Answer: You can prune Mugo pines, but you can't make them a lot smaller than they have already grown. If you reduce their size too much, they won't recover and you'll end up with some pathetic looking plants. You can trim Mugos back in early spring, just as new growth begins. Many of the mugo pines (Pinus mugo) we see for sale are grown from seed and so will show some variation in size and growth habit. Culture can also play a role in that a certain amount of compactness can be induced by breaking off the growth tips in the spring. However, there are different types. Some you might see are "var. mugo", a low growing species under 8 feet tall and twice as wide; "var. pumilio, a prostrate form; "Compacta", a very dense and rounded form, "Gnom", a very short and dark green form, and "Mops" a very dwarf form. You can't encourage a normal sized Mugo to grow into a compact or dwarf form. If your trees are much too large, you might want to consider replacing them with smaller cultivars.
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