Overgrown shrubs with crowded branches or weak or misshapen stems due to previous bad pruning can be rejuvenated. There are two methods: The shrub can be gradually renewed over a period of three years or it can be drastically pruned all at once. Although the first method takes longer to produce a re-shaped plant, some shrubs respond better when gradually renewed. Drastic rejuvenation creates a temporary eyesore, but the goal is achieved more quickly than with gradual renewal.
In gradual renewal all existing branches are removed during a three- to five-year period, thus encouraging new wood to replace the old. Begin by cutting one fifth to one third of the oldest and longest stems at the ground, using loppers or a saw. In following years remove another fraction of the oldest stems until none of the original stems remains after three to five years. It may be necessary to thin remaining branches to restore a balanced shape to the shrub until all old growth is gone.
Some shrubs respond amazingly well when their stems or trunks are cut off at ground level all at once. Shrubs that are declining or suffering from repeated heading often respond to this shock treatment, and it is the fastest way to rejuvenate them. Numerous new shoots soon arise from the ground; select the strongest and most vigorous shoots, thinning the smallest and weakest stems. Do this extensive pruning late in the dormant season before new growth begins.
This method works well with upright shrubs such as lilac or forsythia and with spreading, horizontally branched shrubs such as cotoneaster. When their stems become too woody, cut them to the ground and new shoots will soon take the place of the old branches.
Pruning Large, Overgrown Shrubs into Small Trees
With careful pruning, a gangly, overgrown shrub can be transformed into a picturesque small tree. Among the best shrubs for sculpting into an artistic feature or focal point are flowering crab apple, flowering plum, viburnums such as blackhaw, gray-stem dogwood, magnolia, and winged euonymus. Others that are suitable include bottlebrush, witch hazel, and common lilac.
Begin by selecting several stems to serve as multiple trunks and remove all others. Cut off all lateral branches below the point selected for the tree canopy—usually about 4 to 6 feet above the ground, or higher if desired. Continue to remove new shoots that emerge from the base and below the canopy until the trunks grow large enough to discourage resprouting. Thin the tree canopy as needed.