Answer: You can remove the tallest upright stems individually by reaching into the center of the bush and cutting them off at the ground. On a forsythia, you could do that now as long as you are just selectively pruning some of those oldest stems. However, a mature size of ten feet tall and wide is not unusual for a forsythia bush and you will not be able to keep it small by pruning. If you have a lot of shrubs that are too big for the space, you may want to consider either removing some to leave ample room for the remaining plants or consider replacing them with plants that will mature to a more appropriate smaller size. In general, trying to prune to keep plants far smaller than their normal size is a losing battle.
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