Answer: Next spring right after they have bloomed, would be the best time to thin them. The thinning is done by cutting off main stems at the bottom close to the ground. Take out one third of the old stems next spring. The following year, remove half of the remaining oldest stems. The third year remove the rest of the oldest stems. The next year you may not need to prune at all, but after that, say year five, start removing just some of the oldest stems each year on an ongoing basis.
This will encourage vigorous new growth to come from the bottom of the shrub, maintain its natural shape, and yet keep it from becoming crowded and overgrown. It will not however reduce the overall size of the shrub.
This shrub, as with many of the early blooming shrubs such as forsythia and lilac, should be pruned right after it blooms; if you prune later in the summer or in early spring you will be cutting off the flowering wood.
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