be ample, unless you're aiming for a manicured shape or need to significantly limit the size.
How to Prune Mugo Pine
Step 1: Prepare Your Pruning Tools
Make sure your pruning shears are clean and rust-free to avoid contaminating the plant. Then, sharpen the shears by holding each blade at a slight angle and, moving away from your body, stroking it across a sharpening stone two to three times. Sanitize the blades by wiping them with isopropyl alcohol.
Step 2: Remove Dead Material
Snip off any dead, bruised or snapped limbs and twigs. Dead plant material will have dropped its needles or the needles will have turned a rusty red.
Step 3: Trim the New Growth
Identify the new growth candles and halve them at a 45-degree angle. This will keep the shrub compact and full and prevent it from growing out of control during the season.
Step 4: Snip Into Shape
Train the shrub to grow taller or wider by remove exterior candles at the points where you wish to restrict growth. Do not prune candles in areas where more growth is required. Leave around 1/4 inch outside the branch collar (the shoulder between the trunk and branches).
How to Prune Mugo Bonsai
Mugo pines that have been cultivated as bonsai plants require a more delicate approach to pruning. They will also need to be pruned after each repotting and when wiring the tree into shape, which should be carried out in late fall or early winter (unlike with mugo pines grown as traditional shrubs).
Pinch off the largest candles in late winter or early spring by roughly two-thirds using bonsai pruning scissors. Wait a week and then pinch the smaller candles back by two-thirds.Prune again in fall by gently rubbing away the buds on each branch using your fingertips, leaving just two. At the same time, thin the needles from the top down, with the heaviest removal on top so sunlight can penetrate to the lower branches. Prune the roots by trimming the tough, wiry ones and keeping the soft, fibrous ones.