What is the best way to prune large, overgrown shrubs?

Proper pruning can renew or rejuvenate overgrown, deciduous shrubs.  One option is to prune the shrubs back over a three-year period.  Begin by removing one-third of the largest, oldest stems at ground level in late winter/early spring (March or early April).  The following year (again in March or early April), prune out one-half of the remaining old stems.  Also, thin out some of the new growth.  Retain several well-spaced, vigorous, new shoots and remove all of the others.  Finally, remove all of the remaining old wood in late winter/early spring of the third year.  Additional thinning of new shoots should also be done. 

A second way to prune overgrown, deciduous shrubs is to cut the shrubs back to within 4 to 6 inches of the ground in March or early April.  This type of pruning induces shrubs to produce a large number of new shoots.  The new shoots grow rapidly and may several feet tall by the end of summer.  In late winter of the following year, select and retain several strong, healthy shoots and remove all others at ground level.  Head (cut) back the retained shoots to encourage branching.  Overgrown lilacs, dogwoods, privets, and forsythias may be pruned in this manner.  (Most lilacs rejuvenated by this method will not bloom for two to three years.)  This method is also an excellent way to renew scraggly potentillas and summer-flowering spireas.  For best performance, potentillas should be cut back to within 3 to 4 inches of the ground about every three years.