Answer: Unfortunately, neither of these trees is typically pruned to reduce size. Instead, a smaller maturing variety would be planted. And, at twenty foot heights, the trees are too large for a homeowner, without specialized equipment and without pruning skill, to prune safely or well.
The cherry can be pruned to remove dead or diseased wood at any time, and in some cases a bit of shaping can be done in the spring right after it blooms. The pruning cuts provide an opportunity for insects and diseases to enter the tree, and since these trees are susceptible to such problems, pruning is kept to a minimum.
Pines grown as Christmas trees are sheared or lightly pruned annually to keep their growth uniform and dense. Once they are planted in the landscape and begin to put on some size, this is no longer practical. As the tree grows it will naturally lose its lower branches and the overall look will loosen. With a twenty foot tree, there is no way to make it narrower because of the way the new annual growth occurs only at the tips of the branches. If you cut it back, the cut ends will always be visible.
In either case, if you still want to try to do something, I would strongly suggest you consult with a professionally trained and certified arborist who will have the knowledge-- and the equipment-- needed to do any work correctly and safely.
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