Google keeps sending people to my blog for answers on when to prune a mugho or mugo pine. I know I've mentioned it in posts, but never devoted something specifically to help my fellow gardeners answer that question definitively. Here is that post :)
Don't trim or prune your mugo now, that is, in the fall. That is the short answer. Pines will stop growing from the points at which you prune off their branches (they do not produce new buds from old wood) if you do it out of season. The right season is when the new candle growth begins in the spring. The right way and time to prune a mugo is when the new growth (candles) sprout out and then you simply spruce them up (couldn't resist the pun) by cutting the candles in half. Some people say "pinch growth by two thirds", but I let them get a bit longer and slice them in half with the pruners (secateurs). The main thing is to cut back just the new growth at this time.
If you prune in this way it gives the most natural looking trim. I would keep up with it regularly, since mugos often get bigger than their reputation for being dwarf would indicate. I have two at this property and had several at my city home years ago. There is a lot of variation in the compactness and ultimate size of pine shrubs labeled "mugho" or "mugo". They all look so nice and compact when you get the little ones at the nursery, but they grow into sizes that can overwhelm foundation plantings if you let them. The expert reason given:
Mugo Pine's incredible, yet frustrating, variation is primarily the result of its large native range. Plants with large territories tend to have greater nature/variation than plants with small ranges because they must be flexible enough to adapt to different climactic conditions to survive. Mugo pine's native range is western Europe, eastern Europe, southern Europe, and western Asia. Such a broad range requires a chameleonlike ability to adapt to different situations, which is why specimens of every size, shape and description can be found. Although there are many kinds of cultivated and naturally growing types, all are commonly known as mugo pine.
So there you go- if you happen to have one that grows larger, you can use the pruning technique of halving the candles each year, or so, to control the size. If I were to go shopping for mugo pine shrubs today I would look for the P. mugo cultivars "Mops", "Sherwood Compact" and "Slowmound"... as advised by Anne Pink.
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The previous post (2004) on mugo pines.
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