Kunso Kim, head of collections and curator, suggests you consider these different approaches.
Selective pruning: To shape the shrub or control its size somewhat, go branch by branch, choosing the branches you want to keep and removing those that are too long, superfluous or causing a tangle. Using sharp, clean pruners, prune just above the place where one stem branches off from another; don’t cut in the middle of a branch between joints, which will cause bristly, unsightly growth. Take it slow, pausing often to step back and see whether you have achieved the effect you want.
Renewal pruning: If the shrub is thickly overgrown, try renewing it by cutting some of the oldest, thickest branches down to the ground. Remove no more than one-third of the shrub at a time. In spring, new branches will grow. If you cut out one-third of the oldest branches each year for three years, your shrub will be completely renewed.
Rejuvenation pruning: If a shrub has become such a hulking tangle that you are tempted to give up on it, a drastic step may be in order. Most deciduous shrubs can be entirely cut back within an inch or two of the ground. It will be a shocking sight, but new growth will start in spring, and within a couple of years the shrub will have recovered with new vigor.
Bear in mind that spring-blooming shrubs, such as lilacs, forsythia, azaleas, and bigleaf hydrangeas, will already have formed their flower buds by this time, so pruning off those branches and buds now will keep them from blooming. Wait to prune them until just after they bloom, Kim says.
You can use the branches you prune in indoor arrangements.