Answer: Mahonia aquifolium typically grows quite a bit larger than that, often reaching six feet tall and about four or five feet wide. When a smaller plant is desired, it is probably better (or easier) to plant one of the dwarf forms so the plant can mature in the space available. But if you want to try to keep a larger variety down to a smaller size, the best time to prune would be in late winter or very early spring before the new growth begins for the season. At that point you have two choices. If the plant is an old one and very overgrown, you can rejuvenate it either all at once by cutting it to the ground or in stages over about three years by trimming away a third of the oldest wood each year by cutting it at the base. The new growth will come from the ground or the base of the plant and the end result will be a size reduction for the time being. After that, periodically remove a few of the oldest branches each spring to keep the plant in bounds. Finally, as a side effect, the top pruning typically stimulates the growth of new branches or spreading suckers from the base thus making the plant wider; you may be able to remove some of these spreading bits with a sharp spade.
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